Introduction to lactic acid bacteria in beer: ‘The good, the bad and the ugly’
One only has to perform a cursory literature search of lactic acid bacteria (LAB) to be overwhelmed by available information extolling their industrial importance for fields ranging from human health to food production. Ultimately, LAB are a collection of Gram-positive, catalase-negative, non-sporu- lating, non-motile and acid-tolerant organisms that share the capacity to produce lactic acid as a primary product of sugar fermentation while being incredibly heterogeneous in terms of physiological attributes, metabolic and fermentation capabilities, and ability to inhabit diverse niches (Pfeiler and Klaenhammer, 2007). These diverse attributes greatly increase both their presence and utility in the production of multiple food and beverage products, from wine and beer to cheese, dairy, meat and vegetable products (Makarova et al., 2006). Further, members of the LAB group that naturally occupy food and beverage niches have been ascribed the generally regarded as safe (GRAS) designation, allowing them to be exploited for improvement and preservation of a wide range of food and beverage products, and for the production of probiotics (Klaenhammer et al., 2005). Unfortunately, the unwanted presence or uncontrolled over-growth of these organisms during food or beverage production can occur, posing challenges, as well as opportunities, especially to the brewing industry.
Beer is an unexpected environment to support microbial growth given that beer-spoiling bacteria must simultaneously overcome several physiological hurdles, including the antimicrobial action of ethanol and hop-derived bitter acids, low pH, limited available nutrients, and low O2, with concurrent high CO2 levels (Fernandez and Simpson, 1993; Sakamoto and Konings, 2003; Simpson, 1993b). Nonetheless, LAB probably have always been associated with and/or involved in the production of beer, either as a naturally occurring agent for traditional spontaneous-fermentation styles such as lambic or Weisse beers, producing characteristic ‘sour' flavours via production of lactic or acetic acid (Tonsmeire, 2014) (see Chapter 7), or as an unseen source of spoilage that results in an undesired or poor quality product. Though the participation of LAB in beer-production and spoilage was not appreciated until Louis Pasteur began to isolate these bacterial cells from beer (Suzuki, 2011), their diverse historical role(s) in brewing allows for the general characterization of ‘the good, the bad, and the ugly' outcomes of LAB involvement.
LAB isolates can indeed be helpful, if not necessary components, of specialized fermentations that produce specific beer styles, both traditional and new (Tonsmeire, 2014). This is increasingly important to consider in discussions of beer spoilage-related (BSR) LAB, as the current global beer market is experiencing a significant expansion in the numbers of ‘local' craft beers. The influx of new companies necessitates that breweries distinguish themselves with consumers through unique products and this need has led to innovative use of raw materials and production processes, with the inclusion of both fermenting LAB and non-traditional yeasts. These ‘helpful' fermenting LAB, however, must have several important attributes, chief among them being the inability to overgrow in beer and to not inhibit normal yeast function. Thus, current industry trends highlight an important context in which to consider brewing-associated LAB, and open up interesting avenues for how best to investigate and further the role of LAB in the spectrum of modern beer production.
Putting aside the expansion in modern brew styles, the fact remains that since the industrial revolution, global brewing practices increasingly have focused on producing ‘clean' and consistent brew products, free of bacteria and their metabolites (Tonsmeire, 2014). With the advent of pasteurization and appreciation for hygienic practices during food and beverage production, the average global beer consumer today is probably accustomed to ‘conventional' or non-sour products, beers that are free from characteristic signs of LAB overgrowth. This means the beer should have no cloudy ‘haze', no ‘sour' taste or other unappealing off-flavours such as ‘buttery' diacetyl, and be free of bacterial sedimentation or exopolysaccharide ‘slime' (Back,
2005). Such occurrences in most beer products are unexpected, and encapsulate both the bad and the ugly results of unwanted LAB being present. The outcome of these spoilage events causes a loss of consumer and brand confidence when compromised beer is consumed, or significant revenue and time loss to the brewery in the event of batch contamination. As LAB are attributed with causing 60 - 90% of the brewing spoilage events worldwide (Asano et al., 2009; Back, 1994), significant interest has gone into ascertaining how they spoil beer and how this is best controlled. Despite this interest, incidence of BSR LAB contamination remains difficult to delineate due to under-appreciation of how diverse a group they are, even though relevant research constantly highlights this diversity.