Fruits are one of the many natural sources of supplementary diet; various characteristics of fruits such as nutritional value, processing qualities, flavor and shelf-life determines the quality of fruits (Bapat et. al. 2010). Fruit ripening is a process in which pectic polysaccharides of primary cell wall undergo solubilisation and galactose is lost from the polymer side chain; these two processes occur together while fruit softening. (Redgwell et. al. 1997a). Fruit softening is the change that occurs during fruit ripening, a dry and firm fruit changes to soft, juicy or crispy fruit; chemical and structural changes in cell wall results in fruit softening. Fruit ripening and fruit softening are most important factors which are responsible for perishability in climacteric or fleshy fruit (Bapat et. al. 2010). After few days, fruits are inedible mainly due to over-ripening; softening of fruit, changes in aroma, texture and taste are spoilage that occur once the fruit is softened or over-ripen (Bapat et. al. 2010). There are two strategies which explains more about fruit softening that are fruit firmness decreases with decrease in polymer size and solubility is increased whose wall components are identified, secondly proteins are characterized and are expressed during ripening and its biochemical process is mechanically related. (Vicente 2007) The changes that occur during fruit ripening and fruit softening are irreversible processes once initiated cannot be ceased. It has been found that ethylene plays a major role in fruit ripening, thus altering ethylene biosynthetic pathway have enabled us to prevent or delay fruit ripening and thus controlling fruit softening (Bapat et. al. 2010).
CELL WALL STRUCTURE AND FUNCTIONS:
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Cell wall is made up of three layers namely middle lamella, outer most region of wall and shared by adjacent wall, is made up of proteins and pectin compounds; primary cell wall which gives the rigid skeleton consist of cellulose microfibrils embedded in matrix like structure made up of glycoproteins, pectin compounds and hemicellulose; last layer is secondary wall made up of ligin, cellulose and hemicelluloses imparts compression strength to the wall (Hangarter 2004). The main cell wall components are pectic acid, pectin, cellulose, starch, hemicellulose, structural proteins such as glycoproteins, extension; functional proteins and enzymes such as oxidative enzymes (peroxidases), hydrolytic enzymes (pectinases, cellulases), Expansins (Hangarter 2004). The main functions of cell wall includes providing mechanical barrier from pathogens, inhibit water loss, cell to cell communication and fruit development (Hangarter 2004). Fruit cell wall plays a vital role in determining the texture of fruit, it involves adherence to the adjacent cell walls and its mechanical properties (Hallett et. al. 2005). Cell wall changes associated with fruit ripening and fruit softening are structural and chemical changes which results textures for that particular fruit (Hallett et. al. 2005).
XYLOGLUCAN DEPOLYMERISATION DURING FRUIT SOFTENING:
Large number of cell wall changes occur during fruit ripening and fruit softening, huge amount of literature is available which provides information about the studies conducted to study these changes; major changes involved are swelling of cell wall, number of changes associated with pectins and xyloglucan depolymerisation. Softening of fruit is a genetically programmed process which is a result of alterations in the cell wall structure peculiarly cell wall polymers degradation (Wakabayashi 2000). It was proposed that decrease in tissue strength is a result of fruit softening, therefore many studies were conducted which led to the findings that degradation of particular pectin, polyuronides results in reducing or damaging the adhesion property of the cell wall in turn diminishing the tissue strength (Wakabayashi 2000). However, recent studies using transgenic plant showed that degradation of the compounds other than polyuronides are necessary to bring about fruit softening (Wakabayashi 2000).
Wakabayashi (2000) reviewed about the compound other than polyuronides which is responsible for fruit softening, depolymerisation of Xyloglucans is the key activity require for initiation of softening. Xyloglucan is one of the major components of primary cell wall which is a hemicellulosic polysaccharide (Wakabayashi 2000). Xyloglucans are 400-600 nm in length and they are attached to cellulose microfibril by hydrogen bonds; thus cell wall gets its strength from xyloglucan-cellulose matrix (Wakabayashi 2000). The stress-relaxation method was employed to study xyloglucan breakdown in several fruits. In Melon fruit, whole ripening process takes around 24 to 48 hours whereas tomatoes take 10 to 18 hours; in both the fruits breakdown of xyloglucans were detected during initial stages of softening processes (Wakabayashi 2000). Xyloglucans are degraded extensively resulting decrease in firmness of avocado fruit during softening; molecular mass and the levels of xyloglucans decreased up to 30% and 70%, respectively during softening in persimmon fruit (Wakabayashi 2000). Xyloglucans depolymerisation has been due to activity of hydrolases of cell wall; in tomatoes, action of p-1, 4-glucanase promoted degradation of xyloglucans which in turn increase fruit softening (Wakabayashi 2000). The endotransglycosylation of xyloglucans is another possible explanation for depolymerisation of xyloglucans in which xyloglucan endotransglycosylase activity breakdowns xyloglucans (Wakabayashi 2000).
