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Chapter 8
Storage-Induced Color and Biochemical Changes of Soybeans As Related to Soymilk and Tofu Making Sam K. C. Chang* Department of Cereal and Food Sciences, IACC 322. Dept. 2710, Box 6050, North Dakota State University, Fargo, ND 58108-6050 *
[email protected] Soybeans are subject to storage and shipping after harvest. During this period of time, the physical and biochemical characteristics of soybeans change to various degrees, depending upon the initial soybean moisture and storage/ shipping conditions, including various humidities and temperatures. Inappropriate storage will lead to product quality defects as well as major economic losses. The changes in soybean characteristics may include darkening in surface color, increase in hydration time, reduction in solid and protein solubility, decrease in phytate content, isoflavone structural conversions, changes in protein structures, sugar pattern and lipid oxidation. The coagulant requirement per unit weight of soymilk for tofu making is reduced by improper storage. The yield and quality of soymilk and tofu are also negatively affected by severe storage conditions. Significant correlations were found among quality attributes, including color parameters (Hunter L, a, b, and ΔE), solid extractability (as expressed by soymilk solids content), soymilk pH and protein content, tofu yield, hardness and protein content. We have developed simple statistical equations and kinetic models, which can be used to monitor soybean quality and predict quality changes of soybeans during storage.
© 2010 American Chemical Society In Chemistry, Texture, and Flavor of Soy; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
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Introduction Soybeans are subject to storage and transportation before processing into various food products. They may be stored up to one year or longer after harvest in a wide variety of environmental circumstances. It is well known during shipping in a boat, the temperature of soybeans that are placed in close proximity of the engine compartment may be heated to higher than 50C. Furthermore, the long hot and humid summer months in Southeastern Asia, where soy foods are regularly consumed, cause soybean deterioration if they are not refrigerated. The quality of edible soybeans and the viability of soybean seeds decrease gradually with prolonged storage under certain conditions. The magnitude of the quality deterioration depends on storage conditions, including time, temperature, relative humidity (RH), and microbial contamination. Early studies on grain storage have shown that the initial moisture content is an important factor to influence quality stability during storage at 25C. Furthermore, it has been reported that temperature and relative humidity of the storage environment affect the chemical compositions of soybeans, but relative humidity is more important than temperature at high relative humidity range. Soymilk and tofu are two major traditional soy foods (1). In the Western world, the consumption of soy foods and food containing soy has increased dramatically in the last decade, largely due to the approval of a heart health claim by the US FDA. Depending on storage conditions, the deterioration of soybeans may be reflected by several changes, including a decrease in hydration rate, an increase in the leakage of solids during soaking, an increase in the acidity of soybeans and darkening in bean color (2). The reported quality decreases in soymilk and tofu due to storage of soybeans include darkened soymilk/tofu color, low protein extractability (2), changes in tofu textural properties (2), and less uniformity in the microstructure of tofu (3). Tofu is a complex food system, and requires a series of processing steps from soybean raw materials. Tofu is considered as a protein gel and hence proteins in soymilk affect coagulation during tofu making. However, non-protein component, such as phytate has been reported to play an important role in the coagulation process (4). The coagulation of soymilk is the most critical step to influence yield and texture qualities, because it involves complex interactions among soy proteins, coagulants (usually Ca+2 and/or Mg+2), and non-protein constituents such as phytate, lipids, and carbohydrate. The exact mechanisms of how soybean biochemical compositions change during storage concomitantly to affect tofu yield and quality are still not fully understood. Most storage studies use certain accelerated-aging (adverse) conditions (3, 5, 6). It is commonly known that cold (refrigerated) environment is the best for longterm maintenance of grain quality. Storage under the conditions of 75% relative humidity at the ambient temperature is a critical factor for controlling storage fungi (7). However, discoloration and changes in biochemical constituents do occur during extended storage, even if visible growth of fungi has not occurred; and such changes would result in significant damages on soymilk and tofu quality. Soybeans after harvest are usually stored in uncontrolled ambient temperature and relative humidity conditions, which depend upon the regional environments. Despite the 114 In Chemistry, Texture, and Flavor of Soy; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
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research reported by some researchers, comprehensive studies on maintaining food quality of soybeans stored over a wide range of temperature and humidity had not been carried out previously. Our research laboratory has focused on several objectives to the storage characteristics of food soybeans in the last fifteen years. Here I summarize our studies on the influence of the storage of soybean under various conditions on soybean biochemical components, color quality, coagulant requirement as related to soymilk/tofu making. This article also includes our recent studies on how soybean color changes and relates to the protein extractability, pH changes, and related to soymilk and tofu processing properties. Mathematical equations have been developed to describe these quality relationships as affected by storage temperature, humidity, and time of storage. Influence of Physical Damage on Titratable Acidity and Other Quality Factors during Storage (2) US graded soybeans (No. 1 and No. 2) allow 10-20% of splits. During harvesting and post-harvest handling, seed hull cracking may occur. It was of interest to understand how cracking and splitting affect food quality of soybeans stored under high temperature and humidity conditions. Soybeans were cracked to contain 10% and 20% of splits and seedcoat cracks, respectively, and were stored in 85% RH at 30 C for up to 60 days. Samples were retrieved at 15-day intervals and analyzed for determining chemical components, and soymilk and tofu processing quality. Titratable acidity of soybeans increased significantly from 0.9-1.0% to 1.12-1.23% during the 60 days of storage. Protein extractability in soymilk decreased significantly with storage time from 80% to 68% for the 20% split treatment. In general, the yield of tofu decreased significantly beyond 30 days of storage for all treatment groups and the control group which contained no damages or splits. The 20% split soybeans had a higher loss in tofu yield than 10% splits. Tofu could not be made from soybeans with 20% split and 60 days of storage. The color of tofu darkened, and the hardness, and the force to fracture tofu were significantly (p5000a-fa
>5000a-fa
7
71.0 ± 1.4a-fe
>5000a-fa
>5000a-fa
8
No tofu was made
NA
NA
9
No tofu was made
NA
NA
*
Data are expressed as means ± s.d. Data of yield are means of 2 replicates on a wet weight basis (5.4 % moisture). a-f means with different letters within the same column differ at (p
