STARTER CULTURES AND FERMENTED MILK PRODUCTS

Module 12. Yoghurt

Lesson 18

TYPES OF YOGHURTS, YOGHURT PRODUCTION-SIGNIFICANCE OF EACH PROCESSING STEPS

18.1 Types of Yoghurt

The types of yoghurt that are produced world-wide can be divided into various categories, and the sub-divisions are usually made on the basis of:

1. Legal / Standards/ Chemical composition: FAO/WHO (1973)

i. Full fat > 3% fat

ii. Medium fat 0.5 to 3% fat

iii. Low fat < 0.5%

iv. Balkan yoghurt – 4.5 to 10% fat

2. Method of production

i. Set yoghurt

ii. Stirred yoghurt

iii. Fluid yoghurt – diluted or stirred yoghurt (yoghurt with <11% T.S.)

3. Flavours

i. Naturalor plain yoghurt – traditional type with sharp, acidic taste

ii. Fruit yoghurt- addition of fruits & sweetening agents to plain yoghurt

iii. Flavoured yoghurt – in which synthetic flavouring & colouring agents are added.

4. Post –incubation processing

i. Pasteurized yoghurt

ii. Frozen yoghurt

iii. Dietetic yoghurts- may include less calorie yoghurt, low lactose yoghurt or vitamin/protein fortified yoghurt.

iv. Concentrated(>24% T.S.) and

v. Dried yoghurts (>90-94%).

18.2 Production of Yoghurt

The entire technology of yoghurt manufacture is centered at the ‘care of culture’. The selection of raw material, processing conditions, environment, storage, etc. should be decided in such a way that the culture exhibits predictable behaviour. Here under the influence of each step during manufacture of fermented milk is discussed on the quality of the final product.

The manufacture of yoghurt involves several steps.

1. Selection of raw materials

2. Standardization

3. Homogenization

4. Heat processing

5. Inoculation

6. Incubation

7. Cooling

8. Post incubation processing

9. Packaging

These are given in the form of flow diagram in Figure 18.1

Fig. 18.1 Flow for production of yoghurt

1. The Selection of Raw Materials

Raw milk is the major raw material used in manufacture of yoghurt. During its selection it should always be kept in mind that a live microbial culture is required to proliferate in it. Hence, the raw milk selected for the product must support good growth of the culture. It should have following attributes too:

• It should be fresh,

• have normal composition,

• free from mastitis and other diseases,

• free from antibiotics and other inhibitors,

• free from off-flavours,

• has low bacterial count.

• Free from pre-enzymatic activities

2. Standardization and pre- treatments

Buffalo milk should be preferred for set types of product as it gives firm curd while cow milk may be good for stirred product, which will be smoother and uniform. The manufacturer has to standardize the milk to meet legal requirements for fat and SNF (Solids-not-fat). Technologically, good quality set yoghurt is obtained from the milk having 13-15% total milk solids. Fat do not have significant role to play infermentation, but contributes to integrated pleasant flavour and richness to the product. About 3% fat is sufficient to have good quality product, while SNF can be increased to 10-12%, preferably by concentration or by supplementation with skim milk powder.

The ingredients added during this standardization process should be carefully selected as poor quality ingredients can not make good quality product. If SMP,Whey powder, Butter or any other materials used for standardization has high microbial count, especially spore counts, it will increase load in fermented milk and the product will show defects early.

As such stabilizer is not needed in the product, but to increase the smoothness and decrease whey separation in stirred product, it can be added at the rate of 0.1-0.2%, while heating the milk. This should not add to the total microbial load of the product.

Pre-treatments like filtration/straining removes dirt, dust and extraneous matters that help in reducing the microbial load to a great extent. Fore-warming will lead to activation of microbes, but before they multiply, the milk should be heat processed so that the microbial count does not increase.

3. Homogenization

The milk maybe homogenized at 100 Kg/cm² at 60-70 °C. This is an optional step done in-between the final heat processing of milk. This process is useful as;

• it gives uniform mixing of all raw materials,
• reduce the problem of fat separation in curd,
• improves gel stability and
• improves digestibility.

From the microbiological point of view, homogenizer and the line connecting to it becomes an additional source of contamination. If it is not properly cleaned,it may add to the total microflora of the milk. The process of homogenization also breaks the clumps of microflora in milk and hence, if such milk is tested by plating methods, it apparently shows more cfu/ml, while actually the number of microflora might not have increased.

4. Heat processing

The milk intended for fermented product manufacture must be heated in the range of 80-95 °C for 5-30 minutes. Generally it is practiced at 80C for 30 min, or 85°C for 20 min or 90°C for 10 min or 95°C for 5 min. The heat treatment is made legal from public health point of view, as heat processing should destroy the pathogens likely to come in milk and make the milk safe for human consumption. However, such a high heat treatment is useful from many other angles too as shown below.

