THE EFFECT OF HEAT-TREATMENT ON GLUCOSINOLATES AND NUTRITIONAL-VALUE OF RAPESEED MEAL IN RATS

Samples of rapeseed meal were studied for the influence of heat treatment on glucosinolates and nutritional value. After toasting at 100 degrees C for 15 min, 30 min, 60 min and 120 min, the total content of glucosinolates decreased 24%, 46%, 70% and 95%, respectively. 4-Hydroxy-glucobrassicin was found to be more heat-sensitive than the aliphatic glucosinolates progoitrin and gluconapin. Protein solubility decreased linearly from 85% to 81%, 61%, 52% and 40% after the respective toasting times (P < 0.05). Total lysine content dropped from 5.93 to 5.72 g and 4.91 g per 16 g N after 30 min and 120 min toasting, respectively. Other amino acids remained relatively unchanged except cystine level which dropped 12% after 120 min toasting. The contents of mono-, di- and oligosaccharides decreased after toasting, particularly sucrose. True digestibilities (TD) of rapeseed protein in rats were 77.0%, 73.9%, 72.1%, 72.9% and 71.2%, respectively, after 0 min, 15 min, 30 min, 60 min and 120 min toasting, corresponding to a significant linear decrease (P < 0.01) in TD as a function of the natural logarithm to the heating time. Biological value (BV) and net protein utilisation (NPU) decreased linearly with increasing heating time (P < 0.01). Four commercial rapeseed meal samples, i.e. extracted meal and press cake from Denmark, extracted meal from Germany and extracted meal from China, were also investigated for their glucosinolates and nutritional quality by analysis and rat trial. These samples displayed variable levels of glucosinolates, protein solubility, lysine content and nutritional values corresponding to the heat-treated samples. The results show that up to 30 min toasting corresponding to a protein solubility around 60% will be optimal processing conditions.