Carbohydrate effects on the inducement of the arginine deiminase pathway enzymes in wine lactic acid bacteria : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Microbiology at Massey University
Characterised by a fermentative sugar metabolism resulting in lactic acid as a major end product, the lactic acid bacteria (LAB) may be isolated from a broad range of sources. Dairy products, fermented vegetables, meats and baking products such as sourdough bread involve these organisms in a consistent and intentional manner in present times, no matter how accidental or fortuitous their initial involvement may have been. Alcoholic beverages such as beer, cider and, most pertinently here, wine are also affected by the presence of particular LAB. As conditions differ between nutrient environments so do the LAB found in wine differ to those isolated elsewhere - being both ethanol tolerant to the degree where growth is capable in 10% v/v ethanol and aciduric, able to maintain an active presence at acidic levels as great as pH 4 or less. This ability to remain viable during the primary yeast fermentation of juice into wine leads to these LAB being of no small practical interest in the wine industry. The process of malolactic fermentation (MLF) involves the wine LAB altering the law materials present in the juice and wine further, increasing the intricacies of the winemaking and final product. Primarily encouraged due to its effect of reducing wine acidity, MLF also alters flavour and aroma in what is generally thought to be an advantageous manner when applied correctly. Another factor thought to be of significance is an increase in biological stability. Found, for example, among the lactobacilli, pediococci and leuconostocs, the wine LAB are classed as either homofermentative or heterofermentative. Homofermenters commonly produce two moles of lactic acid per mole of glucose fermented, while heterofermenters form one mole each of lactic acid and carbon dioxide and varied quantities of ethanol and acetic acid from one mole of glucose. Natural or chance occurrences of wine LAB, whether as part of the microbiological community on the raw materials or from other sources - such as inoculation from contaminated equipment - were the original manner in which these organisms were introduced into the vinification equation. With the predilection towards quality control, standardisation and safety in the present day, the use of pure microbial starter cultures to initiate MLF has become increasingly widespread. In order to optimise the manipulation of wine LAB in both the laboratory and industry a thorough insight into their physiology and metabolism is an obvious necessity. Certain areas of interest have undergone more intensive study than others, with, for example, the catabolism of carbohydrates in both wine (Davis et al., 1986) and model wine systems (Liu et al., 1995a) having had a considerable amount of research compared to less primary sources of energy such as nitrogen metabolism.