Brewing is a fermentative process which converts carbohydrate rich raw materials into beer using yeast catalysts such as Baker’s yeast and species of Saccharomyces. Water quality and aeration conditions influence beer quality to a greater extent. Of these two, aeration plays a vital role.
Studies revealed that yeast cells propagation was doubled when cultured under prolonged aerobic conditions as compared to discontinuous aeration. Moreover, increase in cell growth was proportional to aeration volume, mainly due to the synthesis of sterols and unsaturated fatty acids which are important elements of the yeast cell membrane.
Under batch fermentation, aeration affected 1st fermentation; nevertheless, successive fermentation was yeast strain dependent. Chul (2002) observed significant differences in levels of aroma compounds obtained with yeast propagated under different aerobic conditions. However aerobic conditions were not found to affect the levels of diacetyl. According to Chul et al (2007), continuous aeration during yeast propagation directly influenced yeast metabolism, fermentation ability and beer quality.
Microbial contamination risks: Certain groups of micro-organisms pose contamination risk in brewing.
Most hazardous beer spoilage microbial agents are the species of Lactobacilli, Pedicocci and Micrococcus kristinae. Lactobacilli are gram-positive, facultative, anaerobic bacteria. Some species of Lactobacilli which are the common beer spoilage organisms are said to operate using homofermentative metabolism, by lowering the pH of fermentation process by producing lactic acid.
These species are generally resistant to hop compounds. Lactobacillus brevis has the ability to ferment dextrins and starch and its contaminations in beer cause turbidity and super-attenuation (Vaughan et al.
2005). Pediococci are also homofermentative bacteria, which normally grow in pairs or tetrads, and are found in finished beers and wort fermentations.
Presence of Pediococci leaves behind fermentable sugars in beer even after fermentation, thus causing ropiness. Micrococcus kristinae is an aerobic, gram-positive coccus occurring in tetrads or groups of four, forming pale cream to pale orange colonies. The primary habitat is the mammalian skin (Microbial glossary).
Most of the strains are non-pathogens but some tend to be opportunistic pathogens. These species can grow in beer with significantly lower levels of alcohol and hop compounds and at pH values above 4.5 (Vaughan et al. 2005) and produce a fruity atypical aroma.
Gram negative anaerobic bacteria such as Pectinatus cerevisiiphilus, Pectinatus frisingensis, Selenomonas lacticifex, Zymophilus raffinosivorans and Zymomonas mobilis are also potential beer spoilage microbes. Pectinatus species have found to greatly affect non-pasteurized beers rather than pasteurized ones. These anaerobic organisms ideally grow in packaged products between 15 and 40°C with an optimum at 32°C and at a pH of 4.5 (Vaughan et al. 2005).
They are found to produce significant amounts of acetic acid, propionic acid and acetoin in wort and packaged beers. They also produce hydrogen sulphide and methyl mercaptan and cause considerable turbidity and an unpleasant ‘rotten egg’ odor in beer. Zymomonas mobilis is an organism widely used in the bio-ethanol synthesis. However, this species contaminates beer when it produces significantly higher levels of acetaldehyde and hydrogen sulphide.
Gram negative aerobic bacteria such as Hafnia protea and Enterobacter cloacae also other agents which cause beer spoilage and are capable of surviving beer fermentation. Certain species of Saccharomyces are also organism causing beer contamination.