Lactobacillus casei cells were immobilized on fruit (apple and quince) pieces separately and the immobilized biocatalysts were used for 15 successive fermentation batches of whey immediately after their preparation and for fermentations of milk immediately after their preparation and after a long storage period at low temperature. The immobilized biocatalysts used for successive fermentation batches of whey proved to be very effective and suitable for food grade lactic acid production, while no loss of activity at all temperatures (30, 37, 45 degreesC) tested was observed, as very high lactose conversion was reported, Cell immobilization of L. casei on apple and quince pieces was also shown by electron microscopy. In addition, apple and quince supported biocatalysts were used for milk fermentation immediately after their preparation, and after storage for 15, 98, and 129 days at 4 degreesC. No infection was reported during the storage periods. After storage, the immobilized biocatalysts were reactivated very quickly and produced milk fermentation products with a fruity, distinctive aroma which remained during all storage period. (C) 2004 Elsevier Ltd. All rights reserved.