First, what are the factors which determine the level and range of bacterial presence in a distillery?There are a number of elements determining the levels of bacterial presence in a distillery including the environment within the distillery’s washbacks, whether they are wooden or stainless steel; the levels of hygiene and cleaning regimes and mash tuns, as well as the length of fermentation. Meanwhile, malted barley is where it all begins.
“Malted barley has a mix of microbes on its surface when it arrives at the distillery, including various species of lactobacilli and wild yeast.
“That source of bacteria can be influenced by various factors, including harvest conditions,” says Dennis Watson of Chivas Bros technical and scientific director.
The next stage, milling, doesn’t affect bacteria, but mashing certainly does. During mashing the grist (ie milled barley) and its accompanying bacteria are subjected to three (or even four) waters at progressively higher temperatures culminating at 85-90 degrees centigrade.
“Some lactic acid bacteria are more heat-resistant than others, some will be killed by the heat of mashing. If the first water is around 63 degrees centigrade in the mash tun then a number of lactic bacteria will survive, while some will be stunned or killed. The second water will kill more, and the third water will definitely kill more bacteria, but the minute the wort begins to cool down the surviving bacteria will begin to reproduce,” says John Ross, technical manager for William Grant & Sons.
The cleaning regime of the mash tuns is another vital factor affecting the bacterial population.
“If the mash tun is not cleaned very often there can be a significant level of bacteria including lactobacillus on the walls, and underneath the false bottom of the mash tun. Similarly, if the pipe conducting wort to the washback is not cleaned often enough there will be micro-organisms, particularly lactobacillus and wild yeast adding to the bacterial population from the malt,” says Dennis Watson.
Needless to say, the cleaning regime also determines the level of bacteria in the washbacks, with wood harder to clean than stainless steel. One reason for this is that stainless steel provides an even surface, whereas crevices in wooden washbacks provide havens for bacteria.
But however rigorous the cleaning regimes, some bacteria is present in the wort, and when yeast is added to begin fermentation, the bacteria and yeast compete with each other for the sugars and nutrients. The yeast wins this first round, subsequently entering an exponential (ie key growth) phase, before winding down during the stationary (ie little or no growth) phase when the wort becomes an increasingly adverse environment for the yeast. This is due to a lack of nutrients, which means the yeast stops reproducing. Additionally, yeast can’t cope with the growing concentration of alcohol, not to mention the rising temperature of the wort.
Consequently, yeast cells start to die off and autolyse (ie the cell walls rupture), which releases various nutrients from the yeast cells into the wort. This is a vital turning point for the bacteria, as these nutrients provide it with a food source. Wort has a wide assortment of bacteria, and the strains most suited to conditions now begin to grow, while other strains remain inactive. Lactobacillus is the key strain of bacteria which grows is this process, while also producing lactic and acetic acid.
The extent to which lactobacillus grows, and consequently how acidic the wort becomes, depends on how long fermentation is allowed to continue after the yeast has started to autolyse.
Ross explains: “Once the alcohol fermentation is complete the lactic acid bacteria starts to kick off. A shorter fermentation, 50 hours or so, means the bacterial population hasn’t had a chance to reach its maximum level, so the level of acidity will be less. A longer fermentation means a higher bacterial population forming, and so more acid being produced.”
“Different types of lactic acid bacteria are produced during fermentation, especially after the autolysis of the yeast,” adds Watson. “The first, (which we call heterofermentative) grow throughout the process and produce lactic and acetic acid, while the second (known as homofermentative) occur during longer ferments and only produce lactic acid.”
What does the presence of lactic acid and acetic acid in the wash actually mean? Rather than creating flavours in their own right, lactic acid and acetic acid are more significant for the reactions they promote, which results in a higher level of esters (fruity notes).
Watson adds, “An increase in acetic acid can react to increase the fruitiness of the spirit. Additionally, some bacteria modify other compounds which the yeast produces, to provide some volatile phenolic type compounds, even in unpeated malts, though this is a subtle influence compared to the effect of peated malt.”
As with every detail of the production process, how desirable the influence of lactobacillus is, and how acidic the wash should be, depends on the profile of new make spirit a distillery wants to produce. At least the option of utilising a greater, or lesser influence from lactobacillus is always there.
It’s just a question of the time scale allowed for fermentation.