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Yogurt

Overview

Yogurt is milk that has fermented under the action of lactic bacteria. Certain specific bacterial strains convert part of the lactose into lactic acid. The milk coagulates when a sufficient quantity of lactic acid is produced. Today, the curdling is performed by the combined action of two traditional fermenting agents that produce the lactic acid and the components that determine the yogurt’s characteristic flavour: Lactobacillus bulgaricus and Streptococcus thermophilus. 

Legally, in Canada, a product must contain these two specific bacterial strains to bear the name "yogurt". The fresher the product is, the more viable bacteria it contains. Since the virtues of yogurt are associated among other things with the bacteria’s action in the intestine (they help rebuild the intestinal flora), their presence in a sufficient number is important. With respect to this, yogurt should by law contain at least 10 million bacteria per gram at the time it is marketed.

Natural unsweetened yogurt has close to the same nutritional value as the milk with which it is made and is an excellent source of proteins, calcium (100 g provide about 15% of the daily recommended intake), potassium, phosphorus and vitamins A and B. In addition, it provides all of the benefits associated with fermentation while only supplying very few calories (cal./g). Yogurt is quickly digested. More than 90% of the yogurt can be digested within an hour, as opposed to 30% for milk. It provides an interesting nutritional alternative for people with lactose intolerance. Actually, lactose, an enzyme produced by the bacterial cultures, hydrolyzes or predigests from 20 to 30% of the milk’s lactose and continues to act in the intestine; it allows people with an intolerance to avoid upsets (flatulence, diarrhea).

Yogurt is a rich, versatile food capable of enhancing the flavour and texture of cooked dishes.


Characteristics

Yogurt is a living, perishable product. It contains some bacterial cultures that make it unique. Without these bacterial cultures, its beneficial health effects are greatly reduced. Yogurt’s fermenting agents are sensitive to heat. To retain their vitality, the product must therefore not have been subjected to heat treatment. Yogurt has an ideal shelf life of 35 to 40 days when it is stored at temperatures between four and six degrees Celsius. It seems that freezing temperatures do not affect the yogurt’s bacterial cultures. Its ideal shelf life, once frozen, is 1 month. Before use, slow thawing in the refrigerator is recommended.  

Commercial manufacturing produces a firmer yogurt less likely to show signs of syneresis. This phenomenon, sometimes seen on the surface of traditional yogurts, is reflected in the separation of lactoserum, a yellowish liquid, from the curd. This has no affect at all on the yogurt’s nutritional value or safety, but it is nevertheless deemed undesirable by consumers and manufacturers.


Varieties

Today, we find a whole range of yogurts:  

  • Firm yogurt: with a more or less firm consistency, this type of yogurt is fermented right in the container and then cooled. If it is flavoured with natural or artificial products that are normally placed in the bottom of the container.
  • Spun yogurt: with a smooth and homogenous consistency, this type of yogurt, often containing added pieces of fruit or jams, is mixed after fermentation and cooling.
  • Yogurt drink: with a more liquid consistency, this drinkable yogurt is made from fermented milk with added sugar and fruit or fruit-flavoured syrups and is intended as a nutritional alternative to carbonated beverages.
  • "Healthy" yogurt: firm or spun, it contains certain bacterial strains that are referred to as "probiotics". When these living organisms are transported through the human body, they provide certain health benefits in humans by creating a better balance in the intestinal flora. 

It must be noted that if some of these products were pasteurized or processed at ultra-high temperatures (UHT), the bacteria will have been rendered inactive and as a result, many of the benefits associated with these "probiotic" bacteria will have been lost.


Composition

Yogurt is a dairy product obtained through the fermentation of milk, partly skimmed milk or skim milk by the lactic bacteria Lactobacillus bulgaricus and Streptococcus thermophilus with which the lactic bacteria Lactobacillus acidophilus and Lactobacillus casei or the bacteria Bifidobacterium bifidum, Bifidobacterium longum, Bifidobacterium infantis and Bifidobacterium breve may be combined.

The composition standards stipulate that yogurt must contain not less than 0.8% lactic acid, not less than 9.5% non-fat milk solids and not less than 3.0% protein. It may also contain some ingredients that come from milk (either whole or skim milk powder, or concentrated evaporated milk), fruits, fruit juices or extracts, jams, cereals or any other flavouring, sweeteners, a quantity not exceeding 2.0% of texturizing agents (stabilizers, gelling, thickening or emulsifying agents), citric acid, food colouring and, in the case of yogurt with added fruit, fruit juices or extracts or jams, a preservative not exceeding 50 ppm. By definition, a "fat-free" food must contain less than 0.5 g of fat per portion.

The microbiological standards specify that a good quality yogurt must have a maximum of 10 coliforms per gram and 100 yeasts or moulds (or yeasts and moulds) per gram.

Typical composition of Yogurt

 

 

Product (100 g) 

2% natural yogurt  

Fat-free natural yogurt 

Proteins 

4.5 g

4.6 g

Glucides

4.9 g

5.3 g

Fats 

2.1 g

<0.5 g

Calcium 

132 mg

138 mg

pH

4.3 - 4.4

4.2 - 4.3

Energy 

57 kcal

42 kcal 


Various Uses

Yogurt is eaten as is and can be cooked. It is added to salted or sweetened foods. Natural, it replaces sour cream and can be added to mayonnaise or vinaigrettes. It is a choice ingredient, along with cucumbers, of the famous cold dip called Tzatziki served with grilled fish. In Canada, it is used in the making of prepared frozen entrees such as curried chicken. It is also used as an important ingredient in bakery products such as tortes and muffins where it adds moisture to the products. 

It is an important ingredient in Middle Eastern, Oriental, East Indian and Asian cuisine. It has a large number of many traditional uses. First, as an iced beverage with a some water, for cooking meats and vegetables; for seasoning raw vegetable salads; for preparing soups, as well as for sauces for grilled brochettes.

In Europe, yogurt is primarily eaten with sugar, honey, jam, fresh or dried fruits, as a dessert or for breakfast. It is also used to prepare cold or iced snacks, as well as refreshing beverages. Chefs have also discovered its qualities as a fresh cream substitute in cold sauces. In this case, it is stabilized first with a bit of starch if it has to be cooked in soups, stews, broiled toppings, stuffing, etc.


Functional Properties

The well-known health benefits of eating yogurt are the following:

  • that of enabling lactose intolerant people to digest a dairy product;  
  • that of inhibiting the growth of harmful bacteria (for example, Clostridium botulinum causing botulism). This is explained by the fact that yogurt’s fermenting agents produce some bacteriocins which, combined with their acidity, prove beneficial for the intestinal flora.  
  • that of improving intestinal transit.

As an ingredient, yogurt is used in food preparations to: 

  • enhance their flavour: yogurt has a slightly tart characteristic flavour that comes from a mix of many organic compounds released by the fermentation (lactic acid, acetaldehydes, volatile and non-volatile acids, protein derivatives, etc.).   
  • act as a textural ingredient: the addition of yogurt to a recipe adds body and better cohesiveness to cold sauces, vinaigrettes and dips.   
  • reduce the calorie content in certain recipes: substituting sour cream with natural yogurt allows reaching the niche of consumers concerned about reaping the benefits of a low calorie diet.  

 
For more information on yogurt, please visit the University of Guelph’s
 Dairy Science and Technology Web site.


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