2.0 Literature review
The tomato (Lycopersicon esculentum) plant is an herbaceous plant usually grown as an annual. The plant is a vine and can grow at altitudes ranging from approximately sea level to 3,300 m above sea level. It is a warm-season crop that can grow optimally at temperatures of between 21-23oC with a maximum of 35oC. It does not tolerate frosty conditions and temperatures below 10oC are detrimental to the growth of the plant. The ideal soil is the one that is well structured with a high amount of organic matter, well-drained and has a pH of between 6 -7.5. Tomatoes respond well to Nitrogen, Potassium and Phosphorous (NPK compound) fertilizer application but the recommended dosage depends on the type of soil and local conditions (Naika et al., 2005; Abdullahi and Choji, 2009).
Conventionally, tomato plants are raised from seeds which are nursed and later transplanted after 4-6weeks. The seeds are nursed in sterilised soils in seed beds, seed boxes or trays. Conventionally, they are mulched with leaves such as palm fronds from healthy branches or with straw. Watering is done over the mulch material to prevent the seeds from being washed away and the seeds normally germinate in 3-5days.The seedlings are protected from direct sunlight and wind and also from pest and diseases. This is achieved by erecting covers over the seedlings and spraying them with recommended fungicides and insecticides. Light watering is required to help the tender plants establish themselves before being transplanted to the field. The time between planting of the seeds to transplanting them is usually 4-6weeks. A week prior to transplanting, the plants are hardened by reducing shade and water application. The seedlings are transplanted when they reach a height of between 15 and 20 cm and when the stem girth is about the size of a pencil. They are transplanted onto prepared beds at a spacing of 1-2 m between rows and about 50 cm between plants in a row (Centre for Overseas Pest Research, 1983; Jones et al., 1991; Naika et al., 2005). Similar to the conventional method, tomatoes can also be grown in the greenhouse from transplants raised from seeds. However, intensive care is provided and the conditions under which the plants are raised can be monitored and controlled (Hochmuth, 2008)
2.1 Nutrient Composition
100 g of tomato fruit contains about 94% water, 1% protein, 4.3% carbohydrate, 0.3% fibre, 250 IU of vitamin A and 25 mg of ascorbic acid (Purseglove, 1974; Baas, 2006).
2.2 Origin, history and classification
The cultivated tomato is a member of the family Solanaceae also known as the "night shade" family and believed to have originated from the Andes region of South America which covers Peru, Ecuador, Colombia and Chile. Although it is generally agreed among taxonomists and breeders alike that tomato was domesticated in America, the exact place of domestication has not yet been established (Abdullahi and Choji, 2009). The name tomato comes from the Nahuati language"xitomatl" of the Aztecs in Mexico which means the" plum thing". Prior to scientific classification, names of plants were given based on polynomial phrases describing where they were found such as Solanum peruvianum (found in Peru);the morphological characteristics of the plant such as Solanum tuberosum (developed from the tubers) and in honour of people such as Solanum neoricki (named after Neorick). The identification of tomato with the genus Lycopersicon, meaning "wolf peach" in Greek falls in the second category. The genus Lycopersicon was first proposed by Tourmefort in 1694. Later, Linnaeus proposed a classification based on the binomial system and classified tomato under the genus Solanum in 1753. A year later, Miller corroborated Tourmefort's classification of tomato with the genus. Lycopersicon, which gained popularity, acceptability and usage among scientists.. The genus Lycpersicon consists of six species of tomato grouped under two sub-genera, Eulycopersicon and Eriopersicon (M?ller, 1940 as in Spooner and Peralta, 2001). The Eulycopersicon consist of two hairless, red to orange-fruited species with flat seeds, i.e. Lycopersicon esculentum and Lycopersicon pimpinellifolium. The Eriopersicon on the other hand consist of four green-fruited species, i.e. Lycopersicon cheesmani , Lycopersicon grandulosum , Lycopersicon hirsutum and Lycopersicon peruvianum Luckwill (1943 in Spooner and Peralta, 2001) maintain this sub-genera but included L. pissisi in the Eriopersicon group. Rick (1979 in Spooner and Peralta, 2001) also classified tomato under two groups- Esculentum complex and Peruvianum complex; the Esculentum complex consists of seven species while the Peruvianum complex has two species all under the genus Lycopersicon. Later on, Child (1990 in Spooner and Peralta,2001) classified tomato under three series; the Lycopersicon series with three species; Neolycopersicon with one species and Eriopersicon series with five species but all under the genus Solanum (Spooner and Peralta, 2001;Peralta et al., 2006).
The continuous review of the classification is based on novel discoveries that seek to get the most appropriate genus into which the cultivated tomato best fits. Though there has been a common knowledge that the cultivated tomato draws its ancestry from the Solanum lycopersicon var cerasiforme, Nesbitt and Tanksley (2000 in Bai and Linshout, 2007) indicated that the cerasiforme are actually a mixture of wild and cultivated tomato varieties. With recent revelations based on phylogenetic studies using DNA and molecular markers, the genus Lycopersicon has been re-integrated into the genus Solanum . For purposes of consistency, L. esculentum is still being used by many but the appropriate nomenclature for tomato is now Solanum lycopersicon (Peralta et al., 2006; Bai and Lindhout, 2007).
2.3.1 The plant
The tomato plant is a herbaceous perennial usually grown as an annual. It is a succulent vine with typical weak woody stems that allow it to easily climb supports and can grow to heights of up to 3m (Purseglove, 1974)
Based on its growth habit, the tomato plant can be classified into three categories:
1. Determinate type
2. Indeterminate type
3. Semi-determinate type
Feather (2008) described the above-mentioned growth habits of the tomato plant as follows:
Determinate type:- tomato plants that belong to this group exhibit a bushy, compact growth habit where they can grow to a height of between 1m and 1.5m. Flowering and fruiting usually commence when the plants reach this height. The determinate types of tomatoes tend to have a synchronised ripening pattern and are ideal for gardeners who are interested in growing tomatoes for processing because
, all the fruits ripen at the same time and harvesting can be done at once. Although many varieties have erect and self-supporting stems, staking may be necessary under heavy fruit load to prevent fruit contact with soil thereby , preventing infection and spoilage from soil-borne pathogens. By virtue of their height at maturity, the determinate varieties can be grown in containers more easily than their indeterminate counterparts. Some of the determinate varieties include 'Roma VF', 'Better Bush,' 'Bush Early Girl' and 'Mountain Spring.'
