In vitro relative protein digestibility and lipoxygenase activity used as parameters to test and compare quality in five varieties of soybean meals.

Proximate composition, In Vitro Relative Protein Digestibility of 5 varieties of soybean (glycine max), was done. Meals from whole seeds and dehulled seeds were both investigated. Stability to photoxidation was also monitored on the oils over a period after sunlight exposure and finally the extent of oil oxidation was determined after 10 months of ambient storage. There was an intervarietal similarity in proximate composition for the whole seeds as confirmed by the low coefficients of variation, i e, 4.73%; 2.23%; 5.09%; 3.87% and 11.76% for moisture, crude protein, ether extract, ash and fibre respectively. An intervarietal similarity was also shown in dehulled seeds with coefficients of variation at 11.9%, 3.49%, 2.3%, 4.0% for moisture, crude protein, ether extract and ash respectively, except for the fibre content which had a higher coefficient of variation of 31.62%. Significant differences in relative protein digestibility were shown among varieties for both the whole and dehulled seeds. Dehulling significantly improved relative digestibility in all varieties as confirmed by the t-test at a 5% level. Nyala variety gave the highest relative protein digestibility both for the whole and dehulled seeds, at 78.03% and 87.02% respectively. The yield of oil after petroleum ether extraction was highest in the Roan and least in the Duiker seed variety at 20.70% and 17.8% respectively. Stability to photoxidation (lipoxygenase activity) for the oils was least in the Duiker variety, giving an E1%1cm (UV absorbance at 232 nm) of 11.26 after 12 hours exposure to sunlight and 62,0 after 10 months of ambient storage. The most stable oil to photoxidation was from the Soma variety giving an E1%1cm of 6.23 after 12 hours of sunlight exposure and 15.63 after ambient storage of 10 months. The t-test indicated that there was no significant difference (P <0.05) in the rate of oil oxidation between the Roan and the Soma varieties up to a period of 10 months at ambient storage. Ranking for the following quality parameters, protein, relative protein digestibility, oil content and its stability to oxidation were in the following order; Roan, Nyala, Soma, Nondo and Duiker.