A-Z of making yoghurt at home or in small factory

Many at times in my interactions with dairy farmers in the field, I always get asked, “Prof how can we make quality yoghurt?” The process is simple and straighforward as I will demonstrate today.

Yoghurt is a milk product typically fermented with a mixture of two bacteria species — Lactobacillus bulgaricus and Streptococcus thermophilus. These two are among bacteria that produce lactic acid during fermentation, and are  called lactic acid bacteria. Other lactic acid bacteria with certain proven health benefits, such as Lactobacillus rhamnosus, may  be included in the culture to produce probiotic yoghurts. 

Other ingredients such as sugar, fruits, skim milk powder and artificial flavours and colours may be added to improve the sensory appeal. Stabilisers may be used to improve the texture and thickness of the yoghurt and keep fruit uniformly mixed in the yoghurt.

Required ingredients

Fresh milk: This is the main ingredient and can be whole milk, low-fat or skimmed milk.

Starter culture: This is basically the bacteria responsible for the fermentation. Amount used will depend on the manufacturer’s instructions. If too little is used, fermentation will not be effective.

Skimmed milk powder: This is for thickening the yoghurt, and may be omitted if a thin yoghurt is desired.

 White sugar: This is for sweetening the yoghurt and is optional. Amount used depends on the desired level of sweetness.

Artificial flavours, fruit pulps or fruit pieces for imparting different flavours and are optional. When fruit pulps or pieces are used, these may constitute up to 20 per cent of the yoghurt mix.

Stabilisers: A small amount of a stabiliser such as starch (maximum 10g per Kg), gelatin (maximum 10g per Kg) or pectin (maximum 5g per Kg) may be added.

Equipment

1. Pasteuriser: A saucepan can be used

2.Kitchen thermometer for temperature control

3. Sieve: Muslin cloth and an elastic band can be used

4.Measuring cylinder or jar

5.Stirrer: Cooking stick can be used

6. Container with a lid for safe storage of the milk after pasteurizing

7. Incubator for temperature maintenance during fermentation: This can be improvised; for example, a fireless basket insulated with clothes may be used, or a milk can wrapped with a blanket etc

8.Cooler for temperature control after pasteurisation

9. Whisk for blending the yoghurt after refrigeration and addition of flavours

10.Plastic bottles/cups with lids for yoghurt packaging

11Bottle/cup sealers to enhance safety of the yoghurt after packaging

12.Labels for branding and sharing of product information with the consumer

13.pH meter or indicator paper

14. Refrigerators for keeping the packaged yoghurt

KEY STEPS

Step 1: Check Milk Quality

When you receive milk from the supplier, confirm that its quality is suitable for yoghurt processing. This can be simply done by boiling a small quantity in a saucepan; it should be rejected if it coagulates.  Alternatively mix 1 ml of milk to 1 ml of 72 per cent ethanol solution; if the milk does not coagulate, it can be used for yoghurt production. These two tests determine if some fermentation has already started in the milk. To test for adulteration of milk, for example by addition of water, a lactometer can be used to determine the specific gravity of the milk.

If the lactometer is dipped into the milk and it sinks to the point marked ‘M’, it is pure. If it does not sink to that point, it may have been adulterated by water.

Step 2: Mix in ingredients to adjust milk composition

All the ingredients except the culture and flavours are mixed into the milk. After adding in these ingredients, heat the mixture in a saucepan to 40°C and stir continuously to ensure that all the ingredients mix thoroughly.

Sieve the mixture using j-cloth/muslin to get rid of any foreign or undissolved particles.

Step 3: Pasteurise the Milk

The milk mixture is heated at 85°C to 90°C for 30 minutes. The high heat treatment reduces the number of unwanted microorganisms in the milk and allows the proteins to form a more stable gel, which prevents separation of the water during storage. While pasteurising the milk mixture, stir continuously to prevent scorching at the bottom of the saucepan.

Step 4: Cool the Milk Mixture

Cool the mixture to 40 to 42°C to bring the yogurt to the ideal growth temperature for the starter culture.

Step 5: Add Starter Culture

Add starter culture and stir thoroughly to ensure that it blends well with the pasteurised milk.

Step 6: Hold the Milk Mixture at 40 to 42 °C

Hold the milk mixture at 40 to 42 °C by keeping it in an incubator for 4-6 hours. This allows the fermentation to progress to form a soft gel and the distinctive flavour of yoghurt. As the milk ferments, lactic acid is produced and the pH reduces. Fermentation should only be stopped when the milk has curdled and the pH has reduced to about 4 to 4.5.  pH may be estimated by means of  a litmus paper (do not put the paper in the product, rather, drop a small amount to the paper and check its colour change.

Step 7: Cool the Yoghurt

After fermentation, cool the yoghurt to 4°C to stop any growth of microorganisms and avoid over-fermentation which could increase the sourness of the yoghurt.

Step 8. Add flavours

Remove the yoghurt from cold storage and add flavours such as vanilla or strawberry essence and blend well using a stirrer/ whisk until the flavour is well mixed with the yoghurt and a smooth texture achieved. The minimum flavour that gives the best taste should be used. Real fruit can be used in place of the commercial flavours, in which case the maximum fruit content is usually 20 per cent.

Step 9: Packaging the Yoghurt

The yoghurt is packed in bottles or cups with fitting lids to avoid contamination. To enhance safety of the product, the package is sealed and a sticker with product information for the consumer added on the package as appropriate.

Swiss style or set yoghurt

The process of blending fruit after the yoghurt has been fermented and cooled, as described above, gives yoghurt referred to as Swiss style.  Alternatively, fruit may be added into cups before fermentation, at the bottom of the cup and then the inoculated yogurt is poured on top, so that the yogurt is fermented in the cup. Such yoghurt is referred to as set yoghurt.

[Prof Arnold Onyango is an expert on yoghurt technologies at Jomo Kenyatta University of Agriculture and Technology]


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