STARTER CULTURES AND FERMENTED MILK PRODUCTS

Lesson 19. POST PRODUCTION PROCESSING OF YOGHURT INTO VARIOUS TYPES, BIOCHEMISTRY OF YOGHURT

Module 12. Yoghurt

Lesson 19

POST PRODUCTION PROCESSING OF YOGHURT INTO VARIOUS TYPES, BIOCHEMISTRY OF YOGHURT

19.1 Introduction

If yoghurt is heated after incubation, it is called as “Pasteurized yoghurt” or “Heated yoghurt" or “Thermized yoghurt” or “UHT Yoghurt”. Pasteurization of yoghurt can be done to improve the shelf-life. It can increase shelf-life between 4-6 weeks at refrigeration temperature.

The heat treatment given to yoghurt, is less Because yoghurt has low pH, short time of heat treatment is required to achieve in habitation of starter and contaminants. The range of temperature required is 60-70°C for 1-20 min.

19.2 Disadvantages of Heat Treatment

1. Loss of flavour.
2. Increased whey syneresis
3. Loss of vitamins
4. Loss of enzymes, especially Lactase.
5. It destroy live starter flora which may reduce therapeutic value of the product
6. It violates legal FAO/WHO definition of yoghurt, which states that yoghurt should have starter flora in abundance.

19.3 Liquid/Drinking Yoghurt

Yoghurt is cooled to about 20-25°C and then stirred with addition of pasteurized water. The quantity of water depends upon the final total solids desired in the product. Liquid yoghurt generally is defined as the one having less than 9% total solids. This product resembles to cultured buttermilk. It can spiced and taken as a refreshing beverage.

19.4 Carbonated Yoghurt

Carbondioxide is injected in stirred and diluted yoghurt to make carbonated yoghurt.The carbonation of finished yoghurt beverage improves its thirst quenching quality and enhance its refreshing character. It may have higher shelf-life but show more whey separation.

19.5 Freezing (Frozen Yoghurt)

The yoghurt base produced in conventional way and the mix can be prepared by adding several other ingredients including milk, cream, skim milk powder, etc. The mix is then frozen like ice-cream and air is incorporated to have over run. Yoghurt can be hard frozen, or can be served as soft with or without additional flavourings.

19.6 Dietetic Yoghurt

Various types of dietetic yoghurts can be prepared by supplementations of required ingredients and standardization. These may low fat yoghurt, low calorie yoghurt, high calcium yoghurt, etc. Most important form of dietetic yoghurts is the probiotic yoghurt, which is prepared by incorporation of probiotic culture in yoghurt. Some of these varieties are;

Table 19.1 Types of probiotic yoghurts

19.7 Continuous Manufacture of Yoghurt

Continuous method may be of two types

1. Involving continuous incubation and cooling.

2. Involving two stage fermentation

i.e. pre-incubation and in flow inoculation.

19.8 Lactose Hydrolyzed Yoghurt

This product is designed for lactose intolerant people. The milk is first treated with lactase to partially hydrolyse lactose and then finally heat treated and inoculated with yoghurt cultures. The product will have faster growth of starters requiring less incubation time. Yoghurt will be milder and sweeter.

19.9 Biochemical Changes during Yoghurt Production

19.9.1 Acid production

The main biochemical change during yoghurt production is fermentation of lactose by starters to produce lactic acid as the major end product through EMP. The acid produced brings down the pH of the milk,which leads to coagulation of milk proteins forming the gel.

The percentage of lactic acid is affected by

a) Temperature of incubation

b) Starter inoculums rate

c) Ratio of rod: cocci

d) Age of yoghurt

e) level of lactic acid produced

S. thermophilus produces L(+) form of lactic acid while L. bulgaricus produces D(-).

During storage the level of D (-)lactic acid increases due to activity of L.bulgaricus. Similarly as during later part of incubation L.bulgaricus grows faster, over incubated yoghurt has more D (-) type of lactic acid.

19.9.2 Flavour production

The flavour compounds in yoghurt are carbonyl compounds-acetaldehyde, acetone, acetoin and diacetyl. Acetaldehyde is a major flavour compound. The normal concentration is around 13-17 mg/kg, which gives normal yoghurt flavour. Mix culture in symbiosis produces more than in individual. L.bulgaricus is the main contributor to acetaldehyde. Acetaldehyde is produced from various substrates. .It is produced from lactose via pyruvate and acetyl co-A. It can also be produce from milk protein via threonine with the help of enzyme aldolase.

Flavour also comes through thermal degradation compounds of fat, protein, lactose, volatile fatty acids, amino acids.Acetaldehyde to acetone ratio of 2.8 gives the best flavour.

19.9.3 Proteolysis

Yoghurt cultures are weakly proteolytic

Proteolysis in yoghurt brings;

1. Changes in physical structure

2. Contribution to flavour production e.g. peptides, amino acids directly act as flavour compounds or act as precusor for reaction that produces flavour compounds.

3. Yoghurt bacteria increases soluble NPN by 50%.

4. Lactobacillus bulgaricus is more proteolytic and has ability to hydrolyse casein.

5. Beta and kappa casein more degraded than a -casein by S.thermophilus.

6. Ratio of rod: cocci affect proteolysis.

19.9.4 Lipolysis

Lipolysis in yoghurt occurs due to bacterial lipases and esterases.

1. Occurs very slightly.

2. Affects flavour.

3. Increases FFA during storage.

4. Lactobacillus bulgaricus produces more volatile fatty acids than S. thermophilus.