STARTER CULTURES AND FERMENTED MILK PRODUCTS

Module 12. Yoghurt

Lesson 20
NUTRITIVE VALUE, THERAPEUTIC VALUE, SPOILAGE, DEFECTS AND QUALITY ASSURANCE

20.1 Nutritional Value of Yoghurt

Milk contains well-balanced macronutrients including carbohydrate, fat, and protein and micronutrients including calcium, phosphorus, magnesium, and zinc (Table 21.1). Milk proteins have high nutritive value due to the favourable balance of essential amino acids. While fermentation of this milk for making yoghurt, most of these nutrients remain and hence yoghurt has excellent nutritive value.

Table 20.1 Nutrient composition of milk and yoghurt per 100 g

Adapted from Holland et al. (1991) and Buttriss (1997) as cited in

Yoghurt Science &Technology Book by Tamime & Robinson, (1999)

While manufacture of yoghurt, quality and quantity of nutrients are affected by two ways.

• Changes due to modification/fortification

• Changes due to fermentation

Changes due to modification or fortification of supplementation of any ingredient can easily be calculated. However, changes during fermentation depend upon fermentation conditions and the type of cultures.

20.2. Changes due to Fermentation

• During fermentation some nutrients like lactose, protein, fat and some vitamins decrease while nutrients like lactic acid, peptides, amino acids, volatile flavour compounds, vitamins, enzymes and bacterial proteins increase.
• Fermentation improves the digestibility of the proteins. It helps in formation of softer curd and its digestibility is facilitated by presence of lactic acid.
• Availability of some minerals like calcium and phosphorous increases in yoghurt mainly because of the lactic acid and their more soluble forms.
• Vitamin content may increase or decrease depends upon the strain of the culture. Some yoghurt cultures are known to synthesize vitamin B2, B12, Folic acid, Niacin, etc and hence they can improve its nutritive value.

20.3 Therapeutic Value of Yoghurt

Yoghurt and related products have been used for health properties since very longtime. The normal yoghurt cultures do not survive passage through digestive system and do not implant in intestine and hence it do not give the benefits of ingestion of live probiotics. However, the metabolites of and cell contents of yoghurt cultures do confer several health benefits. Some of the applications of yoghurt for therapeutic benefits are listed below.

1. Used as therapeutic agents in gastro-intestinal disorders: Yoghurt flora is known to inhibit several food spoilage organisms and intestinal pathogens due to presence of several inhibitory substance and low pH. This helps in prevention of infections and control of several gastro-intestinal tract illness including diarrhea and constipation.

2. Can be digested by lactose intolerant people: Yoghurt has lower lactose due to fermentation and it also provides lactase from autolysed bacteria in gut and hence it helps in digestion of lactose.

3. May have hypocholesterolemic activity: There are some reports that yoghurt flora can reduce cholesterol in the body by different mechanisms. This can help in reducing the risk of heart diseases.

4. Anticarcinogenic activities: Yoghurt, especially having probiotic cultures are known to have some tumor inhibiting properties. They also reduce the activity of enzymes that convert pro-carcinogen to carcinogen in gut which reduces the possibility of colon cancer.

5. Immunostimulatory properties: Increase in specific and non-specific immune functions have been reported by consumption of fermented milks. Some reports on anti-mutagenic activity are also published.

6. Other applications: Yoghurt and other fermented milk products have been found to be useful in several other health conditions also. However, the therapeutic effect is dependant on the strain of the culture used for the preparation of the product. .

20.4 Quality Testing of Yoghurt

Yoghurt is tested in laboratory for various quality aspects. Various set of tests applied to judge the quality are given below:

• Physical tests: Physical appearance, free whey, presence of gas slits, package conditions, etc.
• Organoleptic tests: Colour & appearance, body& texture, flavour.
• Chemical tests: Titratable acidity, pH, presence of flavour compounds, flavour, etc.
• Microbiological tests: Microscopic examination, starter bacterial count, Coli form count, Yeast-mold count, etc
• Tests for specific properties: If any specific property or function is claimed from the product, a test to justify is required to be done.

20.5 Shelf-Life

Shelf-life of a good quality yogurt is 2-3 weeks at refrigeration temperature. The produce is required to be stored in cold conditions and transported and distributed in cold chain, otherwise several defects develop and the shelf-life is reduced.

20.6 Defects in Yoghurt

Defects in yoghurt can be classified as;

1. Appearance defects

2. Body & texture defects

3. Flavor defects

4. Acidification defects

5. Storage related defects.

Some of the common defects and their remedies aregiven in Table 20.2

Table 20.2 Common defects in yoghurt and their remedies

20.7 Standards for Yoghurt and Quality Requirements

In India, Yoghurt has been defined by PFA as under:

"YOGHURT" means a coagulated product obtained from pasteurized or boiled milk or concentrated milk,pasteurized skimmed milk and or pasteurized cream or a mixture of two or more of these products by lactic acid fermentation through the action of Lactobacillus bulgaricus and Streptococcus thermophilus. It may also contain cultures of Bifidobacterium bifidus and Lactobacillus acidophilus and other cultures or suitable lactic acid producing harmless bacteria and if added, a declaration to this effect shall be made on the label. The microorganisms in the final product must be viable and abundant. It may contain milk powder,skimmed milk powder, unfermented buttermilk, concentrated whey, whey powder,whey protein, whey protein concentrate, water soluble milk proteins, ediblecasein, and caseinates manufactured from pasteurized products. It may also contain sugar, corn syrup or glucose syrup in sweetened yoghurt and fruits infruits yoghurt. It shall have smooth surface and thick consistency without separation of whey. It shall be free from vegetable oil/fat. animal body fat,mineral oil and any other substance foreign to milk. The product may contain food additives permitted in Appendix C. It shall conform to the microbiological requirements prescribed in Appendix D. It shall conform to the following requirements:-

Table 20.3 PFA standards for yoghurt

Provided that titratable acidity as lactic acid shall not be less than 0.85 percent and not more than 1.2 percent. The specific lactic acid producing bacterial count pergram shall not be less than 10, 00,000.

Provided further that the type of Yoghurt shall be clearly indicated on the label otherwise standards of plain Yoghurt shall apply. The Yoghurt subjected to heat treatment after fermentation at temperature not less than 65°C shall be labelled as Thermized or Heat Treated Yoghurt and shall conform to the above parameters except the minimum requirement of specific lactic acid producing count per gram;

Table 20.4 Microbiological requirements specified by PFA