Effects Of Microwave Pretreatment Of Rapeseed On Quality Of Oil Extracted By Solvent From Meal

Rapeseed is one of the most important oil and cash crops in China, however, up to60%of the edible vegetable oil still depends on external import in the current, so the research of efficient rapeseed processing technology for promoting the healthy development of rapeseed industry has become a pressing task. Previous investigations have shown that microwave pretreatment of rapeseed allowed for higher oil extraction yield by cold-pressing, better retention and availability of desirable nutraceuticals, also stronger oxidative stability, but there is still more than10%of the oil left in cold pressed cake, it is advisable to carried this part of residual oil out with solvent extraction for further using. The paper presented a comprehensive research about the effects of microwave pretreatment of rapeseed on major physical and chemical quality, nutritional quality and antioxidant capacity of rapeseed oil extracted by solvent from meal, the main conclusions obtained are as follows:1. The microwave time showed significant effects (P<0.05) on the major physical and chemical quality of rapeseed oil extracted by solvent from meal, including moisture content, acid value, peroxide value and p-Anisidine value, colour and heating test, the influence laws are shown below:(1) Along with the extension of microwave time, the moisture contents of oils by solvent decreased totally, oil pretreated for7min was decreased by19.05%compared to the oil without pretreatment, moisture contents of oils with all microwave times conformed to the requirement of national standard of two-level rapeseed oil (≤0.05%).(2) The acid values of oils by solvent increased firstly, and then decreased with the extension of microwave time, which value was highest when treated for4min, increased by19.05%compared to the oil without pretreatment, the acid values of oils treated for1-7min exceeded the demand of national standard of level4rapeseed oil (≤3.0mgKOH/g). The peroxide values increased first then reduced with the extension of microwave time, which was maximum for oil treated for3min, increased about3times than oil without pretreatment, all oils’peroxide values were below5.0mmolNa2S2O3/kg, accorded with the requirement of national standard of level4rapeseed oil. The p-Anisidine values of increased with extension of microwave time, and value of oil treated for7min was increased by about2.41times than the value without pretreatment.(3) Oils with all microwave time points had precipitates in heating test, a large amount of precipitates was observed in oils treated for0-5min, which didn’t conform to the requirement of national standard of level4rapeseed oil, the amount of precipitates in oils treated for6,7min were relatively low. The colours were gradual deeper along with the duration of microwave time, which didn’t meet the requirement of national standard of level4rapeseed oil (Yellow<35, Red<7.0) no matter treated for any time.2. The microwave time exhibited significant influence (P<0.05) on the nutraceuticals contents of rapeseed oil extracted by solvent from meal, including phytosterols, tocopherol, lutein, β-carotene, total phenol content and free polyphenols, no significant impact was observed on fatty acid composition.(1) The predominant sterol in rapeseed oil is P-sitosterol (51%-52%), followed by campesterol (30-32%) and brassicasterol (13-14%), and a small amount of Δ5-avenasterol (2%-3%) and stigmasterol (1%-2%). The longer microwave time was, the higher phytosterol components and total amount contents that oils obtained, oil treated for6min reached the maximal content, which added by50.45%over the oil without pretreatment.(2) The y-tocopherol was the main tocol (54%-61%) in rapessed oil, followed by a-tocopherol (39%-46%). The tocopherol content of oils increased firstly then decreased with the extension of microwave time, which reached peak for oil treated for4min, the content was increased by29.49%compared to oil without pretreatment, content of oil treated for7min was coming down even24.84%higher than oil without pretreatment.(3) Lutein content of oils decreased gradually with the extension of microwave time, the content of oil treated for7min decresed by49.82%compared to oil without pretreatment; β-carotene content increased maximum and then decreased with the prolonging of microwave time, which was highest when treated for3min, the content was21.42%higher than oil without pretreatment, then the content was decreased gradually, for oil treated for7min, the content was comparable to oil without pretreatment. (4) Along with the extension of microwave time, the total phenolic contents of oils increased significantly, which content of oil treated for7min was3.28times more than oil without pretreatment. Canolol is the most abundant free polyphenols (91%-99%) in rapeseed oil, followed by sinapic acid (0.7%-8%), ferulic acid (0.012%-0.1129%) and cinnamic acid (0.045%-0.357%). Both contents of free polyphenols and canolol increased with the duration of microwave time, the two contents of oil treated for7min were5.50and6.04times more than oil without pretreatment respectively.3. Microwave time had significant influence on DPPH radical scavenging activity (DPPH) and ferric reducing antioxidant potential assay (FRAP) of rapeseed oil (P<0.05). The longer the microwave time was, the higher DPPH and FRAP values the rapeseed oil obtained, and the values of oil treated for7min was2.51and1.29times more than oil without pretreatment respectively. Correlation analysis indicated that DPPH and FRAP values of rapeseed oil were significantly associated with contents of phytosterols, tocopherol, total phenol content, free polyphenols and canolol (P<0.01), among which the canolol made the greatest contribution to the antioxidant capacity of rapeseed oil.