Indeterminate type:- They have unrestricted growth habits under ideal conditions. Hence, they continue to grow, blossom and fruit throughout the growing season until they are killed by frost or by diseases. They produce very tall vines and can easily grow up to 2-3 m tall over the growing season. Indeterminate types by their nature require staking to support the vine in an upright position and to prevent fruit contact with the soil in order to minimise rot by soil-borne pathogens. Staking also promotes air circulation and increases the exposure of leaves to sunlight for photosynthetic activity and thereby increasing the plants' fruit- bearing capacity. Examples of indeterminate varieties include 'Sweet 100' (cherry tomato), 'Big Boy,' 'Brandywine' and 'Beefmaster.'
Semi-determinate or semi-indeterminate:- These are varieties whose growth habit is intermediate between the two classifications mentioned earlier. These types may be referred to as semi-determinate or semi-indeterminate types. They tend to blend the characteristics of the two types and will normally grow larger than the determinate varieties but are less sturdy than the indeterminate ones. Typically, they can grow to a height of between 1 and 2 m and may or may not require staking. Examples of semi-determinate tomatoes include 'Celebrity' and 'Mountain Pride
2.3.2 The leaves and flowers
The leaves (Figure 1.0) of the tomato plant are compound, measuring between 10-25 cm in length. They are odd-pinnate, glandular-hairy and have about 5-9 leaflets. Each leaflet measures about 8cm in length with serrated margins.
The flowers (Figure 1.1) are perfect containing both male and female parts and are borne on the apical meristem. The anthers are fused together along the edges forming a continuous linkage around the style allowing for self pollination and fertilisation. They measure about 1-2cm in diameter and are yellow with five pointed lobes on the corolla. They occur in clusters of 3-12.
2.3.3 The fruit
The tomato fruit is botanically classified as a fleshy berry; in other words, it is a well developed and matured ovary containing ovules. It is green (Figure 2.0) and hairy when it is immature but becomes glabrous (hairless) and shiny upon maturity. It turns red (Figure 2.1) or yellow on ripening, depending on the variety, and it is usually globose (globe-shaped) or depressed at either end and can be smooth or furrowed with an external diameter of 2-15 cm. The fruit has locules which are hollow spaces containing a lot of reniform (kidney-shaped) seeds which are hairy and light brown.
2.3.3a Types of tomato fruits
Cherry Tomato- The term "cherry" usually refers to the shape and size of the fruit. Cherry tomatoes are small in size ranging from 12.5 mm to 16 mm in diameter and may be red or yellow in colour. The fruits are normally found in clusters and can produce on average of up to about 100 fruits per cluster. Owing to their bearing capacity, cherry tomatoes produce high yields per plant compared to other types of tomatoes. Most cherry tomatoes have high-climbing indeterminate vines and the fruits of some have relatively tough skin. Vines can be staked, supported on trellises and/or pruned to two main stems and directed to strings or shoots (Jones et al., 1991; Franco et al., 2009).
Pear Tomato- As the name suggests, pear tomatoes are pear-shaped. Pear tomatoes also have small fruits less than 50 mm in diameter and have nipples at the stem end of the fruit. The vines of this type of tomatoes are similar to that of the cherry tomato in that they are tall and indeterminate but are lower in yield compared to the cherry tomatoes.
Plum and Peach tomatoes:- These types are similar to pear tomatoes but have no nipples. Fruits of all the three types of tomatoes can either be red or yellow with tender skins and juicy endocarp. Some of the tomatoes used in making paste, whether plum- or pear-shaped, are solid and firm (Purseglove, 1974; Jones et al., 1991).
From a commercial point of view, tomato fruits can be classified into three groups according to their shape:
1. Round or spherical - these can further be grouped into round and cherry tomatoes where the latter is smaller.
2. Ribbed - this refers to tomatoes with well differentiated and pronounced ribs around the navel.
3. Oblong/elongated - these are tomatoes with an ovoid or ellipsoidal shape that may be elongated and have smooth skins (OECD, 1988).
The tomato has many culinary uses. Whole tomatoes can be eaten raw as refreshment or as a dessert or cut into pieces and served in salads or sandwiches. The seeds contain about 24% oil which can be extracted and used in salads and also in the manufacture of margarine and soap, while the residue (press cake) is used as livestock feed and as organic fertilizer (Purseglove, 1974; Centre for Overseas Pest Research, 1983).
Gould (1992) cited in OECD (2008) indicated that although consumers normally prefer fresh tomatoes, over 80% of the tomatoes that are consumed are in the processed form. However, it is worthy of notice that no matter the processing method, it is prudent that tomatoes used should be ripe and red in colour, and firm or soft but free from mould, dirt and plant debris. Some of the forms into which tomato can be processed and used in different ways are outlined below:
Canned tomato: - Canned tomatoes can be prepared by simply peeling off the skin of whole fruits, coring them and putting them in cans. Flavourings such as salt, sugar and acid can be added and this can be used in preparing soups and stews.
Tomato puree or pulp: - Tomato is crushed into a form of slurry and the liquid (including the pulp) portion of the fruit is separated from the seeds, and other hard substances. Water is then evaporated from the extract until it is concentrated and retains about 8-24% salt-free soluble solids (4-6% natural solids).
Tomato paste: - Tomato paste is made from tomato puree. The pulped tomato puree is concentrated by further evaporating water from the paste until it contains about 24-39% solids with or without salt, spices and additives. Unremarkably, tomato puree and paste are used interchangeably.
