Freeze-dried immobilized kefir culture in cider-making.

Background: The aim of the present study was to evaluate the fermentation efficiency of freeze-dried immobilized kefir culture on natural supports (apple pieces, delignified cellulosic material) in cider making at various temperatures (5-45 °C) in comparison with freeze-dried free cells. Freeze-dried cells were initially tested in apple juice fermentations at 30 °C, and then the freeze-dried cultures produced with no cryoprotectants were assessed in repeated batch fermentations.

Results: Repeated batch fermentations lasted for longer than 5 months. High malic acid conversion rates (up to 78.5%) and ethanol productivity values (up to 37.9 g L day ) were recorded for freeze-dried immobilized cells. Polymerase chain reaction - denaturing gradient gel electrophoresis (PCR-DGGE) analysis showed that freeze-drying had no effect on the microbial diversity of kefir culture. Higher alcohols were significantly reduced at low fermentation temperatures. Application of principal component analysis (PCA) revealed that both the fermentation temperature and the nature of the freeze-dried kefir culture affected significantly the minor volatiles determined by gas chromatography/mass spectrometry (GC/MS). Notably, all ciders produced were of high quality and were accepted by the tasting panel.

Conclusions: Freeze-dried immobilized kefir culture on natural supports with no cryoprotectants was found to be suitable for simultaneous alcoholic and malolactic cider fermentation at various temperatures (5-45 °C). The high operational stability of the systems was confirmed and the results obtained are of great interest for the industrial sector as they could be exploited for cider, low-alcohol cider, or 'soft' cider production. © 2020 Society of Chemical Industry.