Abstract
Tempeh is a common food in Indonesia, produced by fungal fermentation of soybeans using Rhizopus sp., as well as Aspergillus oryzae, for inoculation. Analogously, for economic reasons, mixtures of maize and soybeans are used for the production of so-called tempeh-like products. For maize, a contamination with the mycoestrogen zearalenone (ZEN) has been frequently reported. ZEN is a mycotoxin which is known to be metabolized by Rhizopus and Aspergillus species. Consequently, this study focused on the ZEN transformation during tempeh fermentation. Five fungal strains of the genera Rhizopus and Aspergillus, isolated from fresh Indonesian tempeh and authentic Indonesian inocula, were utilized for tempeh manufacturing from a maize/soybean mixture (30:70) at laboratory-scale. Furthermore, comparable tempeh-like products obtained from Indonesian markets were analyzed. Results from the HPLC-MS/MS analyses show that ZEN is intensely transformed into its metabolites α-zearalenol (α-ZEL), ZEN-14-sulfate, α-ZEL-sulfate, ZEN-14-glucoside, and ZEN-16-glucoside in tempeh production. α-ZEL, being significantly more toxic than ZEN, was the main metabolite in most of the Rhizopus incubations, while in Aspergillus oryzae fermentations ZEN-14-sulfate was predominantly formed. Additionally, two of the 14 authentic samples were contaminated with ZEN, α-ZEL and ZEN-14-sulfate, and in two further samples, ZEN and α-ZEL, were determined. Consequently, tempeh fermentation of ZEN-contaminated maize/soybean mixture may lead to toxification of the food item by formation of the reductive ZEN metabolite, α-ZEL, under model as well as authentic conditions.
Highlights
Tempeh is a very common food in Indonesia
Results from the HPLC-MS/MS analyses show that ZEN is intensely transformed into its metabolites α-zearalenol (α-ZEL), ZEN-14-sulfate, α-ZEL-sulfate, ZEN-14-glucoside, and ZEN-16-glucoside in tempeh production. α-ZEL, being significantly more toxic than ZEN, was the main metabolite in most of the Rhizopus incubations, while in Aspergillus oryzae fermentations
Whilst the initial soybeans were free of mycotoxins and in the maize ZEN and relatively low amounts of ZEN-14-S were present, the analyses showed for all fermented samples higher amounts and a diverse pattern of conjugates and reductive metabolites
Summary
Tempeh is a very common food in Indonesia. It is made of soybeans, which are fermented by molds and pressed into a compact cake. Tempeh manufacturers in Indonesia do not use specific fungi, but inocula, which consist of a non-specified fungal mixture. The fungal fermentation leads to an enhancement of nutritional value and digestibility, because free amino acids and other water-soluble solids are increased [3,4]. In Europe, the USA, and other industrialized countries, the interest for tempeh is increasing, resulting from a growing interest in health, nutrition, and vegetarianism. As soybeans contain all of the eight (or ten) essential amino acids [5], tempeh is commonly used as a vegetarian source of protein. Triggered by the high nutrient value, a lot of studies focused on the use of tempeh and tempeh-like products with respect to malnutrition in Third World Countries [6,7,8]
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