Effect Of Starter Culture On Bacterial Activity Biology Essay


Yogurt is a cultured, semisolid and curdled food product. It is made from fermented milk by a bacterium. Whereas, fruit yogurts are the milk products that are fermented by special cultures of lacto-bacteria. The consistency of fruit yogurts are jelly like and containing different types of fruits as additives and flavor. Four samples, in three replicates of commercial and home made fruit yogurt will be collected. The purpose of the study is to examine the nutritional quality by proximate analysis and find out microbiological status by total bacterial and plate count of fruit yogurt. The variation in nutritive value and microbiological load among fresh and stored fruit yogurt will also be evaluated. The resulting data will be subjected to analysis of variance (Steel et al., 1997). Significant results will be subjected to Duncan Multiple Range Test (Duncan, 1955) to compare the results.



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Basically, yogurt is a cultured, semisolid and curdled milk food product. It is made from fermented milk by a bacterium. It has slightly lower sugar content than pure raw milk and has more easily digestible proteins. Commercial dairies make yogurt by inoculating sterilized milk with bacteria, Streptococcus thermophillus or Lactobacillus acidophilus and then incubate. The World Health Organization has developed certain standards for the use of yogurt internationally (Fuller., 1995).

Many studies have used probiotic or health bacteria from yogurt cultures. They investigated the role of gut flora as primary immune system. The two bacterial genera, Lactobacillus bulgaricuss and Streptococcus thermophillus were found in yogurt (Pereyra et al., 1991). Lactobacilli and streptococci stimulate an enzyme that helps synthesis of cytokines. The level of this enzyme increases in humans due to ingesting yogurt. Yogurt contains live and active bacterial cultures (Solis-Pereyra et al., 1997).

A special type of fruit preparation is generated by blanching peel of citrus fruit with sugar syrup. For sweetened product, raw material is mostly used. The raw material contains high fructose or blanched fruit. Those fruits which contain high content of fructose for sweetening and flavoring called egute products. Peaches and nectarines contain significantly good amount of major nutrients. They are good sources of beta carotene and vitamin A, which have anti-aging agents. The sweetened fruit preparation is processed by blending whole, sliced or crushed fruits. The ratio between fruit and sugar such as 3:1, 4:1 and 7:1 is mostly preferred. A good quality fruit is used in manufacturing the product. These fruit preparations are mostly used in hot filled packaging. These products are used in yogurt or ice-cream and other dairy products (Hui et al., 2008).

Fruit yogurts are the milk products that are fermented by using special cultures of lacto-bacteria. The consistency of fruit yogurts is jelly like. These contain different types of fruit as additives that may be randomly or homogenously sprinkled in layers form. Fruit preparation through pectin is used in dairy products. Pectin has a stabilizing effect in fruit yogurt. Fruit has been prepared separately from yogurt without any effect of jelly. Pectin provides the smooth and creamy texture in fruit yogurt. They contain high nutritive values and healthy whey proteins (Anon., 2004).

In minimally processed fruit yogurt some phytopreservatives such as vanilli act as antimicrobial agents. Fruit yogurt made from minimally fresh fruits, increases the consumption rates of fruit yogurt rather than plain yogurt. For controlled microbial spoilage of fresh fruit yogurt, the use of vanilli and nisin has been increased. The growth of microbial spoilage suppresses with the addition of vanilli (Penney et al., 2004).

A comparative study on microbiological analyses of fruit-flavored yogurt, plain yogurt, and sugar-added plain yogurt concluded that these types of yogurt should not be stored for longer time period. They should not be stored for more than 7 days. Since contamination of yeast and bacteria will occur likely in all types of yogurt after that period (Con et al., 1996).

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Yogurt is good source of protein, riboflavin, vitamin B12 , calcium, magnesium and potassium as well. Whereas, the fortified yogurt contain high level of vitamin A and D since fruits are good source of vitamin A, C, folate, potassium and fiber (Gibson et al., 2010).

Several health benefits can be had from some species of lactic acid bacteria. They improve the nutritional values of foods. These bacteria also help to control intestinal infections and improve digestion of lactose. It also helps to control some types of cancer and is also good for the control of cholesterol level in the body (Gilliland., 2002). Yogurt in the form of bifidobacteria is good for improving intestinal health. Yogurt appears to be a miracle food for dieters, babies, health nuts or those who prefer natural foods. It carries higher nutritional value though, it is low in calories and fat. Yogurt is also considered to be well-balanced and perfect food (Fuller., 1995).


The project at hand has been planned:

To determine the type of starter culture used for making fruit yogurt.

To determine the activity of bacteria and microbial load on fruit yogurt.

To determine the impact of nutritional value of fruit yogurt.


Keating and White, (1990) were conducted a study on plain and fruit yogurt. They evaluated the effect of alternative sweeteners on plain and fruit yogurt. They concluded that no significant difference was observed to the alternative sweeteners although it increased after prolong time period of storage. They had significantly high level of viscosity. Also they concluded that the time and types of sweetener had significantly affected on growth of the microorganisms. Similarly another study (Anonymous, 1987) showed that viscosity of yogurt had been significantly affected by the addition of sweeteners. Fructose formulated yogurt had highest viscosities. Because of its higher water-binding capacity as compared to other sugars that enhances their viscosity.