Tomato juice: - Tomato juice is made by crushing the fruits and sieving the pulp from the liquid portion. The slurry is then heated and refined into an unconcentrated form. This usually contains fine insoluble solids from the flesh of the tomato. A small quantity of salt is then added and thereafter, the juice can be consumed without dilution. The concentrated form of the juice contains about 20-24% soluble solids. Normally, the juice is not flavoured but beverages that use it as a base contain some seasonings such as sugar, spices, flavourings, citric acid and salt. A high quality tomato juice retains its organoleptic properties and vitamin C content (a minimum of 15 mg/100 ml)
Tomato squash: - Though not a very popular squash due to the fact that consumers are used to squashes from other fruits, it is worth mentioning. Its processing is similar to that of the juice but with added preservatives such as sodium or potassium benzoate. Syrup is also added to increase the concentration of total soluble solids to between 30 and 50%.
Tomato ketchup: - Tomato ketchup (catsup) is made from either the fresh juice or from concentrated pulp. Tomatoes are chopped and the skin, seeds and cores are removed. The pulp is then heated until it is concentrated. Sugar, vinegar, onions, salt and spices are added while it is still being cooked until the required consistency is reached.
Tomato powder: - Traditionally, whole tomatoes are cut into pieces and sun-dried until they are hard and crispy. The dried tomatoes are then milled into powder. In the conventional method, tomatoes are crushed and the liquid component is evaporated using a spray drying technique and heat rollers to dry the pulp to a low moisture content of about 5%. This can then be milled to obtain the tomato power. The powder can be reconstituted as juice for drinking or as an ingredient in soups. It is ideal to store the powder in sealed containers.
Tomato chutney: - Tomatoes can also be made into chutney by cutting the ripe fruits into pieces and boiling them with vinegar until they are soft. Spices such as ground chillies, garlic, ginger as well as seasoning such as salt or sugar are added while boiling at low heat to get the desired consistency (Villarreal, 1980; OECD, 2008).
2.5 Importance of tomatoes
2.5.1 Economic Importance
Tomato has contributed immensely to the per capita Gross Domestic Product (GDP) and export earnings of many countries worldwide including China, France, Greece, Israel, Italy, Portugal, Spain, Turkey and the USA. Owing to its importance as an export commodity, it has generated a lot of trade competition among some of these great nations (World Horticultural Trade and US Export Opportunities, 2005). Between 2002 and 2007, global trade in tomato increased with average exports of 8% per annum ). As shown (Table 1.0) by the Food and Agricultural Organisation(FAO) production figures for 2007, the five leading producers of tomato are China the leading producing country, followed by the USA, Turkey, India and Egypt in that order.
Country Production (tonnes) % Share in the World China 33,645,000 23 USA 11,500,000 9 Turkey 9,919,673 8 India 8,585,800 7 Egypt 7,550,000 6 World Total 126,246,708 100
Tomato production helps in improving the livelihood of growers especially in the tropics through income generation and therefore has a high priority among horticultural crops in these areas. Generally, tomato cultivation generates employment in rural areas, it stimulates urban employment, expands exports, improves nutrition and increases farmers income. In rural areas, the tomato industry generates employment for people throughout the supply chain. This includes input dealers who supply seeds and agrochemicals to growers, nurserymen and women who are engaged in raising the seedlings, those providing labour for land preparation, production and harvesting as well as those involved in the transportation and marketing of the produce. The stimulation of urban employment by tomato production is linked to the production of agrochemicals, packaging materials and related inputs by factories that engage both skilled and unskilled labour. It can also be linked to agencies involved in advertising tomato products. With increases in demand for tomato and its products by non-tomato producing countries, the export base of tomato producing countries expands when they export to these countries. This is so for countries like Egypt, Morocco and Jordan that are exporting tomatoes to Western European countries. Tomato fruits contain vitamins and minerals which are essential in meeting the health needs of humans. Apart from the vitamins and minerals, tomato contains lycopene which has been clinically associated with combating, cancer in humans, preventing osteoporosis, managing infertility in men, reducing blood pressure and hypertension and photo-protection of the skin. Income is generated from the sale of a product. Since tomato farmers get their income from the sale of tomatoes; the greater the output from production, the higher the income obtained by the farmer from the sale of the produce (Villarreal, 1980).
2.5.2 Clinical or nutraceutical importance
Apart from their economic importance, tomatoes and tomato products are vital to human nutrition, supplying folate, vitamin C, potassium, and more importantly, carotenoids (vitamin A precursors with antioxidant activity), the most important of which are lycopene and beta-carotene which protect the cells of the body from oxidative damage (Acedo and Thanh,2006). Available evidence indicates that a newly bred variety of tomato known as the "Purple Tomato" contains high amounts of anthocyanins which can help in fighting cancer in patients. Consumption of the fruit has been shown to extend the lifespan of cancer infected rats (Martin et.al, 2008; Silvia et al., 2009).This assertion is supported by Sharoni and Levy (2007) who indicated that lycopene together with other carotenoids such as phytoene and phytofluene are effective in preventing the proliferation of cancer cells.
Preclinical trials on patients diagnosed with prostate cancer showed that the consumption of 15 mg of lycopene from tomato fruit twice daily for three weeks had a suppressing effect on the prostate cancer (Kucuk et al., 2007). In a related study, Chen et al. (2001) assessed thirty-two prostate cancer patients who were made to consume tomato pasta sauce (30 mg daily) for three weeks. They found a significant increase in serum and prostate lycopene concentrations and an associated decrease in leucocyte oxidative deoxyribonucleic acid (DNA) damage which is related to prostate cancer.
Lycopene is proved to be the strongest antioxidant (Saita and Vitatene, 2008) being 100 times stronger than a-tocopherol and making it instrumental in maintaining human health The benefits of lycopene on human health have and are continuously being explored with exciting results. For instance, studies have been conducted on its effects in the management of male infertility by reducing oxidative stress in spermatozoa (Mohanty, 2007); its effects on reducing blood pressure and preventing hypertension (Paran, 2007); its effects in protecting the skin from ultraviolet radiation (Stahi, 2007) and its role in the prevention of osteoporosis, a disease condition that leads to fragile bones especially at postmenopausal age in women (Rao, 2007).
