Definition,Characterization And Influence Of The Optimum Flavor Range In Food Frying Process

The food frying industry is one of the important industries in the food industry.According to relevant data,only the sales of potato chips in China's market exceed 10 billion each year,and 81.5%of China's"Generation Z"people love potato chips/French fries.Flavor is one of the most important qualities of fried food.It is closely related to the degree of oil oxidation and Maillard reaction during frying.The fried flavor changed with the frying conditions and frying process.Therefore,the inconsistent flavor of industrialized fried food has always been a technical difficulty that plagues the food frying industry,and has seriously affected the sales volume of fried food and the process of industrialization.The underlying reason for this problem is the lack of understanding the reason behind the changes in fried flavor quality and the lack of quantitative indicators.The flavor preference evaluation of French fries indicated that the flavor quality changed in a“wide bell”curve with the frying process,and the flavor scores of French fries gradually improved at the early frying time,and then entered the flavor stabilization period.Finally,the flavor gradually deteriorated with the further deterioration of the frying oil in the later stage of frying.Based on this,we defined the stable period of flavor as the optimum flavor range,and determined the flavor compounds behind the“wide bell”curve.Furthermore,we established the relationship between the optimum flavor range and the physical and chemical indices of the frying oil during the frying process.It was found that the total polar component content difference(?TPC)can well characterize the optimum flavor range.At the same time,the further research found that the unsaturation of frying oil,frying temperature,adding antioxidants will affect the optimum flavor range.The main research contents of the thesis are as follows:Firstly,three representative frying edible oils of soybean oil,palm oil and high oleic rapeseed oil were selected to fry French fries,and flavor preference evaluation of the consumers on different batches of French fries were performed.The results showed that the flavor profile of French fries fried in soybean oil,palm oil and high oleic rapeseed oil showed a“wide bell”curve with the frying process.The 90%of the peak of the curve was set as the critical point t₁ of the rising period and the critical point t₂ of the falling period,and the difference?t was defined as the optimum flavor range.The curve presented three stages of change,which were the induction period,the optimum period and the degradation period.The molecular sensory science method was used to identify the key flavor substances in different frying stages,it was found that the content of the main flavor characteristics(E,E)-2,4-decadienal increased first and then decreased,which changed with the flavor quality curve.In the early stage of frying,as the content of(E,E)-2,4-decadienal continued to increase,the flavor quality curve crossed the induction period and entered the optimal period.The curve extended in a bell shape,and an optimum flavor window appeared;To the later stage of frying,the content of(E,E)-2,4-decadienal gradually decreased,while saturated aldehydes such as hexanal and octanal,low carbonic acid such as hexanoic acid,heptanoic acid,and nonanoic acid,and(E)-2-decenal,(E)-2-undecenal and phenylacetaldehyde increasd gradually.These compounds increased the fat,grass,rancid and stale flavors of French fries,which making the curve quickly entered the degradation phase.The pearson correlation analysis showed that the content of(E,E)-2,4-decadienal in the three types of French fries was significantly correlated with sensory scores(p<0.01),The corresponding time interval when the relative content of(E,E)-2,4-decadienal is higher than 60%can be used to characterize?t.Secondly,the total polar component increase value(?TPC),peroxide value increase value(?PV),the increase value of polar compounds subdivided substances,acid value,anisidine value and total oxidation value of soybean oil,palm oil,and high oleic rapeseed oil were detected during frying.The relationship between the above-mentioned indices and the relative content of(E,E)-2,4-decadienal in French fries was established.When the