Lecture 26 Outline
Why does bread rise and why is it so good in San Francisco? (bread/sourdough); Beer, wine and spirits (yeasts, ethanol fermentation)
Sourdoughs
the fermentation step of sourdough breadmaking includes, in addition to the yeast, lactic acid bacteria
Lactobacilli ferment sugars to lactic acid and this sours the dough
starter cultures of the best strains of yeasts and bacilli for sourdough are closely guarded by bakers
rye breadmaking also involves lactic acid bacteria
various Lactobacilli, Leuconostoc, and Streptococci
Ethanol fermentation
ethanol fermentation in yeasts is analogous to lactic acid and propionic acid fermentation
glucose is converted to pyruvate by a series of enzymes called the glycolytic pathway
glycolysis produces ATP, the energy source in cells that drives enzymatic reactions forward
ATP is required to maintain living processes
during glycolysis the cofactor NAD is reduced to NADH
pyruvate generated during glycolysis is fermented to ethanol in two steps
first pyruvate(a three-carbon compound) is converted to acetaldehyde (two carbons) and carbon dioxide
then the acetaldehyde is reduced to ethanol
both of these are reductive reactions that use the reducing power of NADH
this reoxidizes the NADH to NAD
NAD is required for glycolysis
fermentation is a way to reoxidize NADH to NAD in the absence of oxygen
Beer, Ale, and Distilled spirits
like breadmaking, brewing beer and spirits begin with an enzymatic phase
in brewing beer, barley seeds are allowed to germinate and are then crushed (gristed)
germination of seeds causes the production of enzymes that breakdown the storage polymers of the seed
amylases degrade starches and proteases digest proteins
the gristed product is called malted barley
this is mixed with other grains like rye or wheat producing a mash
during the mashing process, the released barley amylases converts starches to simple sugars
the mixture is then settled and filtered producing the liquid wort
at this point purified amylases can be added to degrade dextrins an making the beer "lite"
the wort is cooked with hops adding flavor and antibacterial compounds
cooking also kills the plant enzymes
after cooking, the wort is fermented
the yeast can be added (pitched) or wild
Saccharomyces carlsbergensis is a beer brewing yeast used for lagers
during settling, this yeast settles to the bottom
Saccharomyces cerevisiae is a top-fermenting yeast used for many ales
these yeast ferment the sugars released from starches into ethanol and carbon dioxide
other fermentation products made in small quantities give the beverage flavor
after the fermentation step the liquid is called green beer
the beer is then aged by storage in casks in prescribed environments
during the aging process, some fermentation continues
this and the precipitation of additional yeasts and particles cause the beer to "mellow"
after aging, the beer is filtered and bottled
at the time of bottling, beer is often carbonated to make it effervescent
some beers are allowed to ferment in the bottle (often with a little added sugar) to produce carbon dioxide bubbles
the making of distilled spirits are an extension of beermaking
the plant starting materials may differ but the series of malting, mashing, and fermenting the wort are the same
the ethanol produced during fermentation is then distilled and collected
in distillation, the liquid is heated above the boiling point of ethanol and below the boiling point of water
along with the ethanol other alcohols and volatile organic compounds are also distilled and flavor the spirit
the liquor is then aged in oak casks often for years
the wooden casks give the final product its rich brown color
wines
winemaking is a process analogous to the two previous processes of breadmaking and beermaking
grapes or other fruits are crushed to form a must
sugars in the must are fermented by added or wild Saccharomyces
again the yeast ferments glucose through pyruvate to ethanol and carbon dioxide
the wine is aged to reduce acidity and mellow the product
dry wines are fermented until no sugar remains
when stored in an anaerobic environment with a moist cork the wine is palatable for many years
if air enters the bottle through a dry cork, unfavorable aerobic reactions occur
the ethanol in the wine is oxidized to acetic acid (vinegar) by Acetobacter
commercial vinegar production follows this scheme intentionally
