The total sugar content of soybean [Glycine max (L.) Merr.] cultivars is important to the soyfood industry. The method of total sugar analysis reported in the literature is labor intensive and time consuming. The objective of this study was to develop a more rapid method for the determination of total sugar in soybean seed. The original method was modified by reducing the amounts of reagents used, eliminating sample filtering, reducing the number of sample dilutions, and increasing the rate of acid hydrolysis. There was a significant difference between the original (187 g kg−1) and the rapid (224 g kg−1) method for the means of 10 samples with a broad range of total sugar, but no significant sample × method interaction was observed. The phenotypic correlation between the two methods for total sugar of the 10 samples was 0.91 (P < 0.05). The number of samples that could be analyzed in 1 d increased from 12 for the original method to 70 for the rapid method, and the estimated cost per sample was reduced from $5.34 to .92. The rapid method can be used to determine the relative differences among soybean samples for total sugar content.

Total sugar content of soybean [Glycine max (L.) Merr.] seeds is considered to be important for the quality of some soyfoods. The objective of this study was to determine the association of total sugar with other seed traits of soybean to facilitate the development of cultivars with appropriate seed composition. Total sugar, protein, oil, fiber, and seed size of 23 conventional and food-grade cultivars were evaluated in replicated tests at eight Iowa locations in 1998. Total sugar content was determined by acid hydrolysis, and protein, oil, and,fiber were measured by near-infrared reflectance (NIR). There were significant differences among the cultivars for all traits. Total sugar was most highly correlated (-0.81) with the sum of protein + oil (P + O). Seed size had a major influence on the association of total sugar with protein, oil, and fiber individually. Analysis of P + O by NIR was not influenced by seed size and should be a useful method of predicting differences among genotypes for total sugar content.

Breeding of soybean [Glycine max (L.) Merr.] cultivars suitable for natto involves selection for seed traits that influence the quality of the product. The objectives of this study were to evaluate the relationship of 11 seed traits to six natto quality traits and to assess the influence of genotypes and environment on the traits. Sixteen small‐seeded genotypes were grown in replicated tests at three Iowa locations during 2 yr. The natto traits evaluated for each plot were water absorption, water loss, hardness of steamed seeds and natto, and darkness of steamed seeds and natto. Seed traits were total sugar, free sugar, sucrose, raffinose, stachyose, protein, oil, fiber, protein + oil, protein + oil + fiber, and seed size. There were significant differences among genotypes during one or both years for all the traits, except fiber content. Significant differences between years or among locations were observed for all traits, except water loss after steaming and stachyose content. None of the seed traits was significantly correlated with darkness of the steamed seeds or natto. All of the seed traits, except stachyose, oil, and seed size, were significantly correlated with one or more of the other natto quality traits. Protein + oil was significantly correlated with natto quality and other seed traits and can be a useful selection criterion in breeding cultivars for the natto industry.