Effect of Rhizopus nigricans (Rhizopus stolonifera)-based novel starter culture on quality and safety attributes of doenjang, a traditional Korean soybean fermented food product

Shruti Shukla,Hae-Kyong Park,Jong-Kyu Kim,Ashutosh Bahuguna,Myunghee Kim

doi:10.1038/s41598-019-57382-y

Shruti Shukla, Hae-Kyong Park + Show 3 more

Open Access

https://doi.org/10.1038/s41598-019-57382-y

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Abstract

We aimed to develop a consortium of starter culture of effective microorganisms to prepare doenjang, a traditional Korean fermented food. Different ratios of Bacillus subtilis TKSP 24 (B), Aspergillus oryzae complex (A), Rhizopus nigricans (also named as Rhizopus stolonifera) (R), and Mucor racemosus 15 (M15) were selected as meju starter cultures to produce doenjang with improved quality. Microbial strain combinations (B: A: R and B: M15: R) were mixed separately at three different ratios [1:1:1 (w/w), 1:0.5:1.5 (w/w), and 1:1.5:0.5 (w/w)] to prepare BAR-1, BAR-2, BAR-3, BM15R-1, BM15R-2, and BM15R-3 doenjang samples. Quantitative analyses included free amino acids, free sugar, volatile and non-volatile organic acids, cellular antioxidant activity along with the presence of biogenic amines and aflatoxins, and microbial counts. Total free amino acids responsible for the sweet taste of doenjang were highest in BAR-2 (322.50 mg/100 g) and BM15R-3 (320.07 mg/100 g). Total volatile organic acid was highest in BAR-1 compared to other preparations. All doenjang samples had biogenic amines, especially histamine, below the toxicity level (500 mg/kg). Also, the aflatoxin and hazardous microbial count in the tested doenjang samples were below the level of toxicity. The findings suggest that use of multiple microbial strains in combination with R. nigricans as a starter culture could be a novel and effective approach to improve the nutrition and safety of fermented soybean food products of doenjang.

Highlights

Doenjang is a protein-rich traditional food product and flavouring ingredient in Korea that is produced by the fermentation of soybeans
The doenjang sample designated BAR was prepared by mixing different ratios of meju starter culture made up of B. subtilis, A. oryzae complex, and R. nigricans
While doenjang samples designated as BM15R were prepared by mixing meju starter culture comprised of B. subtilis, M. racemosus 15, and R. nigricans

Summary

Introduction

Doenjang is a protein-rich traditional food product and flavouring ingredient in Korea that is produced by the fermentation of soybeans. In the preparation of starter culture (meju) for doenjang, soybeans are fermented by Bacillus species in the early stage, followed by the secondary microflora of Aspergillus species such as Aspergillus oryzae and Aspergillus niger[1]. Safety concerns with doenjang include microbial contamination, formation of biogenic amines, and production of aflatoxins, which depend on various factors, such as raw materials, microbial flora, fermentation conditions, and salt content[9]. Several studies have been carried out to produce commercially modified traditional Korean doenjang[1]. No studies have so far been carried out on commercially modified meju prepared by the inoculation of a mixture of various starter cultures, especially Rhizopus strains. The cellular antioxidant activity of the selected doenjang sample was carried out to demonstrate its potential against oxidative stress induced by various reactive oxygen species (ROS)

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