2.6 Constraints in tomato production
Production as well as postharvest constraints are relatively pronounced in developing countries. The production of tomatoes is constrained by factors such as lack of improved crop varieties, inadequate marketing systems, seasonal fluctuation in supplies and prices, inadequate research and, most importantly, postharvest losses known to cause up to about 30% of losses in these countries (Villareal, 1980).
2.7 Postharvest technology
The tomato fruit is considered to be a perishable commodity in that it has high water content and does not store well after harvest without an appropriate handling or storage technique (National Academy of Sciences, 1978).
The postharvest life of fruits and vegetables constitutes all the activities involved in handling the produce immediately after harvest until consumption. The application of the knowledge of biology and physiology of commodities as well as engineering principles in postharvest activities constitutes postharvest technology. Kitinoja and Kader (2002) indicated that the application of postharvest technology is aimed at achieving three main objectives;
1. To maintain produce quality in terms of appearance, texture, flavour and nutritive value;
2. To maintain food safety;
3. To reduce losses between the farm and the point of consumption.
Application of appropriate postharvest technology is an important aspect that requires attention as far as modern agricultural production is concerned. As mentioned earlier, the postharvest stage is a nexus that encompasses several processes that include the integrated functions of harvesting, cleaning, grading, cooling, storing, packaging, transporting and marketing. These activities follow a series of interlinked steps where the produce is handled by different people of different backgrounds between harvest and consumption. The technology of postharvest handling ensures that the consumer receives a commodity in a state as it was before being transported out of the field. Panhwar (2006) observed that the application of improved postharvest practices often results in decreased food losses, improvement in the overall quality and safety of food, and higher profit margin for growers as well as marketers.
2.8 Postharvest losses
Notwithstanding efforts geared towards the development of improved varieties, improvement in the transport system, improvement in the quality and shelf-life of produce as well as improvement in production systems, the tomato industry, particularly in developing countries, still faces significant challenges at the postharvest level due to losses from fruit over-ripening, microbial wastage and physical injuries. As a result, the net return to farmers remains low while retail prices go up.
Postharvest losses can occur either in quantitative or in qualitative terms. These two major causes of losses occur between the farm gate and the table. It is estimated that about one-third of harvested fruits and vegetables do not reach the consumer as a result of these losses. .In order to minimise the losses, it is important to understand the biological and environmental factors involved in postharvest deterioration, and to adopt the appropriate postharvest technology or procedures that will slow down deterioration and maintain quality and safety of the commodities (Kader, 2005).
2.8.1 Quality and safety attributes in fresh produce
Quality is a term that is subjective in description depending on the interest of a particular individual. Quality has been generally defined as "fitness for purpose", i.e. giving consumers what they need, at the right time and at the right price. Quality attributes of fresh produce include: texture, colour, flavour and nutrient content (Kader and Rolle, 2004).
Safety of produce on the other hand can be defined as the "the assurance that food will not cause harm to the consumer when it is prepared and / eaten according to its intended purpose" (CAC, 2003). Aspects of fresh produce that can render them unsafe for consumption could be inherent in the produce such as glycoalkaloids in the Solanum family (potatoes and tomatoes), biological toxins (mycotoxins), pesticide residues, environmental pollutants and other contamination through inappropriate cultural practices and postharvest handling
2.8.2 Quantitative losses
These arise from structural damage or microbial wastage and weight loss through evaporation of intercellular water.
2.8.3 Qualitative losses
These arise from undesirable physiological and compositional changes of the produce that affect the appearance, taste and texture of the produce making it less desirable to consumers. These can arise from the normal metabolism of the produce or its immediate environment (Wills et al., 1998).
2.9 Factors that affect losses in quality and safety of produce
As indicated by Acedo and Thanh (2006) and Kader and Rolle(2004) the factors that affect losses, quality and safety of horticultural produce can be considered in three categories - pre-harvest factors, harvest factors and postharvest factors
1. Pre-harvest factors such as genotypic traits of the crop, climatic/environmental factors as well as cultural practices;
2. Harvest factors such as maturity at harvest, time of harvest and method of harvesting;
3. Postharvest factors such as storage conditions, physiological disorders, pathological disorders and handling.
2.9.1 Pre-harvest factors
Pre-harvest factors that affect the quality and shelf life of horticultural crops have been described by Lee and Kader (2000) as follows:
a. Genotypic traits:-certain crop varieties including tomato exhibit some inherent variations in nutrient composition, quality and shelf life potential. These variations determine the latent quality of the postharvest condition of the produce. Examples are high carotenoids, sugars, acids and vitamin A content in tomato. Additionally, some hybrid varieties contain the rin and nor genes which retard
s ripening and hence contribute to extending the shelf life of these cultivars.
b. Climatic conditions:-the two most important climatic factors influencing the chemical composition of tomatoes are temperature and light intensity. These factors affect the ascorbic acid content of the plant by their effects on photosynthesis from which sugars are broken down into ascorbic acid. Low temperatures reduce ascorbic acid synthesis while high light intensity increases it. On the other hand, high temperature is known to favour ß-carotene at the expense of lycopene synthesis in tomato and hence results in light red or yellow colouration in the fruit. Since plants require water for their metabolic activity, the supply of water to the plant through the soil is influenced by rainfall and this can affect the chemical composition of the harvested produce.
c. Cultural practices:- high nitrogen fertilizer and high water application rates tend to reduce vitamin C content and postharvest shelf life of fruits and vegetables. On the other hand, a high calcium application improves storage life of the crop and confers support against physiological disorders such as blossom-end rot in tomatoes. The use of other agrochemicals such as pesticides also influences the chemical composition of horticultural commodities (Kader and Rolle, 2004; Thanh and Acedo, 2006).
