Jerusalem artichoke (JA), rich in inulin consisting of β-2,1 linkages, is well known for their excellent properties in blood sugar regulation, antioxidant activity, weight management, and prebiotic effects. However, due to their high moisture content and susceptibility to spoilage, JA is challenging to store for extended periods. In addition, JA possesses unpleasant odor and taste, making it more suitable for use in various processed products or as additives rather than consumed raw JA. In this study, we aimed to develop the production process for enhanced funtional yakju with addition of JA to expand the utilization of JA as a funtional food ingredient. However, since Saccharomyces cerevisiae, most common alcohol fermenting yeast, cannot utilize inulin, further researches on various processing method to degrade inulin during the fermentation are required. In order for S. cerevisiae to utilize inulin, it requires the presence of the inulinase enzyme. Some non-Saccharomyces yeasts secrete inulinase, enabling the breakdown of inulin into fructose during mix-fermentation with S. cerevisiae during the alcohol fermentation. Moreover, non-Saccharomyces yeasts can contribute to the production of various volatile aromatic compounds during the early stage of fermentation, thereby enhancing the quality of yakju. In general, steaming rice is required for better saccharification in yakju making process, but this process can make heat flavor and lead to quality deterioration. Hence, studying the influence of JA on the quality of yakju is essential. Furthermore, comparing the effects of microbial-derived inulinase with commercial inulinase on the sensory quality and functionality of yakju is crucial for the development of high-quality yakju producing processes.
In this study, three main experiments were conducted to develop high-quality yakju supplemented with JA. Firstly, we observed the quality changes of the yakju depending on the effect of steam-treatment and amount of added JA. The results showed that as the amount of JA increased, the residual reducing sugars increased, and alcohol contents were increased when JA was steamed before adding to yakju. However, when 10% unsteam-treated JA was added to yakju, it resulted in the most favorable sensory evaluation among all the yakju with JA.
Secondly, we isolated an inulin-degrading strain from nuruk and identified Wickerhamomyces anomalus JS01 as the strain with the highest inulinase enzyme activity. Subsequently, we conducted mix-fermentation with S. cerevisiae KMBL7001 and W. anomalus JS01 for yakju making, followed by analysis of fermentation characteristics and sensory properties. As a result, inulin was more degraded in mix-fermented yakju compared to control yakju, whereas total amino acid content was increased by 20% in mix-fermented yakju compared to control yakju. Sensory evaluation also indicated a higher preference for the mix-fermented yakju. Therefore, it was concluded that the mix-fermentation with S. cerevisiae and inulin-degrading non-Saccharomyces yeasts can contribute to the improvement of yakju when JA is added.
Lastly, we investigated the impact of microbial-derived inulinase on the functionality and quality of yakju compared to commercial inulinase enzyme, in order to confirm the effect of improving the quality of yakju. Since non-Saccharomyces yeasts hardly grow after the initial fermentation stage, inulin degradation is incomplete. To overcome this disadvantage, we used engineered S. cerevisiae contatining inulinase genes to produce yakju. We compared the quality characteristics and functionality of yakju with commercial inulinase enzyme to that produced using microbial-derived inulinase. When using endo inulinase, both microbial-derived inulinase and commercial inulinase enzyme treated yakju samples showed a significant decrease in total amino acid content, and all the yakju treated with inulinase, regardless of endo or exo inulinase, exhibited a substantial increase in volatile ester compound contents. In terms of antioxidant activity, yakju treated with the commercial inulinase enzyme showed the highest DPPH and FRAP activities. However, yakju samples treated with microbial-derived inulinase also exhibited higher antioxidant activity compared to yakju not added with JA. Consequently, the addition of JA for yakju making can enhance its functionality. Furthemore, the increase in volatile ester compound content due to inulinase treatment is expected to contribute to the sensory quality, ultimately resulting in the production of high-quality of yakju.