The effect of lactic acid bacteria on congener composition and sensory characteristics of Scotch Malt Whisky

Abstract

Lactic acid bacteria (LAB) comprise a major part of the natural microflora of Scotch malt whisky fermentations, due to their tolerance of heat and elevated ethanol concentrations. In this study, their effects on the organoleptic properties of the spirit were investigated. Samples from late (>70 h) fermentations were obtained from whisky distilleries throughout Scotland. Bacteria of varying colony morphologies were isolated, purified, and characterised initially using random amplification of polymorphic DNA – polymerase chain reaction (RAPD-PCR). Isolates with differing RAPD patterns were retained and their ability to produce 10-hydroxystearic acid (10- HSA) from oleic acid was determined qualitatively using high performance thin layer chromatography. 10-HSA is the primary precursor of γ-dodecalactone, which is an important flavour compound in malt whisky responsible for the desirable “sweet and fatty” characteristic of the spirit. Thirty-nine isolates had strong or weak bioconversion activity while 89 isolates displayed negligible or no activity. Forty-two strains, largely from the former category were identified using partial 16S rRNA gene sequences. Lactobacillus paracasei was the predominant organism but L. brevis and L. plantarum were also identified. These 42 strains were assessed for their bioconversion capacity in a semi-quantitative manner using gas chromatography – mass spectrometry (GC-MS) and five isolates, comprising L. brevis, two strains of L. paracasei, and two strains of L. plantarum were selected for further study. These isolates were used in laboratory–scale, simulated whisky fermentations with Saccharomyces cerevisiae. Fermentation liquor (wash) was distilled to produce new-make spirit, which was analysed organoleptically by quantitative descriptive analysis. Spirit from fermentations inoculated with L. brevis had an enhanced “sweet” character, probably due to the higher γ-lactone levels detected in this whisky, as well as increased “sulfury” and “meaty” notes, most likely due to yeast autolysis. L. paracasei enhanced the “green/grassy” notes of new-make spirit, while also adding a “sour” aroma probably resulting from the elevated levels of lactic acid detected in the wash. Like L. paracasei, L. plantarum increased the “green/grassy” notes of new-make spirit. Further fermentations were carried out in which L. brevis, one strain of L. paracasei, and one strain of L. plantarum were inoculated into fermentations with yeast comprising 90% S. cerevisiae and 10% Torulaspora delbrueckii, which had been iii isolated previously from Scotch whisky fermentations and shown to enhance the concentration of γ-lactones in new-make spirit. Co-fermentation of L. brevis with S. cerevisiae and T. delbrueckii resulted in a spirit with increased “green/grassy”, “sweet”, and “oily” notes, with decreases in “sulfury” and “meaty” observed when the wild yeast was not present. Spirit derived from co-fermentations of L. paracasei and T. delbrueckii exhibited increased “soapy”, “sour”, and “sulfury” notes. Cofermentation of L. plantarum and T. delbrueckii caused increases in “green/grassy”, “soapy”, “sweet”, “sour”, and “sulfury” notes. Increased concentrations of γ-lactones were detected in new-make spirit distilled from fermentations inoculated with L. brevis, presumably contributing to the enhanced sweet character of this spirit. This effect was further amplified by the inclusion of T. delbrueckii in the laboratory–scale fermentations.