School of Applied Sciences, RMIT University, Melbourne VIC 3000, Australia
Okara is the by-product of soymilk and tofu production. It is reported that 1.1kg of fresh okara is produced from 1kg of dry soybeans during processing to soymilk and tofu (Khare et al, 1995). This reflects both the large amount as well as the high water binding capacity of okara. The product comprises 28% protein, 10% fat and 5% carbohydrates on dry matter basis. It also possesses biologically active compounds, phytoestrogens, which are reported to play a role as cancer protective agents and properties associated with anti-oxidative activity, radical scavenging, lowering serum cholesterol levels and anti-proliferative effects (Coward et al, 1993, Franke et al, 1994).
Okara is considered as a waste product (O’Toole, 1999). There is a lack of information on the potential usage of okara in the food production and the main utilization is currently for animal feed. Okara lacks consumer appeal attributable to its high dietary fibre content and unsavoury taste. It is likely to include trypsin inhibitor, lipoxygenase and isoflavones and factors lead to the development of unattractive flavours.
The purpose of this study was to incorporate okara into yellow alkaline style of Asian noodle products. The resulting attributes particularly as they confer additional viscoelasticity and the reinforcement of textural properties have been evaluated.
Materials and methods
Noodles were made on laboratory scale using 300g flour, 114g water, 3g sodium chloride (BDH Laboratory supplies, England) 3g sodium carbonate (kansui) (Ajax Chemicals, Melbourne), 0.3g sodium acetate (Ajax Chemicals, Melbourne), and 0.1g sodium ascorbate (Merck, Australia). Varying levels of fresh okara were incorporated and the amount of water was reduced correspondingly.
Noodle production method
Yellow alkaline noodles were prepared in the laboratory. This entailed mixing the ingredients for 5 minutes, followed by 20 minutes resting period. The mixture was then formed into a dough and developed by sheeting multiple times to desired thickness and cut into strands. The strands were cooked until the optimum point where the core had just reached the cooked state.
Results and discussion
When the textural characteristics of the cooked laboratory noodles was measured using the TA-TX2 system and a flat probe, some variation in the firmness was observed (Figure 1). In comparisons with the commercial product produced on a factory scale from the same flour and formulation, the factory products were firmer.
Figure 1. Comparison of the hardness of the laboratory scale and factory scale noodles from texture analysis. All data are expressed as mean ± standard deviation; n=5.
When samples of the cooked noodle products were viewed using the Environmental Scanning Electron Microscope (ESEM), the appearance of the noodle surfaces showed starch granules embedded in a gluten matrix. The granules remained obvious even after the noodles were cooked. The okara noodles are shown in Figures 2 and 3.
Figure 2. Appearance of laboratory scale okara noodles by ESEM (1% addition on the left and 5% on the right)
The presence of a cohesive matrix at the highest levels of addition indicate that the hemicellulosic components in the okara did not distrupt the structure as extensively as expected. It is also possible that proteins from the okara may have contributed to the development of the protein network as they confer considerable number of SH groups which in turn enhance gel formation (Obata et al 1993, Obata et al 1996).
Figure 3. Electron microscopy of laboratory scale10% okara noodle
At the higher levels of addition there are spaces within the structure of the noodles None-the-less the protein and the starch granules provided an array (Kovac et al 2004) which had desirable mouthdfeel and textural attributes. Further analyses of the phytoestrogens in the products provided the data presented in Table 1.
Table 1. Comparison of the loss of isoflavone contents in cooked laboratory scale yellow alkaline noodles in different concentrations of okara incorporated. Content data are expressed on a dry weight basis, in mg per 100g, as mean ± standard deviation
Total isoflavone content (mg)
22.9 ± 0.1
24.3 ± 0.3
24.7 ± 0.2
29.0 ± 0.7
Relative loss of isoflavones (%)
These results demonstrate that, for the laboratory scale okara noodles, the minimum loss of isoflavones occurred when 10% okara was incorporated into the noodle. The losses observed in parallel experiments carried out at on the factory scale showed the lowest losses were at 10% addition. However the losses for the factory noodles were greater than those observed for the laboratory products. This may have been caused by the high temperature boiling process used in the factory (Sherkat et al 2001).
The overall results from this study showed no clear difference between laboratory scale 10% okara noodle and normal noodle. The structural changes occurred after okara was incorporated into the yellow alkaline noodle at various concentrations as seen in the electron micrographs. The internal structure was altered as okara at 10% imparted a relatively looser network than 1% okara noodle.
The trials undertaken on okara addition to yellow alkaline noodles indicate that addition up to levels of 10% are the maximum that are practically achievable. It appears that enhanced nutritional value could result and the potential health benefits of isoflavones could be realised with little impact on the organoleptic attributes of the product.
We are indebted to Phil Francis for help with the ESEM
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