More info about yogurt and yogurt bacteria

[Guest guest]

from " Bergey's Manual of Determinative Bacteriology " and other sources.

Two bacteria are involved in making yoghurt: Lactobacillus bulgaricus and

Streptococcus thermophilus. Both are homolactic (homofermentative); that is,

they use lactose as an energy source and produce lactic acid as their sole

fermentation product. Initially, the Streptococcus thermophilus is dominant,

then it is inhibited by the produced acid. In a second phase, the Lactobacillus

bulgaricus continues to ferment the remaining lactose; the pH drops from 6.5 to

about 4.5.

Streptococcus thermophilus

Gram positive spherical or ovoid cells, 0.7-0.9 µm in diameter in pairs t o

long chains. Acid is produced from glucose, fructose, lactose and sucrose; no

acid from trehalose, maltose, inulin, glycerol, mannitol, sorbitol or salicin

and rarely from raffinose, xylose or arabinose. Optimum temperature is between

40°C and 45°C. Growth occurs at 50°C but not at 53°C. No growth at temperatures

below 20°C. Heat tolerance: survives 65°C for 30 min.

This species is easily recognized by its thermal tolerance; unable to ferment

maltose and unable to grow in media containing 2.0% sodium chloride.

Lactobacillus bulgaricus

Gram positive rod, width <1 µm, contains aldolase, is negative for catalase,

indole, nitrate reductase, oxidase, and benzidine reactions, attacks glucose and

produces lactic acid as the major product. Does not ferment adonitol, dulcitol,

erythritol, glycerol, glycogen, inositol, inulin, sorbose, starch. Does not

produce gas from ribose, gluconate or glucose.

Requires niacin, riboflavine and pantothenate. Produces up to 1.7% acid in milk,

does not produce NH3 from arginine. Utilizes lactose, and weakly also fructose,

galactose, mannose. Does not utilize aesculin, amygdalin, arabinose, cellobiose,

maltose, mannitol, melezitose, melibiose, raffinose, salicin, sucrose,

trehalose, and xylose. In contrast to Lactobacillus plantarum, Lactobacillus

buchneri, and Lactobacillus brevis, Lactobacillus bulgaricus grows at 45°C but

not at 15°C. At least 19 strains identical with Orla-Jensen's strain (designated

neotype strain; ATCC 11842, Orla-Jensen's Thermobacterium bulgaricum number 14,

which he isolated from Bulgarian yoghurt) have been isolated from Bulgarian,

Russian, Greek, Syrian, and Armenian yoghurts

-------------------------------------------

The <http://www.foodsci.uoguelph.ca/dairyedu/micro.html#starter>starter culture

for most yogurt production in North America is a symbiotic blend of

Streptococcus salivarius subsp. thermophilus (ST) and Lactobacillus delbrueckii

subsp. bulgaricus (LB). Although they can grow independently, the rate of acid

production is much higher when used together than either of the two organisms

grown individually. ST grows faster and produces both acid and carbon dioxide.

The formate and carbon dioxide produced stimulates LB growth. On the other hand,

the proteolytic activity of LB produces stimulatory peptides and amino acids for

use by ST. These microorganisms are ultimately responsible for the formation of

typical yogurt flavour and texture. The yogurt mixture coagulates during

fermentation due to the drop in pH. The streptococci are responsible for the

initial pH drop of the yogurt mix to approximately 5.0. The lactobacilli are

responsible for a further decrease to pH 4.0. The following fermentation

products contibute to flavour: lactic acid , acetaldehyde, acetic acid ,

diacetyl

------------------------------------------------------------------

Yoghurt is produced by the controlled fermentation of milk by two species of

bacteria (Lactobacillus sp. and Streptococcus sp.). The sugar in milk (called

lactose) is fermented to acid (lactic acid) and it is this that causes the

characteristic curd to form. The acid also restricts the growth of food

poisoning bacteria and some spoilage bacteria. So, whereas milk is a potential

source of food poisoning and only has a shelf life of a few days, yoghurt is

safer and can be kept for up to ten days, under proper storage conditions.

-------------------------------------------------------------------------

Fruit and nuts can be added to yogurt before fermentation begins but care is

needed to ensure that they are thoroughly cleaned and blanched to avoid

contamination.

------------------------------------------------------------------------

The dangers of ready made yogurt:

A common practice in yoghurt manufacture is the addition of skimmed milk powder

to improve the yoghurt's nutritional status and to thicken it. This also

increases the lactose content of the yoghurt, and values ranging from 3.3-5.8%

have been reported. Typically, only 15% of this sugar is utilised during

fermentation by lactobacilli. Consequently such yoghurt may be unsuitable for

those who are lactose-intolerant.

Food technologists have therefore investigated the possibility of making yoghurt

from milk with a reduced lactose content. This procedure involves pretreating

the milk with the enzyme ß-galactosidase (lactase) to hydrolyse its lactose to

glucose and galactose. Two other advantages arise from this. Firstly, yoghurt

made in this way sets more rapidly due to faster initial rates of acid

production (although the final pH is similar to that of normal yoghurt). This

difference has been attributed to increased fermentation of glucose by starter

culture organisms, although the overall numbers and proportions of the bacteria

in the yoghurt are not greatly altered. Secondly, the product is sweeter than

that made with normal milk.

-------------------------------------------------------------

Complex symbiosis:

" It is said that Lactobacillus bulgaricus breaks down milk protein to chains of

amino acids, which are then used by Streptococcus thermophilus to make methanoic

acid (formic acid) which Lactobacillus bulgaricus uses to convert lactose into

lactic acid.

The acidic conditions (pH 3.7-4.3) cause milk proteins to be coagulated, which

provides the thickened texture of yoghurt. Other chemical products from the

bacterial growth, e.g. ethanal (acetaldehyde) contribute to the flavour. "

------------------------------------------------------------------

Keeping qualities

As the yoghurt is cooled, the bacterial growth rate is reduced and the product

can keep for about 10 days at 5 °C. The normal numbers of bacteria are about 108

(100,000,000) per gram. After some time, acid slowly released by the bacteria

gradually kills them and causes the proteins to separate into curds and whey.

Yeasts and moulds can cause problems if they contaminate yoghurt - causing pots

to become " blown " , due to the production of carbon dioxide.

Jan Sultan

http://groups.yahoo.com/group/SCDrecipes/