Lecture 25 Outline
How come those little creams don't need refrigeration? (pasteurization/soured creams); Cheese - moldy and holy
Cheesemeaking I - Pasteurization
Cheesemaking II - Fermentation
Cheesemaking III - Rennin and Curds
Cheesemaking IV - Pressing and Ripening
Pasteurization
toady's milk is pasteurized to kill pathogenic microbes
the pasteurization process requires that milk be briefly heated to high temperatures
non-spore forming pathogens like Brucella, Coxiella, and Mycobacterium tuberculosis are killed by moderate heat
pasteurization can be done in batch by heating to 62.8 deg C for 30 minutes
pasteurization can also be done in flow-through systems that heat to 71.7 deg C for 15 seconds
many pathogens are killed, but the milk is not sterile. It needs to be refrigerated to avoid spoiling
ultrapasteurization occurs with 141 deg C for 2 seconds
this sterilizes the milk so that as long as the milk is in a hermetically sealed container it shelf-life is indefinite
the creams commonly served in diners is ultrapasteurized
Fermented dairy products
fermented products of milk include sour cream, butter, yogurt and cheese
generally, the milk is inoculated with a starter culture containing microbes or purified microbes are added
the fermentation of milk products is a controlled souring that inhibits the growth of pathogenic microbes
sour cream is made from cream by the action of lactic acid bacteria
Streptococcus cremoris is added to produce copious quantities of lactate
Leuconostoc cremoris is added to produce flavor compounds that are found in low concentrations in the finished product
when skim milk is fermented in the same way, the result is cultured buttermilk
ricotta cheese is fermented in this way from whey
Butter
again this is a sour milk product, but it is churned to collect solids
the milk is fermented with Streptococcus cremoris to produce lots of lactic acid
Leuconostoc citrovorum converts milk citrate into flavor compounds
Yogurt
concentrated skim milk is fermented to produce yogurt
Lactobacillus bulgaricus is added to produce a tart flavor compound acetylaldehyde
Streptococcus thermophilus is added to produce lots of lactic acid
note that this is a process that is run at 40 deg C
finished yogurt contains a billion bacteria per gram
Cheeses
milk is separated into curds (solid lumps) and whey (liquid containing sugars and some protein)
milk is first fermented a bit by lactic acid bacteria
then rennet containing the enzyme rennin is added
rennin is casein coagulase obtained from cow stomachs
rennin causes the milk proteins to clump up forming the curds
the curds are then pressed into shape and more whey removed
soft cheeses are high in water (80%), while hard cheeses are low in water (<40%)
if too much water is added during the processing, the finished product may have to be called "processed food" rather than "cheese"
unripened cheeses fermented in a single step
ripened cheeses are fermented more than once
the main players in cheese fermentation are Streptococci and Lactobacilli
unripe soft cheeses include cottage and cream cheese
soft ripened cheeses include limburger and brie
for some cheeses fungi are added to the fermentation during aging or ripening of the cheese
white spore fungi are inoculated onto the surface of brie to give brie its characteristic crust
the semi-soft ripened cheeses like blue-cheese are inoculated with Penicillium spores
the Penicillium strains used are the blue spore fungi giving the characteristic blue veins and adding flavor
hard cheeses like Colby and cheddar are again made by fermentation with Streptococci and Lactobacilli, as are the very hard cheeses like parmesan
the aging process for hard cheese can exceed a year
Swiss cheese
two different fermentive processes take place in Swiss cheese manufacture
first is the standard lactate fermentation of Streptococci and Lactobacilli
towards the end of the ripening process the cheese is inoculated with Propionibacterium
what ensues is propionic acid (propionate) fermentation
propionic acid fermentation is the conversion of pyruvate to propionate
this process is analogous to lactic acid fermentation
conversion of pyruvate to propionate is a reductive reaction
reducing equivalents (NADH) generated from glycolysis (glucose to pyruvate) are reoxidized during propionic acid fermentation
the reoxidizing of reducing equivalents is critical to anaerobic metabolism that relies on glycolysis for ATP and do not have oxygen to oxidize the NADH back to NAD
it is interesting to note that lactate can be oxidized to pyruvate and then this pyruvate can be reduced to propionate
pyruvate can have other fates as well
through a series of reactions, pyruvate can be converted to acetic acid (vinegar) with the production of ATP
the combination of these reactions and their products give Swiss cheese its characteristic flavors
the late burst of respiration also produces carbon dioxide
this gas gets trapped in the cheese and gives Swiss cheese its characteristic holes (cavities)
Finally, PCR
PCR - polymerase chain reaction
uses special DNA polymerase to amplify specific pieces of DNA
short DNA oligonucleotides are used as primers for the polymerase
the sequence specificity of the primers determines what piece of DNA will be copied by the polymerase
many cycles of denaturation, primer annealing, and primer extension are performed
the amount of DNA amplified increase exponentially
even a single copy of a gene of interest can be made into billions of copies
this is a way to produce enough DNA of interest for further study or testing
PCR is used in many medical tests and in forensic science
