Quantitative physiology and aroma formation of a dairy Lactococcus lactis at near-zero growth rates

During food fermentation processes like cheese ripening, lactic acid bacteria (LAB) encounter long periods of nutrient limitation leading to slow growth. Particular LAB survive these periods while still contributing to flavour formation in the fermented product. In this study the dairy Lactococcus lactis biovar diacetylactis FM03-V1 is grown in retentostat cultures to study its physiology and aroma formation capacity at near-zero growth rates. During the cultivations, the growth rate decreased from 0.025 h^-1 to less than 0.001 h^-1 in 37 days, while the viability remained above 80%. The maintenance coefficient of this dairy strain decreased by a factor 7 at near-zero growth rates compared to high growth rates (from 2.43 ± 0.35 to 0.36 ± 0.03 mmol ATP.gDW^-1.h^-1). In the retentostat cultures, 62 different volatile organic compounds were identified by HS SPME GC-MS. Changes in aroma profile resembled some of the biochemical changes occurring during cheese ripening and reflected amino acid catabolism, metabolism of fatty acids and conversion of acetoin into 2-butanone. Analysis of complete and cell-free samples of the retentostat cultures showed that particular lipophilic compounds, mainly long-chain alcohols, aldehydes and esters, accumulated in the cells, most likely in the cell membranes. In conclusion, retentostat cultivation offers a unique tool to study aroma formation by lactic acid bacteria under industrially relevant growth conditions.