2.9.2. Harvest factors
The harvesting stage is a critical stage that determines the quality as well as the length of time the produce can be kept safely in storage without deterioration. It requires an expert acumen to determine the ideal stage to harvest such that the quality of the produce as well as its shelf life is not compromised. In the absence of expert advice, produce can be harvested either overripe, underipe or immature. When harvesting is not carried out properly, mechanical damage at this stage can cause defects in the produce which provide entry points for and invasion by disease-causing organisms during subsequent operations; it can also lead to loss of water and vitamin C. Tomatoes harvested overripe have a shorter shelf life while immature produce fails to develop full colour and flavour, becomes shrivelled, deformed and deteriorates faster during storage (Bautista and Acedo, 1987 as in Acedo and Thanh, 2006). Acedo and Thanh (2006) identified four major considerations that should be adhered to when harvesting tomato. These are: harvest maturity, time of harvest, harvesting method and field postharvest handling.
a. Harvest maturity: crops are considered mature for harvesting either at their physiological climax or by their calendar age/months. Physiological climax is reached when the plants begin to wilt after bearing and which is not associated with disease or moisture stress; when they begin the ripening process and when they begin to lodge depending on a particular crop. The calendar age at which crops are harvested refers to the maturity period as determined by the breeder of that particular crop or variety. However, whether physiological or calendar maturity, Kader and Rolle (2004) observed that some horticultural crops such as strawberries, citrus and pineapple are better harvested ripe on the plant in order to attain maximum quality while others such as apples, mangoes and tomatoes can be harvested and ripened off the plant and still maintain their eating quality. This is a result of their relative production of the ripening hormone (ethylene); the former produce less while the latter produce more ethylene. Harvesting of tomatoes can be carried out at mature green, breaker or turning, pink or red- ripe stage depending on whether they are meant for processing, immediate consumption or for distance transportation. The mature green stage refers to the stage at which the fruit is fully matured but the fruit surface is fully green and when cut the seeds inside the fruit slide without being cut. The breaker stage refers to the stage at which there is a break in the green colour of the fruit by a tannish-yellow, pink or red spot not more than 10 percent of the fruit surface. At the turning stage, the colour change in the fruit is more than 10 percent but not more than 30 percent of the surface while at the pink stage, the colour change is more than 30 percent but less than 60 percent and at the red stage, the colour change is between 60 and 100percent of the fruit surface area (USDA, 1997). However, maturity at harvest has an impact on the nutritional composition and postharvest life of the crop. For instance, tomatoes harvested at the red ripe stage contain more ascorbic acid and overall flavour than those harvested at green or breaker stage and ripened afterwards (Lee and Kader, 2000; Kader, 2002)
b. Time of harvesting: it is ideal to harvest tomatoes early in the morning or under cooler conditions to minimise heat load on the fruits. This reduces respiration rates, ethylene production and associated ripening. Harvested produce gets heated especially on warm days. It is ideal to hold produce in a cool place on-farm. This allows it to cool and reduces ethylene evolution and hence enhances storage life. As indicated by Thomson et al. (2001 in Kitinoja and Kader, 2004), when produce is harvested and left in the sun, its internal temperature rises above the external temperature by about 4 - 6oC hence increasing the metabolic rate.
c. Method of harvesting: tomato harvesting is usually carried out manually by hand. This means that there is the tendency by harvesters to tear the fruit from the pedicel when the fruit is pulled from the plant. Therefore during harvesting of tomatoes, it is necessary to ensure that the fruits are not pulled from the plants in order to prevent tearing of the pedicel as this helps prevent water loss and aids in gas exchange.
2.9.3 Postharvest factors
a (i) Field handling: tomato being a perishable crop requires careful handling especially at the field level to ensure minimal damage to the fruits. By virtue of negligence or ignorance, some harvesters throw the fruits into collecting containers such as pans or boxes. This practice of fruit dropping or throwing needs to be avoided in order to prevent impact bruising. The harvested produce should be held in a shaded or cool area to remove heat from them before transportation.
Proper handling of horticultural produce is critical during transportation in order to avoid damage and to ensure a longer shelf life of the produce. Careless or rough handling and transportation of produce on bad and bumpy roads may cause mechanical damage to the produce. With fruits such as tomato, such a rough handling practice will trigger the production of ethylene which will enhance ripening and early senescence of the fruits. Additionally, when produce is transported in open trucks over long distances without a covering on the containers, the fruits may be sun-scalded especially on bright sunny days. Against this backdrop, the produce should be protected from the sun by covering with an appropriate material and a provision made for enough ventilation. In the case of vegetables, severe moisture loss and wilting can result under such harsh condition. When the produce is loosely packed, it easily gets damaged in transit through bruising and abrasions as the commodity moves and hits each other or the container during the transportation process (Kader, 2005).
When produce is harvested, especially late in the morning or early afternoon, its temperature is high resulting in increased metabolic activity and hence requires immediate cooling. Delay in pre-cooling during field handling often leads to rapid deterioration and reduction in quality as a consequence of increased ethylene production. Improper control of storage conditions for a particular commodity can also result in faster deterioration and/or a poor quality product. Avoidance of mixed storage helps in preventing spoilage and subsequent senescence. For instance, ethylene production from ripening fruits can promote senescence in leafy vegetables when these are mixed-stored. Storage at temperatures that are too low may induce physiological disorders or chilling injury (Kitinoja and Kader, 2001).
a (ii) Retail handling: marketing of agricultural produce involves the physical process of bringing the produce from producers to consumers and retail sale. Knowledge of marketing strategy of agricultural produce should be combined with available technology for enhancing the quality and storage life of the produce. Displaying produce for lengthy periods in retail outlets leads to a reduction in quality. Also, displaying produce at different stages of ripening or displaying commodities that produce ethylene with those that do not often leads to an overall reduction in quality and faster deterioration. For instance, Wills et al., (1989) observed that displaying potatoes in relatively bright light results in greening and synthesis of solamine, a toxic glykoalkaloid. In addition, many of the above practices are aggravated by pathological problems resulting from rotting by fungi or bacteria. High humidity conditions are conducive for microbial growth especially in produce damaged during harvesting and handling.
b. Losses due to physiological disorders
Physiological disorders involve tissue breakdown in the produce that may not be associated with disease or pest attack or mechanical damage. They may arise in response to pre- or postharvest conditions such as nutrient imbalance during the development of the fruit or irregular storage temperature respectively. According to Wills et al. (2007), physiological factors can be sub-divided into five categories namely: nutritional disorders, temperature-related (low and high) disorders, respiratory disorders, senescence and miscellaneous.
