Study On The Mechanism And Process Of Microwave Puffing Of Raspberry Crisps

As one kind of small berry,raspberry is soft and juicy,and rich in anthocyanins,vitamins and other bioactive substances,which cause its unique medical value and health benefits.However,raspberry is perishable due to its high moisture,which limits its market promotion.At present study,microwave puffing technology was utilized to produce raspberry crisp with a high nutritional value,crispy and distinctive puffed products.Considering the disadvantage of raspberry crisps,such as soggy and partial scorching.Fresh raspberry slices were selected as the raw materials,the effects of additives(xanthan gum,soybean fiber and monoglyceride)on the puffing properties of crisp crisps were analyzed.The raw material formula of raspberry slices was optimized to obtain crispy taste and puffed slices.Fuzzy evaluation method was employed to optimize the phased varying power microwave puffing of raspberry crisps to solve the problem of overheating and scorching in the later stage of the puffing raspberry slices.The energy distribution,heat and mass transfer,internal pressure and volume change during microwave extrusion of raspberry crisps were analyzed by simulation technology to reveal the mechanism of microwave extrusion of raspberry crisps.The main conclusions of this study are as follows.(1)The effects of three additives(xanthan gum,soybean fiber and monoglyceride)on the puffing characteristics of raspberry crisps were investigated by single factor experiment.The results showed that the effects of three additives on the expansion rate,hardness and brittleness of raspberry crisps were similar,and the puffing rate and brittleness of raspberry crisps initially increased and then decreased,while the hardness firstly decreased and then increased.The addition ranges of xanthan gum,soybean fiber and monoglyceride were 0.4%?0.8%,4%-8%and 0.4%?0.8%,respectively.A three-factor five-level central composite experiment was designed considering level of single factor.According to the fuzzy evaluation method,the optimum formula parameters to achieve the highest expansion rate of 3.70±0.08,the maximum brittleness of 77±1,and the minimum hardness of 4380.25±152.17 g from raspberry crisps were obtained under the xanthan gum of 0.63%,soybean fiber of 5.26%,and monoglyceride of 0.62%.(2)Compared with the phased variable power method,the hardness and brittleness of crisps under constant power extrusion were 8236.24±99.23 g and 39±1,respectively.The four kinds of anthocyanin monomers of crisps in terms of delphinidin,cyanidin,peonidin and malanin,and the retention of these anthocyanin monomers were 16.52,27.06,8.17 and 3.69 mg/100g,respectively.And the hardness and brittleness of crisps were 2653.23± 102.15 g and 80±2,respectively.Total retention of anthocyanin monomers of crisps was 55.44 ± 1.08 mg/100g,and the retention of delphinidin,cyanidin,peonidin and malanin of crisps were 21.35,35.32,13.47 and 3.69 mg/100g,respectively.Compared with the that under constant power,the hardness decreased by 67.79%,the brittleness increased by 51.25%,and the anthocyanins retention increased by 33.17%.The effects of processing parameters of the phased varying power microwave puffing on the puffing characteristics of raspberry crisps were further investigated.The microwave intensity range of process in 1st stage was determined to be 20?40 W/g based on single factor experiment,moisture content in stage conversion was 14-18%,and the microwave intensity in 2nd stage was 10-30 W/g.(3)Based on the phased varying power single factor experiment,1st-the microwave intensity,moisture content in stage conversion and 2nd-the microwave intensity were taken as the influencing factors,and the expansion rate,brittleness and sensory evaluation of raspberry crisps were taken as the target factors.The optimization of fuzzy evaluation indicated that the process parameters of phased variable power microwave extrusion were:1st-microwave intensity 32.38 W/g,moisture content in stage conversion 14.93%,2nd-microwave intensity 23.57 W/g,expansion rate of crisps 4.02±0.08,brittleness 80±2,sensory evaluation value 9.26±0.37.The order of the influence of the experimental factors on the comprehensive quality of expanded raspberry crisps was moisture content in stage conversion,1st-microwave intensity and 2nd-microwave intensity.(4)A multi-field coupled mathematical model of electromagnetic field,heat transfer field,water field and pressure field during microwave puffing was established to reveal the mechanism of microwave puffing of raspberry crisps.It was found that the distribution of electromagnetic field in the process of extrusion was characterized by high electric field intensity at the bottom and low electric field intensity at the top.The temperature distribution of fresh slices was high at the center and low at the edge.The moisture content on the surface of fresh slices was lower than that in the interior of fresh slices during extrusion.The pressure inside the fresh tablets increased gradually with the expansion time,which caused the expansion of the fresh slices.The expansion rate of brittle crisps could be divided into constant volume stage(pre-expansion stage),rapid volume expansion stage(mid-expansion stage)and constant volume stage(post-expansion stage).Mufti-physical field coupling analysis showed that microwave energy absorbed by moisture in fresh slices increased temperature,which caused phase change of moisture in fresh slices.At this time,the pressure produced by moisture evaporation was less than that of the internal binding force of fresh slices,which resulted the unchange volume of slices.The absorption of microwave by fresh slices caused the temperature rose rapidly and evaporated of moisture in the middle period of expansion(30-120 s),which produced pressure gradient in fresh slices,and the pressure acted as a driving force to promote the rapid expansion of fresh slices,which was the fastest in this stage.The ability of fresh slices to absorb microwave energy was weakened owing to the removal of moisture evaporation from fresh slices in the later stage of expansion(120-150 s).The volume of raspberry crisps did not increase because the steam pressure was not enough to promote the expansion of fresh slices.That is to say,microwave induced the evaporation of moisture in fresh slices to produce pressure,which acted as the driving force to cause the volume expansion of fresh slices,and form porous structure of crisp slices.In this study,the effect of additives on the puffing characteristics of raspberry crisps was clarified,and the formula of microwave puffed raspberry crisps with crispy taste was obtained;The phased variable power microwave extrusion process was optimized,which solved the problems of local coking and low retention rate of nutrients in raspberry crisps caused by local high temperature of microwave heating.At the same time,the microwave puffing process of raspberry crisps was analyzed by numerical simulation,and the mechanism of microwave extrusion of berry crisps was revealed.The research results can provide theoretical basis and technical reference for the actual production of microwave puffed berry crisps,and also provide a new idea for industrialized production of berry crisps.