Aroma defect, beer

The taste and odor profile of a beer, including possible aroma defects, can be described in detail with the help of 44 terms grouped into 14 general terms, as shown in Fig. 20.3. Apart from a great variety of terms for odor notes, the terms bitter, salty, metallic, and alkaline are used only for taste and the terms sour, sweet, body etc. are applied to both taste as well as odor. 

Esters. Esters are extremely important aroma compounds and there are many reports that esters are biosynthetic products of bacterial action. Thus, the fruity flavor defect sometimes found in cheddar cheese is due to the presence of esters, principally ethyl butyrate and ethyl caproate (25). Similar esters can be found in beer in which both fusel alcohols and the short chain fatty acids, acetic and butyric, are also present. These materials can undergo esterification, which in this case is mediated by the enzyme alcohol acetyltransferase present in the yeast used for beer fermentation (26). There are a number of esters present in wine which are metabolically produced by the yeast. Of these,... 

The most important off-flavour and aroma associated with the lactic acid bacteria is the sweet, butterscotch or honey note provided by diacetyl and related vicinal diketones. It can be discerned readily in lager beers at concentrations as low as 0 5 (xg/ml. The defect was formerly called Sarcina sickness after Sarcina, the outdated generic name for brewery gram-positive bacteria. 

Besides the yeast conversion of phenolic adds into flavour-active volatile phenols that may have an adverse impact on beer aroma as discussed above, there is an interest in the contribution of phenolic acids to beer antioxidant activity (Piazzon et al 2010). Indeed, the use of hydrolytic enzymes (e.g. esterases) to release phenolic adds during mashing has been reported (Szwajgier, 2011). It must be cautioned that boosting the level of phenohc acids snch as fendic acid in the wort may inaease the content of volatile phenols in the finished product, and a flavour defect may ensue. 

Citrate is present in milk, fruit, and vegetables. It can be co-metabolized with sugars by citrateutilizing LAB. Citrate utilization results in an excess of pyruvate, which is thus converted to diacetyl (2,3-butanedione), acetoin (2-hydroxy-3-butanone), and 2,3-butanediol to equilibrate the redox balance of cellular metabolism (Collins 1972 Bartowsky and Henschke 2004). Some LAB can also synthesize 2,3-pentanedione from pyruvate and threonine (Ott et al. 2000). Diacetyl and 2,3-pentanedione are associated with a buttery aroma, which positively contributes to the flavor of a range of fermented dairy products such as butter (MalUa et al. 2008), yogurt (Routray and Mishra 2011), and cheese (Curioni and Bosset 2002). Diacetyl also contributes to wine style, while it is responsible for flavor defects in beer. Diacetyl is widely produced by LAB, including species of the Lactococcus, Streptococcus, Leuconostoc, Lactobacillus, Pediococcus, and Oenococcus genera. 

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