Nutritional Evaluation of Brassica Meals Processed by Various Methods
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Meals of the Brassica species generally have a low market value, per unit protein, compared with the more commonly used protein supplement, soybean meal. This inferiority can be attributed in part to the glucosinolate contents and the methods of processing. These studies were undertaken to gather more information on the harmful effects of glucosinolates, both with and without dietary myrosinase, on the metabolic rate, the growth response and the enlargement of the liver and kidneys of mice. Investigations were also carried out on methods of removing glucosinolate products from enzyme-inactivated rapeseed meal. To assess the effects of meals of Brassica species on metabolic rate of mice, a simple automatic respirometer was designed and tested. The experimental results showed that the apparatus was capable of detecting abnormalities in thyroid activity with a high degree of reliability. The effects of synthetic thyroid-active iodinated-casein, synthetic goitrogenic thiouracil and natural glucosinolates on the metabolic rate of mice were studied. Feeding meals of Brassica species containing thioglucosides and thiouracil resulted in a reduction in oxygen consumption, whereas iodinated-casein caused an increase in consumption. Myrosinase supplementation of the means of Brassica species consistently depressed oxygen uptake, but there was a wide range of response. This variation can be attributed to the differences in levels, types of glucosinolates and other unknown toxic factor(s) in the different meals. It has been found that adding a certain amino acid(cystine) to rapeseed (Brassica napus) meal in acidic (pH5) media will neutralize glucosinolate products, hydroxynitrile, isothiocyanate and other known toxic factor(s). An improved Brassica napus meal has been developed by using L-cystine to give a Significantly better nutritive value and acceptability. Growth response and organ weights (liver and kidney) of mice comparable to those obtained on Bronowski meal were possible when a certain quantity of this amino acid was added to the meal. Glucosinolate contents, their hydrolysis products, isothiocyanates, goitrin, hydroxynitrile, and biological quality were investigated in rapeseed (Brassica napus) meals treated according to various techniques developed during these studies. Particular attention was given to the formation and toxicity of hydroxynitrile. According to the present methods, a process is provided for the treatment of rapeseed meal containing a high level of glucosinolates to produce a rapeseed meal with a feeding value equal to casein. The feeding Values of the rapeseed meals treated by these techniques were of the same magnitude as Casein, while untreated enzyme-inactivated meal was 40 percent below casein. Aqueous extraction procedures for the removal of glucosinolates (as their non-enzymatic hydrolysis products) from rapeseed ueal are described. Enzyme-inactivated meal is thoroughly mixed with certain levels of ferrous sulfate and L-cystine, then wetted with pH5 buffer solution to allow non-erlzymatic hydrolysis of glucosinolates to occur. The meal slurry is then subjected to extraction. The residual meal contained small amounts of isothiocyanates, goitrin, hydroxynitrile and unknown toxic factor(s) of sufficiently low concentrations to be readily tolerated by laboratory animals.