Low oxygen and/or high carbon dioxide (CO2) in or around the harvested produce usually result in respiratory disorders especially when it is enclosed in a packaging material. An example of such disorders is black heart in potato and midrib browning in lettuce caused by low oxygen and high CO2 respectively.
An example of a disorder associated with senescence is mealiness in apple which is a result of over-maturity and/or long storage.
Some of the disorders considered miscellaneous include greening in potatoes exposed to light as well as rooting of onions in high humidity.
c. Losses due to pathological disorders
Pathological disorders (Table 2.0) are caused mainly by fungi such as Aspergillus, Botrytis, Alternaria, Fusarium, Penicillium among others and bacteria such as Pseudomonas and Erwinia. With the exception of Colletotrichum, these pathogens can only invade damaged tissues of the produce (Wills et al., 1989).
Major postharvest diseases of fresh fruits and vegetables (Source: Wills et al., 2007).
Crop Postharvest disease Causal agent Apple, pear Grey mould Botrytis cenerea Blue mould Penicillium expansum Papaya, mango Anthracnose Colltotrichum glooesponioides Potato, leafy vegetables Bacterial soft rot Erwinia carotovora Banana Crown rot Colletotrichum musae Peach,cherry,strawberry Rhizopus rot Rhizopus stolonifer
2.10 Effects of handling on ethylene production
Generally, living organisms respond to external stimuli in various ways. In plants, the response to physical damage may trigger the production of ethylene as a defence tool or to repair the damaged tissues. In handling horticultural produce such as fruits and vegetables, there is always the tendency to cause some physical impact on the produce. This impact causes feedback reaction which is a spontaneous imitation of deteriorative processes such as the production of ethylene, increased respiration and softening. Any of these processes will alter the quality and shelf life of the produce. Thus, Pangaribuan et al. (2003) conducted a study to establish the effect of physical damage (by slicing some fruits) on ethylene production in tomatoes. Some tomatoes were sliced and treated with 1-MCP (1-methylcyclopropene) while other cut slices and fruits were not treated with 1-MCP. They found out that those tomatoes treated with 1-MCP before slicing had reduced ethylene production compared to those slices that were not treated with 1-MCP. They observed that 1-MCP inhibited ethylene production by 31% in fruits and 24% in slices resulting in firmer fruits and slices in the treated samples
2.10.1 Ethylene and postharvest losses
Reid (2000) indicated that ethylene alone can cause postharvest losses of between 10%-20% through undesirable acceleration of ripening, undesirable defence response (russet spotting and bitterness), abscission and senescence.
Ethylene is a colourless gas that has a faint sweetish smell. It is produced naturally by plants as part of their normal metabolic processes. It can be synthesized in almost all tissues of higher plants, and serves as a plant hormone that controls many of the plants metabolic processes (Ishida, 2000). At room temperature, it is a gas that can easily diffuse between tissues of the plant or between different commodities. Understanding the effects of ethylene is very important in postharvest technology in order to be able to maintain the quality and shelf life of fruits and vegetables. Thus, Martinez-Romeo et al., (2007b), Serrano et al., (2008), cited in Martinez et al., (2009), indicated that most postharvest technologies for tomato are geared at controlling the production of ethylene and its associated effects so that the shelf life as well as quality can reasonably be maintained until it reaches the consumer.
2.10.2 Classification of crops based on their ethylene production
Fruit ripening is an important process for the postharvest technologist. The ripening of the fruit is triggered by endogenously produced ethylene, but can also be induced artificially by the application of exogenous ethylene.
Based on its ethylene production, fresh produce has been grouped into two classes: climacteric and non-climacteric. Climacteric fruits such as tomato are those that show a sharp rise in respiration with associated ethylene production as they ripen after harvest while non-climacteric produce do not exhibit a rise in respiration or ethylene production after harvest. The distinction here is that climacteric fruits typically ripen after harvest by softening, changing colour and becoming sweeter. Conversely, non-climacteric produce do not change significantly after harvest and do not change to improve their eating characteristics (Jobling, 2009). Since the increase in respiration is indicative of a rise in carbon dioxide production, Thanh (2006) linked the rate of carbon dioxide production to ethylene evolution as shown in Table 3.0.
2.10.3 Effects of ethylene on fruits and vegetables
Ethylene is used in handling perishable crops. It is instrumental in the entire life cycle of fruits and vegetables stimulating seed germination shoot elongation, healing and defence responses, leaf senescence, flower induction, and enhancing ripening among others.
Albeit many vegetables produce small amounts of ethylene, some are sensitive to ethylene at different storage temperatures. In their work, Bower and Mitchan (2001) found that, when broccoli is exposed to ethylene at a temperature of 10oC, its shelf life can be reduced by half even when treated at moderate concentration. They indicated that similar effects have been observed on beans stored at 5oC at a concentration of 0.1 ppm. They have also shown that ethylene concentrations less than 0.1ppm can reduce the shelf life of many green vegetables. One of the main effects found with ethylene is its influence on an increase in respiration in horticultural produce. This rise in respiration indicates a rise in metabolic activity. Therefore, any treatment that can decrease respiration will also help in extending the storage life of the commodity. Ethylene has been shown to increase the degradation of chlorophyll which results in yellowing of tissues in immature fruits and vegetables as well as ornamental plants (Kader and Rolle, 2004).