Salih and Sandine., (1990) investigated three different freeze-dried microgards which played inhibitory action against the spoilage organisms in fruit (strawberry)-flavored yogurt. They focused their study on the acid tolerant and gram negative bacteria and evaluated the activity of microgards in commercial yogurt. They found the activity of the microgards protected from spoilage by bacteria in yogurt.

Martin., (1994) reported that the Lactobacillus acidophilus had been more acid tolerant as compared to bifidobacteria. Similar work was also carried out previously by Klaver and Kimgma., (1992); Laroia and Martin., (1991) Holcomb and frank, (1991); Kim., (1988); and Hull et al, (1984); These studies concluded that the bifidobacterial population might have dropped to greater extent than that of lactobacillus acidophilus at 10 0C. Similarly, Sakai et al., (1987) concluded that the most important factor in yogurt is pH. In bidobacterial mortality, lowering the pH of yogurt and effect on the storage temperature.

Dave and Shah, (1996) assessed the viability of yogurt and probiotic bacteria during manufacture and storage condition by using four different commercial starter cultures. They concluded that the level of acidity, pH, and oxygen content had increased or decreased during the manufacture and storage of yogurt but the concentration of hydrogen peroxide increased in the preparation of product with the cultures that contained Lactocbacillus bulgaricus. They further observed that the viability of bifidobacteria exhibited better stability rather than other cultures that were used in yogurt manufacture. The temperature of yogurt for storage was also found to affect the viability of bifidobacteria but not of Lactobacillus acidophilus. Several studies have been conducted on natural or plain yogurt but very few studies are available on fruit yogurt. They have no any specific data on the effect of added fruits on the survival of probiotic bacteria (Micanel et al, 1997).

A study (Lubbers et al., 2004) had concluded that polymerization of casein in skimmed milk by adding fruits might be prevented from disrupting the protein network. A similar study demonstrated that the addition of fruits may had increased the acidity of yogurt (Talwalkar and kailaspathy., 2004). Another study (Al-Kadmany et al., 2003) was determined the shelf-life of concentrated yogurt with the help of some microbiological and physiochemical parameters. They concluded that increasing the storage temperature, the pH had decreased and lactic acid had increased.

Kailasapathy et al., (2007) conducted a study to evaluate the effect of commercial fruit yogurt on the viabilty of probiotic bacteria during storage. They concluded that 5 or 10g fruit preparation had no significant effect on the viability of two probiotic bacteria that are Lactobacillus acidophilus and bifidobacterium. However, after addition of 96% fruit preparation had greater effect on shelf life and viability of bacteria in fruit yogurt as compared to the plain yogurt. This study showed that by the presence of fruit pulp, the post-storage pH of yogurt and its viability of probiotic bacteria were affected. This study also showed that the formulation and properties of fruit yogurt may influence the viability of probiotic bacteria.

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Singh and Muthukumarappan., (2008) conducted a study to see the influence of calcium fortification on sensory, physical and rheological characteristics of fruit yogurt. They prepared calcium enriched mango yogurt mixed with 50mg Ca/100ml of calcium lactate and examined there physical and rheological characteristics. They concluded that the water holding capacity of fortification of yogurt with calcium lactate had increased by 2.99%. The water holding capacity of fortified yogurt was higher than control samples of fruit and plain yogurts. Viscosity and shear thinning of fortified yogurt was found to be significantly less than control.

Gonzalez et al, (2010) conducted a study to determine the effect of prebiotic or synbiotic ingredient on the properties like sensory evaluation and consumer acceptability of flavored yogurt. They used six yogurt drink samples and found significant differences in flavored yogurt. The yogurt containing prebiotic ingredient had no significant differences as compare to the control sample. The synobiotic samples contain high sour and yeasty aroma and lower sweetness than control samples. So the consumer had negative impact on acceptance of synobiotic yogurt that was made with skimmed milk beverages. The consumer evaluation indicated that the whole milk based yogurt is most preferable than skimmed milk-based yogurt that contain synobiotics.

Karaaslan et al., (2011) conducted a study on phenolic fortification of yogurt using grape and callus extracts. The study included extracts of grapes of four different varieties were used to determine the antioxidant activity and total phenolic substances. It concluded that the grape callus yogurt showed significantly higher antioxidant level. Grape-callus culture plays an important role in reducing the risk factors of chronic diseases like cancer and cardiovascular disease.


It would be a comparative study in which the nutritive value (Proximate analysis) and microbiological status (total bacterial count) of fruit yogurt will be determined a fresh as well as stored for 7 days.

Sample Size

In all 4 samples of commercial fruit yogurt in to three replicates will be collected from the local market of Faisalabad. Whereas, equal (4) samples of fruit yogurt, in three replicates will be prepared at home.

Area of Research

The fruit yogurt samples will be brought and processed in the Nutrition Research Laboratory of the Institute of Animal Nutrition and Feed Technology and Microbiological Laboratory of the Institute of Veterinary Microbiology, University of Agriculture, Faisalabad.

Proximate Analysis

The proximate analysis of fruit yogurt for protein, fat, crude fiber, ash and soluble carbohydrates will be done by using the method given by AOAC (1990).

Microbiological Analysis:

The total and viable number of bacteria per gram will be assessed by Hussain et al, (2007)

Statistical Analysis

The resulting data will be subjected to analysis of variance (Steel et al., 1997). Significant results will be put to Duncan Multiple Range Test (Duncan, 1955) to compare and interpret the results.


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