Effect of lactic acid bacteria fermentation on lipidic and phenolic profiles of tofu

Soy foods have long been a staple of the human diet in Asia and tofu may be the most popular food made of soy worldwide. Tofu is prepared from soymilk curd and it can also be further processed (grilled, frozen, or fermented) modifying taste, texture and end-uses compared to traditional tofu. Fermentation also prevent undesired microbial and chemical reactions improving the stability and the quality of the product. Moreover, in addition to be natural, nutritious and safe, epidemiological studies have shown that fermented soybean products exhibit healthy effects. In fact, soybean and its products contains biologically active compounds including isoflavones, essential fatty acids, sterols, phospholipids and tocopherols which may contribute to prevent chronic diseases such as hormone-dependent cancers, cardiovascular diseases and osteoporosis. Due to the influence of processing on the bioactive compounds content of soy food products, soybean, traditional tofu and tofu obtained by soymilk fermentation were studied by GC–FID and HPLC coupled with fluorimetric, light scattering and electrospray ionization mass spectrometry detector, in order to analyse their lipidic profile (fatty acids, tocopherols and sterols) and isoflavones content and composition. In particular, for isoflavones analysis, a new method was set up. Fermentation was performed using specific strains of lactic acid bacteria (LAB), such as Lactobacillus casei and Lactobacillus acidophilus. Fatty acids profile of soybean and tofu samples were very similar, even in term of trans fatty acids and omega-3 fatty acids, so fermentation cannot differentiate tofu from soybean by fatty acid composition. The most abundant fatty acid was linoleic acid, followed by oleic acid, palmitic acid and α-linolenic acid, as reported by other authors. The processing conditions did not significantly affect neither tocopherol composition of tofu (traditional and fermented) compared to soybean and in all products the main tocopherol was γ-tocopherol followed by δ-tocopherol and α- tocopherol. As concerning phytosterol content, ß-sitosterol was the main sterol found with the phytosterol composition that was similar in soybean and traditional and fermented tofu. The main effects of fermentation were on isoflavone profile: in fact, significantly differences were seen in fermented tofu isoflavones content compared to soybean and to traditional tofu. In particular, fifteen isoflavones were identified as aglycones and glycosides: the aglycone forms were significantly higher in fermented tofu compared to the traditional one. These results suggest that fermentation by LAB cannot discriminate the fatty acid, tocopherol and phytosterol composition of fermented tofu from the traditional one and from soybean, but it plays an essential role not only on shelf-life extend, but also in the increase of the content of aglycone isoflavones with important biologically implications on human health.