Study On Preparation Of Whole Wheat Crisps From Whole Wheat Grains And Changes Of Nutrients

At present,whole wheat grains are rarely used in preparation of whole wheat products,which are mainly made from whole wheat flour.Parts of wheat bran and germ will be lost after milling.And during the common processes,such as baking and extrusion,nutrients in wheat will be lost to a further extent.In this study,the unmilled whole wheat grains were used as raw materials,mild extrusion conditions were applied to fully mix and gelatinize the wheat grains and the extrudes were dried under microwave vacuum drying to obtain whole wheat crisps.The extrusion and dehydration processes were optimized and the changes of physicochemical properties and nutrients of wheat before and after processing were investigated.The variation of water absorption and crispness of whole wheat crisps during soaking in medium was also studied.The results are as follows:Firstly,the water absorption rules of wheat grains,the effect of pre-steaming time,screw temperature and screw speed on the properties of extrudates and the quality of crisps were studied.It was concluded that the saturated water content of wheat grains was 36%.The brightness of extrudates first increased and then decreased as the pre-steaming time increased but it decreased as the screw temperature increased.The effect of pre-steaming time on the gelatinization degree of extrudates was the most significant.The gelatinization degree of the unsteamed wheat extrudates was only 70.34%.As the pre-steaming time increased to 10 min,the gelatinization degree of extrudates reached 94.39%and then remained stable.The correlation between the texture of extrudates and the quality of crisps was not significant(p>0.05),so the texture of extrudes wasn't suitable to be used as an indicator for the following processes.The hardness and crispness of crisps measured by the texture analyzer(TA)and the sensory evaluation were fitted well(R~2:0.9394,0.9538,respectively),so hardness and crispness of crisps measured by TA could be used as indicators for further process optimization.Next,hardness and crispness of crisps were used as evaluation indexes,the pre-steaming time,screw temperature and screw speed were optimized by orthogonal experiment.The optimal extrusion parameters were:8 min of pre-steaming time,90°C of screw temperature and65 r/min of screw speed.Similarly,hardness and crispness of crisps were used as evaluation indexes during orthogonal experiments of microwave vacuum drying.The optimal dehydration conditions were:55°C of microwave temperature,450 W of microwave power and 25 min of dehydration time.The hardness and crispness of whole wheat crisps prepared under the optimum conditions were 4.93 kg and 22.66 respectively.This whole wheat crisps were suitable for casual snacks.Then,the changes of physicochemical properties and nutrients of wheat before and after extrusion and dehydration were studied.The results showed that the brightness of crisps decreased compared with the control.The micro structure showed that the holes appeared in the extrudes and got larger in crisps.The starch of crisps reached complete gelatinization.The secondary structure of protein remained intact before and after extrusion and dehydration.The relative activities of polyphenol oxidase and peroxidase in crisps were completely inactivated.In terms of changes of nutrients before and after extrusion and dehydration,the water solubility index and water absorption index of wheat starch increased;the content of rapidly digested starch and slowly digested starch increased whereas resistant starch decreased;the in vitro protein digestibility in crisps increased from 69.31%to 76.55%;the fatty acid value decreased;the soluble dietary fiber increased from 2.06%to 2.64%.Finally,the kinetic model of water absorption and the change of crispness of whole wheat crisps in hot whole milk and skim milk were studied.The water absorption curve of the crisps in the whole and skimmed milk could be fitted by the Peleg model(R~2:0.9864,0.9915,respectively).The crispness of self-made whole wheat crisps and the commercially available whole wheat breakfast crisps performed similarly when immersed in milk.After 3 min of immersion,the crispness went downward linearly and then remained stable.In addition,the total number of colonies,mold and E.coli in the ready-to-eat whole wheat crisps were 30 CFU/g,10 CFU/g,<0.3 CFU/g respectively,which were much lower than the regulation of microbial content in the cereal products of GB19640-2016.