Yogurt is produced through the fermentation of milk by lactic acid bacteria, usually lactobacillus bulgarius and Streptococcus thermophilus.
The milk is firstly heat treated, homogenised and is then cooled to allow the addition of bacteria or starter culture.
Given the right conditions, i.e. correct temperature and moisture, the bacteria are able to ferment the milk sugar (lactose), producing lactic acid.
The milk proteins then coagulate and set, to form yogurt.
A colourless liquid called acetaldehyde is also produced during fermentation and gives yogurt its distinct flavour.
Yogurt can be made from different types of milk, including skimmed, semi-skimmed, whole, evaporated or powdered forms.
Varieties of yogurt
The market now offers a vast array of yogurts to suit all palates and meal occasions. They come in a variety of textures (e.g. liquid, set, smooth), fat contents (e.g. luxury, low-fat, virtually fat-free) and flavours (e.g. natural, fruit, cereal), can be consumed as a snack or part of a meal, as a sweet or savoury food and are available all year-round.
Fermenting milks with different micro-organisms has also provided an opportunity to develop a wide range of products with different flavours, textures, consistencies and, more recently, health attributes. These include:
- Live yogurts, which contain harmless bacteria that are added to the milk and are still present and alive.
- Probiotic yogurts, which contain live probiotic micro-organisms that are suggested to be beneficial to health.
- Bio yogurts, which are very popular and are made using bifido bacterium bifidum (bifidobacteria) and/or lactobacillus acidophilius. Bio yogurt has a milder, creamier flavour which is less acidic than some other varieties and has shown to aid digestion and promote good health.
The nutritional composition of yogurt is affected by many factors including the species and strains of bacteria used in the fermentation, the source (whole, semi or skimmed) and type of milk solids added before fermentation, the temperature and duration of the fermentation process as well as the addition of milk solids non-fat, sweeteners and fruits.
Despite the variation that occurs between products, yogurt naturally contains many of the same nutrients as milk and is also considered to be a nutrient rich food due to the array of nutrients it provides relative to its fat and energy content.