Nutrition Meets Food Science

Fermented Health Food from Milk: Yoghurt

Yogurt or dahi has been prepared in different parts globally be bacterial fermentation of milk, the most common being cow’s milk. Although earlier, yogurt was prepared by natural microbes present in the milk, using previous fermented milk, FSSAI defines commercial sold yogurt to be one fermented by Lactobacillus bulgaricus and Steptococcus thermophilus. Other lactic acid bacteria may also be used in addition. On the other hand dahi does not have that restriction. (1)

When dahi is prepared at home, previous dahi is used. This dahi usually contains a large number of microbes, some lactic acid bacteria but there may be others. Commercially, pure cultures are preferred so the results of fermentation are predictable. S. thermophilus and L. bulgaricus are both capable of growing and fermenting milk at warmer temperature of around 40ºC, which not only avoids contaminants that may be harmful to humans and for these organisms but also fermentation is faster.

During fermentation by lactic acid bacteria, the milk sugar lactose is converted to lactic acid which lowers the pH of milk and milk becomes thicker and sour as more acid is formed. The microbes also produce vitamins and a very pleasant flavour.

Health Benefits (2)

Yoghurt provides some important nutrients that body needs. Just one cup has almost half the daily requirements of calcium that is necessary for healthy bones and teeth. It also provides good amounts of B vitamins especially B12 and riboflavin. Besides, it is rich in phosphorus, magnesium and potassium, the minerals needed for regulating blood pressure, metabolism and bone health.

Yoghurt is rich in high quality protein, about 3.5 to 5.5% in plain yoghurt and about 7 to 9% in Greek yoghurt. Fruit yoghurt may contain a bit lower amounts. Addition of fruit products improves the acceptability, but such yoghurts also add sugar.

Proteins not only provide the essential nutrients but also helps in appetite regulation as it activates production of hormones that signal satiety and help in weight management. Greek yoghurt is more effective in this sense.

One significant benefit of yoghurt is the presence of healthy bacteria which benefit digestive health. They also help in suppressing unhealthy microbes and some even reduce the symptoms of irritable bowel syndrome especially in yoghurts containing Bifidobacteria. Lactic acid bacteria in yoghurt generally strengthen immune system and reduce inflammation.

Many people with lactose intolerance can consume yoghurt without difficulty. This is because firstly some of the lactose is converted to lactic acid so there is less lactose in yoghurt. Secondly, the bacteria produce lactase which can hydrolyse lactose to reduce it further. Finally, while yoghurt is being digested some bacteria still remaining will continue to digest more of lactose so it would cause little problem. However, those with lactose intolerance should be careful in trying yoghurt.

Making Yoghurt (3)

The main ingredient for yoghurt is milk, which can be whole milk, low fat, and skim, etc. Other dairy ingredients like milk powder, unfermented buttermilk, concentrated whey, whey protein/powder etc could also be used. Yoghurt may contain sugar, corn or glucose syrup, and fruit products in fruit yoghurt. It may also contain stabilisers.

Milk composition is adjusted by adding milk ingredients to have proper fat and solids content. Dry milk solids may be added to increase protein content to have a better firmer texture. Milk is pasteurised and homogenised to mix all the ingredients for uniform consistency. Milk is then cooled to warm temperature (about 40 to 42ºC) that is ideal for growth of microbial cultures used for fermentation.

The cultures are then added and mixed into milk. Besides the main microbes S. thermophilus and L. bulgaricus, other microbes may be added for better flavour and some probiotics may be added for additional health benefits.

Fruit and other ingredients may be added at different times. For set style, fruit may be added first at the bottom. On top the inoculated yogurt is poured and allowed to ferment. In mixed style, yoghurt is mixed with fruit after fermentation.

After the fermentation, yoghurt is cooled to 7ºC to stop further fermentation.

Different Types of Yoghurt (4)

Earlier only plain natural yoghurt was available but for some it was too sour so it was sweetened to make it more acceptable. Later fruit was also added for better flavour. Consumers liked varieties so different formats were offered.

Set yoghurt comes with set consistency which is uniform through. The stirred yoghurt will have the set mass broken up by stirring especially with added fruit ingredients. There are also drinking yoghurts which are like stirred yoghurt but having a thinner consistency so it could be poured easily from a bottle or aseptic pack.

Greek yoghurts have become very popular recently as they are considered healthier. These are strained after fermentation to remove some of whey portion. This results in thicker consistency with higher protein and other nutrient contents. It also has lesser sourness as some acid is removed through straining.

Another popular variation is frozen yoghurt. This is prepared with same ingredients as ice cream but will have yoghurt cultures and may also contain less fat than ice cream as it is made from milk rather than cream. After fermentation the product will be frozen. It may be slightly less hard than ice cream and will be more sour.

Finally, yoghurt has become quite popular globally and in Asian countries because of its health benefits. Several new forms may appear along with variations of flavours and ingredients. There may be low sugar variants as health conscious consumers may want lower calories. Market will certainly be very active in this category of dairy products.

Dr Jagadish Pai

Editor, PFNDAI

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