Utilization of cider by-products for microbial fermentation: optimization of low-cost culture media for mannitol production

This study explores the sustainable valorization of three cider industry by-products -apple pomace, cider lees and rotten apples - for developing low-cost culture media for heterofermentative lactic acid bacteria fermentation to produce mannitol . First, a medium derived from apple pomace, supplemented with yeast extract and manganese sulfate, favored the growth of the seed cultures of the bacteria used in this study, thus avoiding the use of commercial media. Second, rotten apple juice was used as the main carbon source for mannitol production due to its high fructose content, being supplemented with either yeast extract or cider lees as nitrogen sources. After 48h, the strain Fructobacillus fructosus CECT 7088 exhibited an average fructose conversion of 70 % in the presence of yeast extract, while the strain CECT 8046 displayed a higher fermentative capacity, even with cider lees. To further enhance productivity, cryo-concentration was applied to the rotten apple juice to standardize and increase initial sugar levels. Using Leuconostoc mesenteroides CECT 8146 with yeast extract, approximately 98 % of the fructose was transformed, yielding 102.8 ± 1.8 g/L of mannitol. In contrast, cider lees did not prove to be an efficient nitrogen source due to their low content of primary amino nitrogen, which limited fructose conversion to mannitol and resulted in reduced process efficiency. Overall, these findings highlight the potential of cider by-products (apple pomace and rotten apples) as substrates for microbial mannitol production, offering a circular approach to valorize currently unvalued products.