|Abstract: ||The soybean variety of Djakal was stored in 4 conditions: room temperature (~ 22
°C) aerated with nitrogen (RN), room temperature aerated with air (RA), high
temperature (45 °C) aerated with nitrogen (HN) and high temperature aerated with
air (HA) with very low relative humidity (RH). Samples were stored for 20 months,
and were taken every 5 months. Soybean sealed in plastic bags and stored at 4 °C
was used as the control sample.
Stored soybeans were made into soy flour and soy products (soymilk, okara, tofu
and whey). The protein contents of the soy flour and soy products were determined
by Elemental vario MAX CNS analyser. The sulfhydryl and disulfide contents of
the extractable proteins of the soy flour were determined by spectrophotometry.
SE-HPLC was used to examine the molecular weight distributions of the soluble
soy proteins of the soy flour, soymilk and whey. The fatty acid and soluble sugar
compositions of the soy flour were examined by GC and RP-HPLC respectively.
The yields, protein contents and solid contents of the soymilk, okara, tofu and
whey were determined. The texture of tofu was examined using a texture analyser.
A novel application of the Rapid Visco Analyser (RVA) to examine the pasting
properties of soy flour was also explored in this study.
During storage, the low relative humidity condition caused substantial water loss
in soybean. The protein extractability decreased in high temperature stored
soybeans. The decrease of free sulfhydryl content and increase of disulfide content
indicated a transformation from sulfhydryls to disulfide bonds during storage. A
decrease on the total sulfhydryl contents (include sulfhydryls and disulfide bonds)
indicated the possibility of other reactions of sulfhydryls and disulfide bonds other
than the interaction between the two. A decrease of low molecular weight proteins
and an increase of aggregated proteins were found in the SE-HPLC profiles of
stored soybeans. Decreases in the contents of unsaturated fatty acids, oleic, linoleic
and linolenic acids, were observed. The contents of the main sugars in soybean,
fructose, sucrose, raffinose and stachyose, decreased during storage.
Storage caused a decrease in the yield, solid content, protein content and protein
recovery of soymilk, accompanied by an increased yield and protein recovery of
okara. The tofu made from stored soybeans had a lower yield, protein recovery and
water content. The hardness of tofu made from the high temperature stored
soybeans increased in the first 15 months, followed by a substantial decrease in the
last 5 months, indicating the variation on the protein gelling property of high
temperature stored samples. Other texture indices of tofu curd (springiness,
resilience, cohesiveness and chewiness) underwent significant (P < 0.05) variations
during soybean storage in all conditions.
A smooth and repeatable RVA profile of soy flour was obtained in this study. The
addition of sugars and salts (CaSO4 and MgCl2) caused a decrease in RVA
viscosities, while the defatting process of soy flour increased the RVA viscosities.
Storage of soybeans also led to significant (P < 0.05) changes to their RVA
parameters, however, the mechanism was not clear. Strong correlations were
observed between RVA parameters and tofu texture, protein content, protein
extractability and total sugar content, implying the possibility of RVA application
in assessing the freshness of stored soybeans, and predicting the quality of tofu.
Compared to the surrounding atmosphere, high temperature played a more
important role in accelerating the variation of protein, sugar, and the properties of
soymilk and tofu. However, nitrogen exhibited great ability on improving yield
and quality of soymilk and tofu after soybean storage in high temperature for more
than 10 months. In addition, nitrogen could partially prevent oil deterioration
during soybean storage, and was thus an effective way to protect soybean used for