Evaluation of seed chemical quality traits and sensory properties of natto soybean
Introduction
Natto is a fermented soyfood mainly consumed in Japan (Taira et al., 1982). Natto is made by fermenting seeds with a bacterium, Bacillus subtilis (natto), which provides a unique flavour and stickiness (Watanabe, 2006). The natto market is relatively small compared to other soyfoods, such as tofu and soymilk. Also, the unfamiliar aroma, flavour, and texture of natto have kept it from becoming a popular international soyfood (Hosoi and Kiuchi, 2003, Zhang et al., 2008b). However, natto is very popular soyfood in Asia, especially Japan, which has a stable market that provides the US soybean growers very profitable opportunities.
Seed quality evaluation is essential to facilitate soybean breeding for natto production in the United States, where we have very limited information on seed chemical quality traits and sensory properties. Measuring protein and oil content is simple and easy for indirect natto selection (Geater, Fehr, Wilson, & Robyt, 2001). One of the important traits for the natto soybean is sugar content, but sugar analysis is time consuming and costly. Since protein and oil content have a strong negative correlation with total sugar (r = −0.81), the selection of genotypes with lower protein and oil content should produce sweeter natto (Geater & Fehr, 2000).
Total sugar and sucrose content advances the taste of soyfood. During the fermentation of natto, high total sugar and sucrose content help to produce natto with better flavour (Taira, 1990).
Negative correlations have been reported between glucose and sucrose, fructose and sucrose, and fructose and stachyose content (Hou, Chen, Gray, Giannoccaro, & Wang, 2006). On the other hand, total sugar content was positively correlated with sucrose, raffinose, and stachyose content. There was also a positive correlation between raffinose and stachyose and between glucose and fructose contents (Cicek, Chen, Saghai Maroof, & Buss, 2006). In addition, Geater and Fehr (2000) found that natto hardness was negatively correlated with the total sugar content.
Few studies have specifically reported an association of minerals, such as calcium, manganese, and boron with natto quality. However, it is important to examine if there is a correlation between these minerals, other soybean chemical quality traits, and natto quality. For example, calcium performs structural roles in the cell walls and membranes (White & Broadley, 2003); manganese is important to photosynthetic oxygen evolution in chloroplasts within most plants (Kuwabara & Murata, 1983); and boron maintains the integrity of cell walls. However, there is no information regarding the effect of these minerals on soyfood quality.
Fermentation by B. subtilis (natto) is a crucial step in natto production. B. subtilis (natto) is about 0.8 × 3 μm in size and can be found everywhere in nature, especially in soil and crop residue (Watanabe, 2006). During the fermentation by B. subtilis (natto), amylase and protease help to produce natto with a unique flavour, soft texture, and stickiness. The stringiness and stickiness of natto is caused by poly-glutamic acid and fructan during fermentation (Yoshioka, Sekine, & Otobe, 2007).
In addition, natto quality is affected by its shelf life, which is determined by changes in appearance, stickiness, flavour, and texture over time. As natto ages, the taste of natto will drastically deteriorate and white crystals, consisting of tyrosine and struvite, or ammonium magnesium phosphate (MgNH4PO4), will form on the surface, which is not accepted by consumers. Tyrosine is one of the components of the umami taste, but it does not adversely affect natto taste. Struvite, however, worsens the flavour of natto and dramatically reduces natto’s shelf life (Muramatsu, Yasui, Suzuki, & Kiuchi, 2000).
Although natto breeding has been conducted at several universities and food processing companies, there is still very little research on natto seed quality and sensory analysis. Sensory evaluation is necessary for food products due to its psychological factors rather than field or laboratory factors (Fuller, 2005). Although it is difficult and expensive to measure sensory properties among samples and use sensory panel, it is too vital to be omitted from crop breeding and food product development (Fuller, 2005).
The specific objectives of this research were: (1) to evaluate natto seed chemical quality traits including seed protein, oil, protein plus oil content, minerals (calcium, manganese, and boron), sugars (glucose, fructose, sucrose, raffinose, stachyose, and total sugar), and sensory traits of 15 natto soybean lines across locations and year, and (2) to determine the heritability and correlation of seed chemical quality traits and natto sensory properties.
Section snippets
Plant materials
The natto soybean genotypes evaluated included three superior natto varieties used in commercial natto production (ARK1, ARK2, and ARK3), nine moderate natto varieties with intermediate marketing value (MO8109, MO8750, R02-1983, R05-1953, R05-1989, R05-2206, R05-2629, R05-2734, and SS 516), and three unacceptable natto varieties determined by Japanese natto marketers (R04-245, R05-1298, and R05-1679). ARK1, ARK2, ARK3, and SS 516 are commercial natto cultivars. ARK1 was developed by Kaneko Seed
Genetic and environmental effect on seed chemical quality traits
Analysis of variance showed the significance on genotype × year × location on protein, oil, and protein plus oil content (Table 1). In a similar study, Geater, Fehr, and Wilson (2000) indicated that genotypic effects on seed protein, oil, and protein plus oil content were significant, while non-significant effects were observed in year, location, and genotype × location interactions at three Iowa locations during two years, which are in agreement with the results of the current study. Genotype
Conclusions
Seed chemical quality traits are essential to determine suitable natto varieties to produce high quality natto. Combined data analysis showed no significant year or location effect on variation in seed quality traits, but a significant genotype × year × location interaction effect on protein, protein plus oil, calcium, and manganese content. Results of the comparison of three natto quality types showed that superior natto types had lower protein, protein plus oil, calcium, manganese, and boron
Acknowledgements
We would like to give special thanks to Blue Horizon Inc., Cabot, AR, which provided most of the equipment, time, and space for our research. We would also like to thank Dr. David R. Walker, USDA-ARS, Urbana-Champaign, IL for his support in academic research and study.
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