Claire Feiertag
Takehome #9
Friday, December 6, 2013
I happen to have a mixture of German and Irish background, and I chose to
talk about the microbiology of beer. Beers are made from corn, wheat, rice, and
barley but they are classified by the type of yeast that are used in their production.
Barley is the source of the fermentable sugars in modern beer. Malt is formed by
germinating the barley seeds. The plant cells convert starch in the endosperm to
fermentable sugars. Plant proteases reduce the total protein and make amino acids
available for the yeast. The malt is then dried and roasted. The alcohol in beer is
produced by yeast fermentation. When the oxygen is depleted, the yeast will switch
to fermentation and produce ethyl alcohol. Carbonation is added to beer by
bubbling in CO2 or by a secondary fermentation in which sugar or unfermented
‘wort’ is added to the bottled beer. Commercial beer is aged several weeks and
pasteurized in the bottle.
Some beers are filtered rather than pasteurized to remove contaminating
microbes. Off-tastes and odors are produced most often by wild yeast and lactic acid
bacteria (Lactobacillus and Pediococcus). Micrococcus kristinae is the only aerobic
bacterium reported in beer spoilage. During the 1990s, gram-negative strictly
anaerobic bacteria including Pectinatus, Selenomonas lacticifex, Zymophilus, and
Megasphaera were isolated from spoiled beer. Some yeast strains produce
acetaldehyde which gives a rotten-apple taste to beer. Diacetyl, dimethyl sulfide, cis3-hexanal, and organic acids are the most frequent products of contaminating
bacteria.