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During fruit softening there are physical and chemical changes that take place. Usually in some fruits primary cell wall reduces appear diffused and swollen. Redgwell et. al. (1997b) conducted experiment on nine different fruits to check the cell wall swelling using microscopy techniques. The fruits that were taken for the experiment were avocado, apple, kiwi fruit, strawberry, nashi pear, watermelon, black berry, plum and persimmon (Redgwell et. al. 1997b). It was observed that the greatest changes that took place in softening in fruits like kiwifruit, strawberry and avocado and it was seen that the least softening in fruits like nashi pear, apple and watermelon (Redgwell et. al. 1997b). It was also seen that plum and persimmon showed uneven swelling and unexpanded in cell wall was observe. Persimmon showed a higher degree of splitting of cells and it was also seen that blackberry and avocado swelled moderately (Redgwell et. al. 1997b). The experiment was conducted in vivo and in vitro changes in cell wall during fruit softening and it was observed that fruit which swelled in vitro did not show same swelling in vivo (Redgwell et. al. 1997b). There are physicochemical changes which lead to the swelling in the fruit and mechanism of pectin solubilisation and cellulose xyloglucan complex were also modified. There was a correlation between swelling and degree of pectin solubilisation (Redgwell et. al. 1997b).
Most of the dicotyledonous plants generate fruits which are composed of galacturonans, xyloglucans and cellulose but there are exceptions such as banana which is commelinoid monocot composed of arabinoxylans and galacturonans (Shiga et. al. 2011). Banana softening studies are based on expression levels of enzymes which degrade cell wall as well as expansins which also play an important role in softening (Shiga et. al. 2011). During softening, large quantity of starch is generated in pulp and this starch is degraded throughout the ripening process; along with degradation of these non-structural carbohydrates, soluble sugar is also accumulated (Shiga et. al. 2011). Carbohydrates from cell wall of dessert banana cultivars and the pulps of one plantain were extracted to study the softening of banana and were analysed at different stages of the ripening and it was demonstrated that reduction in starch levels along with accumulation of soluble sugars results in softening of banana (Shiga et. al. 2011).
GALACTOSE LOSS DURING FRUIT SOFTENING:
One of the important changes that occur in cell wall during softening is loss of galactose and pectin solubilisation. Redgwell and Harker (1995) studied that softening of Kiwi fruit and proposed that softening is mainly due to decrease in amount of galactose and pectin solubilisation. A study was conducted in which numbers of discs were removed from the outer region of pericarp of kiwifruit which is not fully ripe which was kept over 72 hours for softening to take place (Redgwell and Harker 1995). After 72 hours of softening, discs were observed and examined, it was found that softened discs showed decrease level of galactose in cell wall and pectin solubilisation was enhanced (Redgwell and Harker 1995). To check whether softening was due to loss of galactose or not, 50mM galactose solution was added to discs, galactose loss was completely ceased after 72 hours and fruit softening was delayed but pectin solubilisation was not affected (Redgwell and Harker 1995). Ethylene biosynthesis in discs was inhibited using and inhibitor namely aminooxyacetic acid; it was found that pectin solubilisation was completely retarded but no loss of galactose was observed (Redgwell and Harker 1995). These results indicated that cell wall associated changes during kiwi fruit softening which is loss of galactose and pectin solubilisation are two separate processes (Redgwell and Harker 1995).
In peach fruit during, it was believed that increase in ethylene production was responsible for its ripening process and subsequently softening of peach, but tissue strength and ethylene biosysthesis in honey peach were examined after storage at 5Â°C and 20Â°C (Chang-Hai et. al. 2006). It was found that during storage ethylene biosynthesis was far behind the peach softening which means that factors other than ethylene production are present which are responsible for peach softening (Chang-Hai et. al. 2006). It was proposed that changes in cell wall materials (CWM) and pectins caused softening of peach fruit; loss of galactose and arabinose were also discovered in CMW residues and pectins (Chang-Hai et. al. 2006). Thus these results shows that peach softening after harvesting is mainly due to degradation and solubilisation of pectin side chains and CMW-residue fraction; possibly due to rise in the activity enzymes related to cell wall polysaccharides (Chang-Hai et. al. 2006).
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It is seen that different fruits have different changes occuring into the cell wall of fruit at different stages and different extents. Fruit softening spoils the fruit quality and excessive softening leads to the reduction of shelf life of the fruit and limitation towards storage and transportation and it get infected by pathogens easily. It is also seen that cell wall changes in the fruit is complex biochemical process and modifies enzyme and proteins. There are various methods to avoid fruit softening. UV-C irriadiation method was conduction on strawberry fruit and it was onserved that it delayed the process of fruit softening and it was also seen that UV-C treated strawberries showed higher firmness than non treated strawberries. It also change the expression profiles of genes and proteins and enzyme also get modified. (Pombo et. al. 2009). There was other experiment done on peach fruit with ACC treatment in which single synthesis spray applications of ACC which allowed ethylene. ACC is 1-aminocyclopropane-1-carboxylic acid and its precussor of ethylene and it is used for fruit maturation. Peach industry were able to control ripening and firmness was increased. ACC concentration is economic and convenient way to avoid fruit softening. (Hayama et. al. 2008). Fruit industry should take appropriate techniques or methods to avoid fruit softening and increase shelf life of fruit so that it can be eaten and stored for longer time. All precautions should be taken to decrease fruit softening like storing at proper temperature mostly in the cooled climate which decreases fruit softening.