MCB2004L Exercise 11 (Microbial production of Yogurt/Sauerkraut)

Naturally-occurring microorganisms on cabbage plants

-many are lactic acid bacteria that can ferment CHO's to lactic & other kinds of organic acids -usually confined to surfaces of the plant *however if a cabbage leaf is cut, the microbes on the surface may enter the tissue fluids of the plant, where they find sugar & other nutrients that they can use for metabolism/growth

The microbial succession that occurs during sauerkraut fermentation occurs in 3 stages:

1) Early Stage 2) Intermediate Stage 3) Final Stage

What are the 2 steps involved in yogurt production?

1) S. thermophilus homolactic bacterium is added to milk at a temperature of 46-48 degrees Celsius (which favors this bacterium's growth) to ferment lactose at a rapid rate ==> dec. pH of the milk to 4.6, at which casein protein in milk begins to coagulate & denature to form a *curd* (semisolid substance) 2) When fermenting milk reaches desired texture in the 1st step of the process, it is refrigerated at a low temp of 4 degrees Celsius, causing S. thermophilus growth to slow & eventually stop while L. bulgaricus bacterium begins to ferment the lactose that had not yet been fermented by S. thermophilus when temp. began to drop; fermentation by this bacteria produces lactic acid as well as other volatile organic compounds that give yogurt its characteristic flavor & aroma

What are the 2 bacteria involved in yogurt production?

1) Streptococcus thermophilus 2) Lactobacillus bulgaricus

The salt used in sauerkraut production serves to

1) extract nutrient-containing fluids from the leaf tissues (making it easier for microbes to get at the nutrients) 2) suppresses growth of unwanted microbes that cannot tolerate high-salt env.'s 3) specifically favors growth of salt-tolerant lactic acid-fermenting bacteria 4) enhance the flavor of the final product

Steps in sauerkraut production

1) shred cabbage leaves into narrow strips 2) mix shredded leads w/ measure amt. of salt (salt conc. 2-3% by wgt. in container in next step) 3) pack salted leaves tightly into a sealed container 4) a weight is placed on top of the cabbage-salt mix. in fermentation container to keep the mixture beneath the surface of the brine (formed while container is being packed)--readily produces & maintains anaerobic environment throughout fermentation process

How are diff. fermented milk products w/ diff. flavors & textures obtained?

1) using diff. types of milk or cream 2) using diff. species of lactic acid bacteria 3) diff. fermentation conditions

It is necessary to keep the temp. in the container b/w ............... during fermentation in order to promote the growth of the right types of lactic acid bacteria (the ones that produce right amt.'s of acid/other flavor-enhancing organic products)

15-24dC

The first step of yogurt production process is carried out at a temperature of .........., which favors S. thermophilus growth & lactic acid fermentation, while the second step is carried out at a temp. of .......

46-48 degrees Celsius; 4 degrees Celsius (refrigeration)

Production of sauerkraut is done by

Creating enrichment conditions for the growth of only the lactic acid bacteria that occur naturally on the cabbage plant by 1) adding NaCl (salt) to cabbage leaves 2) removing O2 to maintain an anaerobic env.

Give an ex. of a lactic acid bacterium commonly used to produce yogurt

Streptococcus thermophilus

Streptococcus thermophilus

a lactic acid bacteria that has the ability to rapidly ferment lactose (milk sugar) to produce lactic acid, essentially the only compound produced during the fermentation process (b/c the organism is a homolactic fermenter)

Microbial succession

a situation in which diff. types of microorganisms grow up & then die off one after another (successively) as the conditions in the environment change over time; occurs during sauerkraut & yogurt fermentation processes

Enrichment culture

a specialized environment that favors the growth of only a single kind of bacteria

How is sauerkraut made?

by pickling (fermenting) cabbage using microorganisms

Yogurt produced commercially in North America is made from

cow's milk

What is meant by calling a lactic acid bacteria a "homolactic" fermenter?

it produces lactic acid as its primary fermentation product

How long does the first step of the yogurt-making production process proceed?

only until the coagulated milk reaches the proper texture (as a result of curd formation)

How long is the second step of yogurt production process allowed to proceed?

only until the temp. of the refrigerated milk drops below the point at which L. bulgaricus can grow & produce lactic acid (as well as the volatile compounds that contribute to the yogurt's flavor & aroma); the yogurt will then remain unchanged as long as it is kept refrigerated

Fermented milk products

products made by allowing lactic acid bacteria to ferment some type of milk or cream

Brine

the fluid that is extracted from the cabbage tissues by the salt during fermentation process

Casein

the primary protein in milk, which also gives the milk its white color, that occurs as a colloidal suspension of its calcium salt (calcium caseinate) at the normal 6.6 pH of milk

What happens to casein protein in milk once lactic acid fermentation reduces its pH to ~4.6?

the soluble casein begins to denature, coagulating as it does, resulting in the formation of a semisolid curd

Original purpose of making sauerkraut

to preserve (prevent the spoilage of) cabbage (now can be preserved by refrigeration)

What happens if the first step of yogurt production is allowed to continue for too long?

too much acid will be produced & the final product will be too sour for most people

Fermented milk products incl.

yogurt, sour cream, buttermilk, kefir, & cheese