2.10.4 Effects of ethylene on ornamental plants
Ornamental plants, commonly referred to as ornamentals are plants that are grown for their aesthetic value. The beauty of these plants may either be from their flowers or foliage. Any interference in the aesthetic nature of the plants also affects their market value. The response
of ornamentals to ethylene is evidenced in their growth, abscission and senescence. For instance, when poinsettia (Euphobia pulcherrima) is exposed to ethylene, it responds by epinastic (downward curvature) growth of the leaves and bracts. Also, the Christmas mistletoe (Phoradendron tomentossum) responds to ethylene exposure by abscission of its fruits, leaves and stem segments. Similarly, senescence by way of premature discolouration and wilting of flowers results in carnations and cymbidium orchids when exposed to ethylene (Wills et al., 2007)
Tomatoes are sensitive to exogenous ethylene and exposure of mature-green fruit to ethylene will trigger ripening. Ripening tomatoes produce e
thylene at a moderate rate and mixed storage or shipment with sensitive commodities, such as lettuce and cucumbers, should be avoided (AVRDC, 2006).
2.11 Handling postharvest losses
Postharvest handling of fruits and vegetables is as important as production practices itself because producers realise its benefits after harvest (Villareal, 1980). Poor handling practices can result in large losses in which labour, material and capital resources have been committed to produce.
Seung and Adel (2000) indicated that because fresh fruits and vegetables are living tissues, they are subjected to continual changes after harvest. Such changes cannot be stopped but can only be maintained or controlled within certain limits by using various postharvest technologies or procedures
2.12 Technologies for maintaining the quality and shelf-life of tomatoes
Packaging has been variously defined by different bodies and in different references. The definitions by two institutions are considered here
.i.e.the Packaging Institute International defines packaging as the enclosure of products or items in a wrapped pouch, bag, cup, box, cans etc. to perform one or more specific functions.
The UK Institute of Packaging defines packaging in three ways:
1. A coordinated system of preparing goods for transportation, distribution, storage, retailing and end-use.
2. A means of ensuring safe delivery to the ultimate consumer in sound conditions at minimum cost.
3. A techno-economic function aimed at minimizing costs of delivery while maximizing sales(and hence profits)
Whatever the definition, packaging is supposed to fulfil one of four universal functions- containment, protection, convenience and communication.
Containment:- this function ensures that the produce can be enclosed together in a manner convenient to allow for easy handling and distribution. The package should provide adequate space to contain the shape of the produce.
Protection:- the package should be able to protect the produce from damage either mechanical or microbial. This implies that the package needs to be rigid enough to fulfil this requirement.
Convenience:- the package should allow for the produce to be handled easily in several units during transportation and distribution without having to take them in singles.
Communication:- the package allows the product to be identified by both manufacturers and consumers. Information about the product such as ingredients, preparation and usage as well as date of manufacture and expiry are all provided on the package (Coles, 2003; Robertson, 1993).
Types of packaging materials
According to Peleg (1985), packaging of fresh produce can be grouped into consumer/retail packs and wholesale /bulk packs, the choice of which depends on the marketing requirements or characteristics of the product.
Consumer packs:- these are meant to be used for small quantities of consumer size produce displayed at retail outlets. There are several and varied packaging materials designed to contain small quantities of produce. They include:
Plastic films/pouches - these are made of flexible plastic materials which could be low density polyethylene (LDPE), polyvinylchloride (PVC), and polypropylene (PP) and cellulose acetate films. Normally, these are used as pouches (Figure 3.0) with ventilation holes to allow for exchange of gases between the packaged product and its exterior.
Trays - plastic trays or moulded pulp trays are used to contain some horticultural produce. These trays are normally overwrapped with transparent food grade plastic film
Plastic punnets- they are made either of PVC or PP that may be transparent and have ventilation holes for easy air circulation. They are food grade material that can be recycled.
Net bags or sleeves - these are generally made of high density polypropylene or polyamide, tubular and can contain different sizes and shapes of produce. They allow for air circulation around the produce and help maintain freshness of the produce.
Wholesale packs:- they are intended for bulk packaging of produce that is usually transported from production areas to market centres. Some of these packs were originally used for harvesting but are now used to contain produce for shipment and for storage. These materials are designed to withstand physical impact during transportation. They are made of rigid materials such as wood, corrugated fibre boards or plastic or flexible materials such as sacks.
Corrugated boxes - these are made from fibreboard or plastic (PP or high density polyethylene) and are used for transporting or shipping produce.
Wooden boxes - these are rigid boxes made from lumber and confer much protection to produce during transportation. The boxes are staggered such that air can circulate easily within the produce.
Plastic crates - they are made either of high density polyethylene or polypropylene and can easily be cleaned and disinfected. Their capacity varies between 20-40 kg and they are either stackable, stack-nest or collapsible in design.
Sacks - they are made from jute, plastic, or cotton and are normally used for transporting fresh produce from fields They are liable to physical impact damage.
(Coles, 2003; Gast, 1991; Robertson, 1993)
It is well known that fruits and vegetables are living tissues that continue their normal metabolism even after harvest. They respire by giving off carbon dioxide and taking in oxygen, hastening their ageing process. However, when the amount or the level of carbon dioxide increases and that of oxygen drops, this ageing process is slowed down and the produce shelf life is extended (Wills et al., 1989, 2007).
Modified Atmosphere packaging (MAP) /Controlled Atmosphere (CA) packaging
Air is a mixture of gases and in the natural environment; it is composed of nitrogen (78%), oxygen (21%), carbon dioxide (0.04%) and traces of inert gases. As indicated earlier, at high levels of oxygen and low carbon dioxide, deterioration of harvested produce is enhanced as the produce is living and continues to respire. For this reason, different forms of packaging - Modified Atmosphere Packaging (MAP) and Controlled Atmosphere (CA) packaging have been adopted to alter the relative proportions of the atmospheric gases around the produce in order to extend the shelf life of perishable products.
