Influence of Soaking and Steaming Conditions on the Hardness of Steamed Soybeans and Fermented Soybeans (Natto)

Abstract

To evaluate the effect of processing conditions on the hardness of natto, physical properties of steamed soybeans and fermented soybeans (natto) were investigated by instrumental rheological analysis and sensory evaluation. The boron content in the elution of steamed soybeans was also determined as boron is a potential indicator of cell wall fracturing, which is induced by steaming and may affect hardness reduction. The elastic modulus (E) and the storage elastic modulus (E′) of soaked soybeans significantly decreased with increasing steaming temperature, independent of the soybean variety. The sensory-evaluated hardness negatively correlated with both E (-0.75) and E′ (-0.67). On the basis of this correlation, natto with E> 200 kPa or with E′>800kPa was determined to be ‘hard’. To prevent the production of ‘hard’ natto, the optimal processing conditions were determined to be as follows : steam soybeans containing more than 80% water content at a minimum steaming temperature of 121°C for at least 15min. Increases in water content and steaming temperature were accompanied by elevation in boron elution and therefore acceleration in the rate of cell-wall fracturing due to steaming. Except for steamed soybeans with E>300kPa, there was also a negative correlation between the elution level and E, confirming that boron content positively correlates with cell wall fracturing.