1. It supports good growth of the culture as it destroys other competing microflora giving free ground for the starters to proliferate.
2. It inactivates natural inhibitory substances in milk,
3. drives out oxygen
4. produce some growth stimulating agents for starters
5. Higher heat treatment denatures whey proteins which help in improving the gel stability

Heat treatment is the only severe treatment applied during the entire process of fermented milk manufacture, which can destroy microorganism. After this step,there is no other step which can destroy microorganisms and hence care should be taken that after heat treatment, the milk should not get contaminated by extraneous microorganism. That is why heat treatment is considered as critical control point (CCP) in HACCP program for yoghurt.

5. Cooling

Just after heating, the milk is cooled to incubation temperature, which is around 42-45°C. The cooling is essential before inoculation, because if the culture is added in hot milk, obviously it will die. Care should be taken that milk after cooling should be inoculated as early as possible. If the time gap before inoculation is more, the survivors of pasteurization will grow. The microbial count just after cooling should be minimum.

If set-product is to be prepared, the colour and flavour in appropriate concentration is mixed at this stage. In case of stirred product, flavouring is done while stirring. The milk is now ready for inoculation.

6. Inoculation

The milk is inoculated with active yoghurt cultures, S. thermophilus and L. delbrueckiis ubsp. bulgaricus at the rate of 2% (v/v) of milk. Usually both the cultures are added in equal proportion (1% each).

7. Filling in retail packs for set yoghurt

If set yoghurt is required, the inoculated milk is required to be packed in retail containers before incubation. The packing material must efficiently be treated, sanitized to minimize contamination. Risk of mould and bacterial spores decreasess helf-life.

8.Incubation

After adding culture in the milk, it is uniformly mixed without aeration. It is then incubated in bulk or in the same tank, if stirred product is to be made. If set-product is required, the milk is filled in retail containers before incubation. Incubation is purely a biological process during which the culture grows and brings necessary transformations in milk to get a desirable fermented product. Incubation temperature should be kept 42°C. The period of incubation varies between 3-6 h,depending upon the rate of acid production by the culture in the milk. However, the best end point to stop fermentation is just after the milk sets. Setting takes place at about 0.6% acidity and the remaining acidity required in the product can develop while cooling. During incubation, the milk is very sensitive to mechanical disturbances and other changes. Hence, it should not be disturbed.

9. Cooling

As soon as the curd sets or desired acidity in the product is achieved, it must be cooled. Cooling is done to reduce the rate of multiplication of starter cultures and stop their growth at the end of cooling. This is essential to avoid over acidification in the product. The final acidity desired in the product and total count of starter cells excepted in the product will depend on the rate of cooling and how much times it takes to reduce the temperature below 5°C.

The rate of cooling affects the quality characteristics of the product and should be decided according to the per cent lactic acid expected in the final product. Rapid cooling may lead to more contraction of gel and separate more whey, while too slow cooling may sour the product. In yoghurt, two stage cooling is preferred, i.e. in first stage cooling from 42°C to 20°C and in the second stage from 20°C to 5°C in cold store.

In the stirred products, cooling and stirring are simultaneously done. It is advisable to stir the product at lower temperature to reduce the problems of wheying-off. In most cases, the product is stirred at about 20°C and also blended with colour, flavour, fruits, nuts and other additives and then packed in retail containers. The product is to be stored at less than 5°C, until its consumption.

10. Packaging

The packaging for set product is done in retail containers before incubation. However, for stirred product, the packing is done during cooling. The packaging materials may be polythene pouches, polystyrene cups,bottles or cartons. All such packaging materials serve as additional source of contamination and their sterility should be ensured before packaging. Packaging materials should also prevent the contamination during storage and distribution. Yoghurt packaging machines are based on one of the following principles.

1. Volumetric level filling- When fluid yoghurt is poured in to glass bottles.

2. Volumetric piston filling- As applied to the packaging of stirred yoghurt in the plastic container.

This is more widely used, but the piston pump can cause some shearing of the coagulum. To minimize this reduction in viscosity, it is recommended to use low speed of filling and the use of a filling nozzle with a large orifice. It is also important that the design of the filling head should allow for a high standard of hygiene.

11. Storage

All packaged retail units are packed in larger cartoon and stored in cold stores. The yoghurt must be stored at less than 5°C to ensure inhibition of growth starters and non-starter microorganism. If there are temperature fluctuations or temperature increases during storage, the growth of culture as well other microorganisms may take place and it will make the product sour or produce other defects. Hence, maintenance of temperature during storage is very important.

The distribution of the finished product should always be through cold-chain.

A good quality yoghurt has shelf-life of 2-3 weeks days at 5°C.