| Tiger nut is an emerging oilseed resource with a wide distribution,high environmental adaptability and high yield per acre,rich in nutrients such as oil,starch,protein and minerals(phosphorus and potassium).In addition,as a raw material for oil extraction,the oil is light in colour and flavour and has a fatty acid composition comparable to that of olive oil,making it an emerging oil resource that can reduce our dependence on foreign oilseeds.Tiger nut can also be used as an auxiliary ingredient in various food products,such as soya milk powder,ice cream,biscuits and beverages.Therefore,it has potential development value and broad market prospects.However,the thick skin,high moisture content at harvest and the high starch content of tiger nut can lead to lower quality or lower oil yield in processed products.In addition,the north-east is a very important tiger nut growing region,but the cold weather can cause frost on tiger nuts.Research on tiger nuts is still in its infancy and the problems mentioned above have rarely been studied.Therefore,in this project,four processing methods,namely Peeling process,drying methods,steam treatment and freeze-thaw treatment,were selected to systematically study the tiger nuts,to investigate the effects of different processing methods on the tiger nuts and its oil quality,and to provide the necessary basic parameters and theoretical basis for the development of tiger nuts.The main research content and conclusions of this topic are as follows:(1)The effect of peeling processing on the quality of tiger nut and its oil.The peeling process was carried out using a peeling machine to examine the changes in the main constituents,compression and shearing characteristics and oil quality of the tiger nuts.The results showed that the starch,crude fat and crude protein in the kernels were 17.9,3.30 and1.80 times higher than those in the skins,respectively.This indicated that there was a significant difference in the composition of the tiger nut kernel and the tiger nut skin.The compressive and shearing forces of peeled tiger nuts were significantly lower compared to unpeeled tiger nuts.In addition,the R(red)value of the oil from tiger nut peel was 1.80 times higher than that of the oil from tiger nut kernel.The highest phospholipid content(342.81mg/kg)and the longest oxidation induction time(45.44 h)were found in tiger nut kernel oil compared to tiger nut skin oil.This means that the peeling process is beneficial for improving the colour and oxidative stability of the oil.In conclusion,the peeling process is beneficial for increasing oil yield,improving mechanical properties and improving oil quality.The results of this study provide a theoretical reference for the lower pressure required to press oil from peeled tiger nuts compared to unpeeled tiger nuts,and provide the necessary theoretical basis for the improvement of tiger nut products and oil quality.(2)The effect of drying methods on the quality of tiger nut and its oil.Four drying methods,namely sun drying,oven drying,microwave drying and vacuum drying,were used to compare and analyse the effects of different drying methods on the quality of the tiger nuts and its oils.The results showed that the tiger nuts dried sun and under vacuum had a desirable colour.The morphology of the starch granules in the oven-dried and microwave-dried tiger nuts changed considerably.In addition,the highest oil yield(22.26%)was obtained from microwave-dried tiger nuts,while the fastest oil yield was obtained from vacuum-dried samples.The kinetics of oil extraction from tiger nuts by soxhlet extraction can be well described by an exponential model.In terms of oil quality,sun drying was the best option for retaining carotenoids,sterols and unsaturated fatty acids in tiger nut oil.Microwave-dried tiger nut oil showed the highest degree of oxidation and the longest oxidation induction time(53.27 h),while the opposite was true for the vacuum-dried samples.There were no significant differences in the acid value and vitamin E composition of tiger nut oil between drying methods.The results of the study provide guidance for the selection of drying methods for tiger nuts according to the specific processing purpose.(3)The effect of steam treatment on the quality of tiger nut and its oil.The effect of steam treatment on the oil yield,microstructure and oil quality of the tiger nuts was studied.The microstructure showed that steam treatment(100°C and 120°C)destroyed the cell structure of the tiger nuts and caused the starch granules to aggregate.Compared with the untreated tiger nuts,the oil yield of steam-treated tiger nuts increased by 8.72% to 18.85%.In terms of oil quality,the steam treatment(120°C/60 min)significantly increased the oxidation induction time,total phenol and vitamin E content,and decreased the acidity and carotenoid content of the oil compared to the untreated oil.In addition,steam treatment had a minor effect on the fatty acid composition of tiger nut oil.Steam treatment had a significant effect on the volatile composition of tiger nut oil.A total of 60 volatile components were detected in the untreated and steam-treated oil of tiger nut,with the highest content of aldehydes,followed by esters and phenols.In conclusion,steam treatment may be a viable pretreatment method for improving the yield and quality of tiger nut oil.(4)The effect of freeze-thaw treatment on the quality of tiger nut and its oil.The 24 h freeze-thaw treatment was used to simulate the cold weather in the northeast of China,and the changes of freeze-thaw treatment on the oil yield,microstructure and oil quality of the tiger nuts were analysed.The results showed that the freeze-thaw treatment damaged the cell structure of tiger nut and caused the surface of starch granules to become rough,forming a microporous structure.The freeze-thaw treatment increased the oil yield of the tiger nuts,with the maximum value(19.23%)obtained at the 12 th freeze-thaw treatment.In terms of oil quality,the freeze-thaw treatment increased the degree of oxidation and vitamin E content,prolonged the oxidation induction time and reduced the total phenolic content of the oil compared to the untreated samples,but had no significant effect on the fatty acid composition.In conclusion,the experimental freeze-thaw treatment conditions improved the quality of tiger nut oil.These findings provide data references for the development and application of tiger nuts to avoid wastage of tiger nut resources. |
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