Bacteriological characteristics of fermentation tests according to Dueggele.
Plane type. Low-microbial milk, Streptococcus lactis, coli-aerogenes, ziarniaki.
Type gl. Streptococcus lactis and cocci are predominant, butyric acid candies, coli-aerogenes, flurescens.
Typ s. Streptococcus lactis in larger amounts, bloating bacteria and cocci.
Type z. Usually Streptococcus lactis, next to him alternating coli-aerogens, ziarniaki.
Type in. Coli-aerogenes predominate, next to them Streptococcus lactis, few grains, Bact. house.
Próba f e r m e n t a c y j n o – r e d u k c y j n a (reduction). Ferment reductase is produced in milk by some groups of bacteria during metabolism, and not to the same extent. For this reason, only the approximate number of bacteria in the milk can be deduced from the amount of reductase enzyme produced. Despite this inaccuracy, the reductase test is very important in practice. The reductase has this property, that it discolored, that is, it reduces methylene blue. So the shorter the time, where a predetermined amount of methylene blue is discolored, the more bacteria the given milk contained.
The fermentation test and the reductase test are complementary. The first indicates roughly the quality and direction of fermentation, the second amount of bacteria. Hence the thought, to combine both samples into one so-called. fermentation-reduction test. This saves a lot of time and resources. In this case, for fermentation samples containing po 40 cm3 of milk you only need to add a little methylene blue.
It is most convenient to use methylene blue tablets. One tablet is dissolved in boiled and then cooled water,’Then topped up with the same water to 200 cm3. From this it comes to the containing sample 40 cm3 of milk 1 cm3. The methylene blue solution stored in the darkroom will remain the same 14 days, then you need to prepare fresh.
Another way, according to O. Jensena and Barthla: w 20 cm3 of alcohol dissolve a few grams of shiny green methylene blue and shake it through 2 hours. From a saturated solution is taken 5 cm3 and diluted 195 cm3 of water.
The milk is evaluated according to the following indications (according to O. Jensena and Barthla).
1. Good durability:
maintains color for over 5½ hours, number of bacteria below 500000 w 1 cm3.
2. Average durability:
stays color for 2½ to 5½ hours, number of bacteria from 500000 do 4 million in 1 cm3.
3. Low durability:
retains color for 20 minutes to 2 hours, number of bacteria over 4 do 20 million in 1 cm3.
4. Bad durability:
discoloration before expiration 20 minutes, number of bacteria over 20 million in 1 cm3.
Bacterial numbers are approximate.
When assessing the reductase test, the color of the upper third of the test tube contents should not be taken into account, because here the deoxygenated methylene blue is oxidized again on exposure to air.
Pay attention to this, that the milk is healthy and clean, and therefore low in lactic acid bacteria, may give an unfavorable result in the fermentation test, because over time harmful bacteria developed there, not inhibited by competition from lactic acid bacteria. Contrary, perishable milk, but rich in lactic acid bacteria, muffling all the others, sometimes creates a beautiful type of gl. Such illusions are often rectified by the reductase test, thus also a fermentation-reduction test, which also shows the approximate amount of bacteria.
Milk taken from the entire contents of the boiler should always be subjected to the fermentation-reduction test, before the rennet is added, the other - already renneted and without the addition of methylene blue. Boiler milk not renneted as far as vertebrate cheeses are concerned, it should not clot before it expires 8 hours, but also no later than within 20 hours. In the first case, the amount of bacteria would be excessive, while in the second it is too small. These requirements are well illuminated by the reductase test.
For practical reasons, the fermentation test or the reduction test is performed:
1. samples of milk delivered by individual suppliers,
2. using centrifugal force, that is, using a centrifuge. rennet him,
3. a sample of collective milk from tanks before and after delivery in order to check for infections via transport vessels,
4. milk from individual cows to detect udder diseases (coli).
Depending on the need, there may be more such combinations.
A simple and cheap fermentation device that works like a water bath. It is a tin tank divided into two parts. Heated water is poured into the upper one 33 ~ 39°C, he inserts a spirit lamp into the lower one, whose flame must be so regulated, so that the temperature does not drop below 38 ° C, a did not rise above 40 ° C. Tubes with milk are placed in the heated water, covered with capsules or plugged with cotton wool to protect against condensed steam. The water level should be 2 do 3 cm below the tube opening, wki. After inserting the milk tubes, the tank is closed and the temperature is monitored with a thermometer reaching into the water through the hole in the lid.. Due to the risk of fire, the fermentation device is placed on a fireproof floor or on a stand. The water changes after each attempt. More expensive, but thermostats with a hot water tank or electrically heated are convenient and precisely maintaining the temperature (see "Ripening of rennet mortar”).