Anti-nutritional Factors and Ruminal Dry Matter and Crude Protein Degradability of Gamma and Microwave Irradiated Native Rapeseed

Native rapeseed (NRS) is planted in some parts of Iran because of climatic condition. The consumption of NRS in animal nutrition is limited by anti-nutritional such as phytic acid and glucosinolate. Moreover, the protein of NRS is highly degraded by rumen microorganisms. Several processing methods have been used to enhance the nutritive value of whole oilseeds, including extrusion, roasting, toasting and Jet-Sploding. However, most heat processing methods adversely affect protein digestibility in the small intestine. Recently, other processing methods such as processing by gamma and microwave irradiation have been noticed. Therefore, this research was carried out to evaluate the effects of gamma irradiation (15, 30 and 45 kGy) and microwave irradiation (800 W) for 2, 4 and 6 min on ruminal dry matter (DM) and crude protein (CP) degradability, in vitro CP digestibility, anti-nutritional factors (glucosinolate and phytic acid) and chemical composition of NRS.
Chemical composition (DM, CP, EE and Ash) of untreated and irradiated NRS was determined by AOAC methods. Then, sufficient water was added to the sample to increase the moisture content to 250 g/kg. Gamma irradiation was completed by using a cobalt-60 irradiator at 20 ºC. The dose rate determined by Fricke dosimetry was 0.36 Gy/s. Another three samples (500 g each) were subjected to microwave irradiation at a power of 800 W for 2, 4 and 6 min. Phytic acid and glucosinolate contents of untreated and irradiated samples were determined by standard methods. Degradation kinetics of DM or CP were determined according to in situ procedure. Six grams of untreated or irradiated NRS were incubated in the rumen of three ruminally fistulated Taleshi bulls for 0, 2, 4, 8, 16, 24 and 48 h. Bags were placed in the rumen just before the bulls were offered their first meal. After retrieval from the rumen, bags were thoroughly washed with tap water until the rinsing water was clear. The same procedure was applied to two bags to obtain the 0 h value. The residues were dried and analyzed for DM and CP to determine degradation kinetics of NRS. Digestibility of rumen undegraded CP was estimated using the three-step in vitro procedure. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) was used to monitor protein subfractions and the fate of true proteins of untreated and irradiated NRS in the rumen.
Irradiation had no significant effect on chemical composition, but decreased the total glucosinolate and phytic acid of NRS. This elimination of phytic acid by irradiation is probably due to chemical degradation of phytate to lower inositol phosphates and inositol, by the action of free radicals, which have lower chelating power, or cleavage of the phytate ring itself. The glucosinolate content of untreated NRS in this study, 122.8 mol/g, was reduced up to 30 kGy, but further irradiation had no effect. Major deleterious effects of glucosinolate ingestion in animals include: reduced palatability, decreased growth and reduced production. However, according to our study, gamma and microwave irradiated NRS may be fed to dairy cows at high levels without glucosinolate induced detrimental effects. Gamma and microwave irradiation decreased the washout fraction, degradation rate and effective degradability (ED) of DM and CP and increased potentially degradable fraction of DM and CP of NRS. Decreasing CP degradability as a result of irradiation is due to the occurrence of cross-linking of polypeptide chains, denaturation and protein aggregation. Gamma irradiation at doses of 30 and 45 kGy and microwave irradiation increased in vitro CP digestibility of NRS. Irradiation may induce unfolding of the protein and its denaturation, thereby exposing hydrophobic amino acids (especially aromatics) that are positional groups for the active sites of pepsin and trypsin enzymes. Moreover, the improvement in CP digestibility may be attributed to reduction of phytic acid. Electrophoresis results indicated that major proteins of NRS were cruciferin (globulin 12S) and napin (Albumin 2S). Electrophoresis results indicated that in untreated NRS, three subunits of cruciferin and in gamma and microwave irradiated NRS, three subunits of cruciferin and two subunits of napin consisted of bypass proteins.
In conclusion, gamma irradiation at doses of higher than 15 kGy and microwave irradiation more than 2 min reduced ruminal degradability of CP, increased in vitro CP digestibility and reduced anti-nutritional factors of NRS. Subsequently, in vivo studies are required to investigate effect of feeding irradiated feedstuff on lactation performance of dairy cows.