Modified Atmosphere Packaging (MAP):- It is defined as" the packaging of a perishable product in an atmosphere which has been modified so that its composition is other than that of air" (Hotchkiss and Hintlian, 1986) cited in McDowell and Mullan (2003). It is a simple and straightforward method that involves the use of permeable films as the packaging material and storing the packaged produce in ambient temperature. As the product is hermetically sealed (Figure 4.0) in the package, it continues to respire using oxygen (O2) and releasing carbon dioxide (CO2) and because the packaging film serves as a barrier, the amount of oxygen in the package drops while that of carbon dioxide rises. Therefore, the build up of carbon dioxide slows down respiration and for that matter ripening, leading to an extended shelf life of the product (Cantin et al., 2008). Jobling (2001) indicated that the high levels of carbon dioxide and reduced oxygen during storage can reduce product sensitivity to ethylene, slow chlorophyll loss and also decrease fungal growth. Kader and Rolle (2004) have shown that levels of O2 and CO2 at 2-5% and 8-12% respectively have been adopted and used in MAP profitably on fresh-cut fruits and vegetables. An experiment conducted on tomato fruits at various developmental stages showed extended shelf life when they were kept in a 44.4 micron thick polyethylene (PE) films and stored at 20oC. Also, tomatoes kept in 50micron thick polyethylene and 25micron thick polypropylene (PP) and stored at 13oC were still firm after 60 days with less weight (AVRDC, 2006).
Controlled Atmosphere (CA) packaging:- With CA packaging, the gas composition, which might include nitrogen, carbon dioxide and oxygen, as well as humidity and temperature around the produce are continuously monitored and adjusted appropriately. Unlike the MAP which is used in consumer packaged produce, the CA is used for bulk storage and transportation at refrigerated temperature (Kader and Rolle, 2004).
2.12.2 Heat treatment
In a bid to decrease the use of postharvest chemicals, heat treatment has evolved as a substitute and offers protection to produce in a non-damaging physical treatment (Lurie, 1998). Heat treatment has been used commercially in different countries for maintenance of postharvest quality, for disease control and as a quarantine technology. There are different techniques adopted in heat treatment and these include hot water dips, hot air treatments and hot water brushing (Ferguson et al. 2000).
In an attempt to extend the shelf life of tomato fruit, it is equally necessary to ensure that the quality and safety attributes associated with the produce are maintained. Heat treatment is one aspect that has been explored to extend the shelf life and to maintain tomato fruit quality by suppressing the activity of enzymes and microorganisms. (Yildiz and Baysal, 2006) subjected tomato fruits to alternating currents (AC) of between 36 and 108V/cm to assess its effects on pectin methylesterate (PME) and Aspergillus niger. They observed that tomato fruits treated at higher electric field strengths especially at 108V/cm over longer periods showed decrease PME activity and that Aspergillus niger.
Barkai-Golan and Philips (1991) cited in Lurie (1998) showed that heat treatments control many postharvest plant pathogens that cause decay during handling. Heat treatment has also been shown to be effective in controlling some physiological disorders as indicated in Table 4.0.
Physiological benefits of thermal treatments in horticultural crops. (Source: Lurie, 2008).
Crop Disorder prevented Treatment Temperature/time Apple Scald HAT1 8oC/4days Avocado Skin browning HAT then HWT 238oC/3-10h then 40oC/30mins. Internal browning, pitting HWT 38oC/60mins. Cactus pear Rind pitting, brown staining HAT or HWT 38oC/24h or 55oC/5mins. Citrus Rind pitting HWT 50-54oC/3mins. HAT 34-36oC/48-72h HWB3 59-62oC/15-30s Mango Pitting HAT 38oC/2days HWT 47oC/90-120mins. Persimmon Gel formation HAT 50oC/30-45mins Green pepper Pitting HAT 40oC/20h Cucumber Pitting HWT 42oC/30mins. Tomato Pitting HWT 38oC/2-3days HAT 48oC/2mins Zucchini Pitting HWT 42oC/30mins.
1Hot Air Treatment, 2Hot Water Treatment, 3Hot Water Brushing.
2.12.3 Ethylene scrubbing
This involves the removal of ethylene as it is produced by the ripening produce in an enclosed environment. Active packaging helps achieve this by including the scrubbing material in the package or head space to absorb the ethylene produced by the produce. This packaging method helps in improving the storage life of the produce as well as maintaining attributes such as sensory, safety and quality aspects of the food item. Hence, active packaging can be designed to include components of packaging systems that are capable of scavenging oxygen; absorbing carbon dioxide, moisture, ethylene and/or flavour/odour taints; releasing carbon dioxide, ethanol, antioxidants and/or other preservatives; and/or maintaining temperature control and/or compensating for temperature changes (Day, 2008).
Active packaging has been employed to control in-package conditions like ethylene production and how they vary when the produce is stored over a period of time. Ethylene can be scrubbed from packages using potassium permanganate or polymers impregnated with tetrazine. Unlike potassium permanganate, tetrazine is relatively specific with respect to volatiles like ethylene. Other chemicals that can be used for ethylene scrubbing include ozone and brominated activated charcoal but these can be hazardous if not handled properly. Ethylene scrubbers are unlikely to work properly unless positioned well within the package. The ripening fruit continues to produce ethylene and the scrubber should be able to absorb the ethylene produced or else it becomes a waste of resources and time (Wills et al., 2007)
Return on investment for ethylene scrubbing
The need to control postharvest losses can be justified if the benefits or net returns on investing in postharvest technology are considered. In view of this, the University of California at Davis conducted a study to estimate the benefit gained on investment in ethylene scrubbing technology.. They found that scrubbing ethylene during storage resulted in a net benefit that could offset the cost of the ethylene scrubber. Similarly, loss of vegetables and fruits as a result of ethylene damage during transportation was calculated to be about six times the cost of an ethylene scrubber. This implies that it is worth preventing postharvest losses by investing in appropriate technologies that can extend the shelf-life of produce (Kader, 2006).
2.12.4 Ethylene inhibition
This involves the prevention of ethylene production in the produce. There are two ways to achieve this, i.e. either the use of ethylene receptor site blockers such as silver thiosulphate (STS) or 1-methylcyclopropene (1- MCP) or ethylene inhibitors such as aminoethoxyvinylglycine (AVG) or aminooxyacetic acid (AOA) (Wills et al., 1998).
a. Silver thiosulphate (STS)
Silver thiosulphate (STS) is a compound that is known to block the action of ethylene in plant tissues. It can normally be prepared in a ratio of 1:4 of silver and thiosulphate respectively. The silver present in the sol
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