Like cheese, yoghurt originated as a vehicle to preserve the nutrient value of milk. Through time, the product has evolved to a foodstuff richly diverse in flavour, texture and functional properties. Thus, the formulations may now incorporate components such as fruits, grains and nuts, as well as having a range of textures.
Yoghurt is only one of a series of fermented dairy products (Table 11.1). Sour cream comprises cream (>18% milk fat) fermented with specific lactic cultures, perhaps with the use of rennin, flavours and materials to enhance texture. Kefir and kourmiss are fermented milks from Russia and Eastern Europe. In their production, yeast accompanies bacteria with the impact of producing alcohol and carbon dioxide. Rather than seeding with organisms, an endogenous microflora is employed and this supposedly contributes to the health value of such products. The organisms employed are listed in Table 11.2.
Basically yoghurt is a semisolid foodstuff made from heat-treated stabilised milk through the action of a 3: 1 mixture of Streptococcus salivarus ssp. thermophilus (ST) and Lactobacillus delbrueckii ssp. bulgaricus (LB). Their relationship is symbiotic. In some countries, other organisms are also used, namely L. acidophilus and Bifidobacterium spp.
Starter cultures are purchased either in a freeze-dried, liquid nitrogen or frozen form. They are used either as is or receive further propagation. This is in liquid skim milk or a blend of non-fat dry milk in water (9-12% solids). Media may also include citrate, which is a precursor of the diacetyl that makes a major contribution to flavour.
The milk used originates from a range of animals, but is chiefly from the cow. To achieve the desired consistency, the milk is fortified with dried or condensed milk. Vitamin A (2000 IU per quart) and vitamin D (400 IU per quart) may also be added. Other additions sometimes used are lactose or whey to increase the content of non-fat solids; sucrose, fructose or maltose as sweeteners; flavourings, colour, and stabilisers.
Milk is the natural habitat for a range of lactic acid bacteria. Milk of course will spontaneously sour, but the uncontrolled nature of this means that starter cultures are nowadays the norm.
Table 11.1 Examples of fermented dairy foods other than cheese.
Acidophilus milk Chal
Low-fat milk. Heat-treated and inoculated with
Lactobacillus acidophilus or Bifidobacterium bifidum Camel's milk yoghurt
Skim cow's milk heated, homogenised, cooled and inoculated with Streptococcus cremoris, Streptococcus lactis, Streptococcus lactis ssp. diacetylactis, Leuconostoc cremoris Whole cow's milk pasteurised, homogenised, cooled, fermented with ropy strains of Streptococcus cremoris and other organisms used for cultured buttermilk. The polymers giving ropiness are important for the slimy texture Acidic and mildly alcoholic effervescent milk. Goat, buffalo or cow milk heated to 90-95°C for 3-5 min, cooled and inoculated in an earthenware vessel with Kefir grains or starter comprising Lactobacillus casei, Streptococcus lactis, Lactobacillus bulgaricus, Leuconostoc cremoris, Candida kefyr, Kluyveromyces fragilis, etc. Similar to Kefir, from horse milk and frequently served with cereal Sour drink consumed salted with herbs and spices or sweetened with honey. Low-fat acidic soft cheese eaten fresh. Fresh milk pasteurised, cooled, treated with rennet and starter culture of lactic acid bacteria (similar population to cultured buttermilk) Hard cheese from whey, used as whipped dessert or for making of gnocchi or lasagne. Whey, perhaps with added skimmed, whole milk or cream, salt and Streptococcus thermophilus and Lactobacillus bulgaricus, followed by heat treatment and curd collection
As raw milk contains heat-sensitive microbial inhibitors, notably the enzyme lysozyme and agglutinins, it is either heated at 72°C for 16 s or auto-claved for 15 min at the onset of the process. This heating also degrades casein, liberating thiol groups and it also encourages the shift of lactose to lactic acid.
The non-fat solid content of milk varies seasonally and this in turn impacts the microflora, with greater growth of lactic acid bacteria as the solid content increases.
The bacteria are also at risk of bacteriophage infection, for which reason chlorine (200-300 ppm) is applied to processing equipment, and culture rooms are fogged with 500-1000 ppm chlorine. Culture media may also incorporate phosphate to sequester the calcium that is needed for phage growth.
The production of lactic acid must be sufficient to lower the pH to a level where acetaldehyde and diacetyl (amongst other flavour-active components) are generated sufficiently.
Table 11.2 Organisms involved in making fermented milks.
Acidophilus milk Lactobacillus acidophilus
Cultured buttermilk Lactoccus lactis ssp. cremoris Lactococcus lactis ssp. lactis Lactococcus lactis ssp. lactis biovar diacetylactis
Lactococcus lactis ssp. lactis Lactobacillus delbrueckii ssp. bulgaricus Lactobacillus helveticus Lactobacillus delbrueckii ssp. lactis Lactobacillus casei Lactobacillus brevis Lactobacillus kefir Leuconostoc mesenteroides Leuconostoc dextranicum Acetobacter aceti Candida kefir
Kluyveromyces marxianus ssp. marxianus Saccharomyces cerevisiae Torulospora delbrueckii
Kumiss Lactobacillus delbrueckii ssp. bulgaricus
Lactobacillus kefir Lactobacillus lactis Acetobacter aceti Mycoderma sp. Saccharomyces cartilaginosus Saccharomyces lactis
Yoghurt Lactobacillus delbrueckii ssp. bulgaricus
Streptococcus salivarius ssp. thermophilus
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