Flashcards in Yogurt Deck (24)
what sources of nutrients does milk have?
3.carbohydrates (in form of lactose)
4. vitamins A and D
5. minerals (calcium, potassium, and phosphorous)
Problem with milk?
at standard ambient temperature, it spoils rapidly through microbial activity and becomes unpleasant
what is the oldest way to preserve milk?
fermentation of milk to produce yogurt or yogurt-like products
common mechanism for fermented milk products
fermentation of the sugars in milk by lactic acid bacteria such as lactobacillus, lactococcus, and streptococcus.
what specific sugar and protein does milk contain?
1. a simple sugar lactose
2. milk protein casein (as calcium caseinate)
What makes up lactose? and what type of bond is shared making lactose
glucose and galactose connected by a beta-glycosidic bond
1st step in making yogurt? and why do we do it?
milk is scalded to kill spoilage bacteria and then cooled to 46-48 degrees celcius
2nd step in making yogurt? what bacterial strain is preferred at this step?
milk is inoculated with an appropriate amount of yogurt once it reaches 46-48 degrees celcius.
Thermophiles such as streptococcus thermophilus, and mesophiles such as L. acidophilus
Third step in making yogurt?
inoculated milk is maintained at 46-48 degrees celcius for 2-3 hours or until it has curdled sufficiently.
Why is the 46-48 degrees celcius temperature important to maintain?
it encourages growth and metabolism of streptococcus thermophilus, a high temp loving, homolactic fermentative bacterium.
Characteristics of streptococcus thermophilus and function?
1. active fermenter
2. capable of transporting lactose from the milk into the cell where it is hydrolyzed to simple sugars by beta-galactosidase
3. copious amounts of acid will be produced
What happens to the lactose once it is hydrolyzed by beta-galactosidase?
it turns into glucose and galactose.
from there they are converted to glucose-6-phosphate and then they enter glyocolysis.
What happens to the pyruvate (result of glycolysis)
it is converted to lactic acid by the fermentation pathway.
what happens to the protein (casein) levels in result of the huge amount of acid?
The pH wil decrease rapidly, converting calcium caseinate (colloidal suspension) to free caesin. . the free casein denatures and forms a white precipitate (curdling)
What happens if this milk is left incubated at 46-48 degrees celcius for too long?
it will become too sour for taste
4th step in making yogurt?
curdled milk is moved either to the refrigerator (4 degrees celcius) or left at room temperature (22 degrees celcius).
Why is the curdled milk left to cool at room temperature or cool in the refrigerator?
As the temperature of the milk drops, the activity of S.thermophilus begins to diminish, while the activity of the MESOPHILIC organism (lactobacillus bulgaricus) begins to increase.
significance of mesophilic organism lactobacillus bulgaricus
1.its either acid resistant or acidophilic and can continue to carry out the fermentation and curdling begun by the previous thermophilic bacterium.
2. they can produce volatile organic compounds that give the yogurt its specific taste and aroma.
What happens if you refrigerate the curdled milk after the first incubation at 46 degrees celcius instead of leaving it in room temperature?
the temperature continues to drop below mesophilic range, the activity of the mesophiles also diminishes and acid production stops.
What is the purpose of refrigeration ?
significance of low pH in yogurt production?
prevents growth of spoilage bacteria.
what are the 2 overall achievement for this method of two incubation temperatures for yogurt production?
1. preserving the milk and its nutrients from spoilage
2. introducing new flavors to milk
How do you make kefir ?
small amount of kefir grains containing a mixture of lactic acid bacteria and yeast is added to milk.
mixture is left at room temperature for 24 hours (mesophilic) with occasional agitation.
Resulting product is slight sour, frothy, and mildly alcoholic