Food Microbiology

Dairy

Cultured Butter

Cultured butter is fresh cream butter to which lactic acid cultures have been added for the development of a particular flavor. This product is most popular in European countries; whereas in the United States, cultured butter is not a large percentage of butter production. Cultured butter is made from milk fat to which a mesophilic starter culture has been added to enhance its flavor, and diacetyl, made from citrate by LAB, enhances storage properties.
Cultured butter does differ in tasted from sweet cream butter. Cultured butter has a more pronounced, distinct flavor that stems from starter cultures added to the cream during churning. As a result, flavor compounds from cultured butter are superimposed on those of sweet cream butter, creating a full-flavor effect. Starter cultures are typically mixtures of flavor concentrates produced by one strain or mixed strains of bacterial cultures. Streptococcus diacetilactis produces diacetyl, the flavor most commonly associated with flavored butter and Streptococcus lactis is used to produce lactic acid, which contributes to the acidic flavor typically associated with cultured butter.

Link for Cultured Butter Production: http://www.butterisbest.com/book/bkeysteps.html

Cheese

The making of cheese is essentially a dehydration of milk, resulting in a six to twelve fold concentration of milk proteins (caseins), fat, and minerals. Although varying cheeses each have differing manufacturing protocols, the basic steps common for most varieties are:
1. Acidification
2. Coagulation
3. Dehydration
4. Salting
The starter bacteria function mostly in acid production. Lactic acid is responsible for the fresh acidic flavor of unripened cheese and is important in the coagulation of milk. Starter cultures also function in the production of volatile flavor components (diacetyl, aldehydes). Furthermore, they are involved in the synthesis and release of the intracellular proteolytic and lipolytic enzymes during cheese ripening, and the suppression of pathogens and spoilage microorganisms. The flavor of bacterial-ripened cheeses relies on LAB. Many of the details of exactly how LAB contribute to cheese flavor have yet to be worked out. However, current knowledge suggests that LAB may develop cheese favor by three key mechanisms:
1. Hydrolysis of proteins into smaller peptides and free amino acids
2. Metabolism of free amino acids
3. Hydrolysis of milk lipids into free fatty acids and the conversion of these fatty acids to esters.
The hydrolysis of caseins, into smaller peptides and free amino acids is widely accepted as one of the most significant processes during the maturation of most bacterial-ripened cheeses.

Yogurt

Yogurt is the product of milk fermented by two species of LAB, Streptococcus thermophilus and Lactobacillus bulgaricus growing at about 45ºC. Streptococcus produces much acid, resulting in pH to about 3. Lactobacillus is responsible for much of the flavor and aroma of the yogurt. The entire process takes only a few hours. The production of lactic acid can be summarized as follows:

lactose ------> glucose ------> pyruvic acid ------> lactic acid

Over 90% of the fresh yogurt marketed in the United States is flavored with fruit has sugar added, which reduces the sharp taste. The nutritional value of yogurt depends on the fat content of the milk used, the percent of skim milk used, and the additives. The addition of fat improves the flavor and unfortunately also increases the calories. The firmness of the gel depends on the concentration of skim milk solids present. Lesser amounts will give a more delicate gel. Gums such as carrageenans, alginates, and agar can be added to make a firmer gel but will not add as much nutritive value as using more skim milk solids. In making yogurt, the liquid milk is pasteurized to kill any contaminating organisms. The bacteria needed to bring about the fermentation are present in supermarket yogurt , which can be used as a starter culture. Less than the desired quality of yogurt products can be the result of contamination by other bacteria and/or the wrong concentration of milk solids.