relative content of(E,E)-2,4-decadienal in French fries reached 60%,the coefficient of variation of the physical and chemical indexes of frying oil corresponding to t₁ and t₂ was compared to select the suitable index to characterize?t.The results showed that the smaller coefficients of variation at t₁ and t₂ are?PV and?TPC.However,?PV firstly increased and then decreased with the frying time.Therefore,the large decrease of?PV in the later stage of frying will lead to misjudgment of t₂.On the contrary,?TPC showed a good linear increase trend,and the coefficient of variation at t2 was the smallest(0.02).Therefore,?TPC is more suitable to characterize t₁ and t₂ compared to other physical and chemical indices.The French fries prepared by cottonseed oil,palm olein,rapeseed oil,high oleic sunflower oil and sunflower oil were further verified that the?TPC values at t₁and t₂were not affected by oil types.It is finally determined that the?TPC at t₁ is 3.00±1.08%,and the?TPC at t₂ is 20.57±0.51%.When the?TPC reaches this value,the corresponding time difference represents?t.Thirdly,the effects of iodine value of frying oil,frying temperature,the ratio of oil to sample,frying batches,and fresh oil replenishment frequency on TPC of frying oil were studied,respectively.Furmore,their effects on?t were investigated.The results showed that?t was negatively correlated with iodine value(x),and followed the equation,?t=-0.41x+71.54(R²=0.8242).So when the iodine value increased by 10 g/100g,the?t decreased by 4.1 h.The palm oil has the lowest iodine value content(56.03 g/100g),and linseed oil has the highest iodine value content(174.50 g/100g).The order of?t from big to small is palm oil,high oleic sunflower oil,high oleic rapeseed oil,sunflower oil,soybean oil and linseed oil.?t was negatively correlated with the frying temperature.When the temperature increased by10°C,the?t of soybean oil,palm oil and high oleic rapeseed oil decreased by 1.9 h,4.1 h and2.8 h,respectively.The?t of soybean oil,palm oil and high oleic rapeseed oil all firstly increased and then decreased with the increase of ratio of oil to sample and frying batches.The optimum ratio of oil to sample is 30:1,which is the most suitable for frying.The frying batch is 2 batches/h.Replenishing the fresh oil can prolong?t,and the faster frequency would prolonged?t.When the fresh oil replenishment frequency was 6 times/day,the?t of soybean oil,palm oil and high oleic rapeseed oil were prolonged by 84.61%,43.39%and 68.75%,respectively.Through multiple linear regression analysis,a predictive model of?t is obtained:?t=-0.934(iodine value)-0.102(frying temperature)+0.197(fresh oil replenishment frequency)-0.035(the ratio of oil to sample)-0.032(frying batches).According to the regression coefficient of the model,the iodine value of frying oil has the greatest influence on?t,followed by the fresh oil replenishment frequency.The adjustment of?t is achieved by changing the process parameters in the model.Finally,the prolongation effect of adding tert-butyl hydroquinone(TBHQ),L-ascorbyl palmitate(L-ap)and rosemary extract on the?t of soybean oil was investigated.The results showed that the?t of soybean oil added with TBHQ,L-ap and rosemary extract was extended by 31.0%,15.4%and 7.7%,respectively compared with the control group.Three kinds of antioxidants can effectively inhibit the oxidation of frying oil.Besides,the characteristic flavor compound(E,E)-2,4-decadienal can maintain a high content in a longer frying time,thereby prolonging?t.Antioxidants reduced the production of saturated aldehydes such as hexanal,heptanal,octanal and nonanal,which are oily at high concentrations,and the production of off-flavor acids such as hexanoic acid,heptanoic acid,octanoic acid and nonanoic acid,which have rancid odors,thereby delaying the time of entering the degradation period.L-ap and TBHQ are more effective than rosemary extract in inhibiting the formation of saturated aldehydes and off-flavor acids.Among them,L-ap has a better inhibitory effect on the oxidation products of linoleic acid(hexanal,heptanal,hexanoic acid and heptanoic acid).TBHQ has a better inhibitory effect on the oxidation products of oleic acid(octanal,nonanal,octanoic acid and nonanoic acid).In summary,the paper found that the flavor quality of French fries showed a“wide bell” curve with the frying process,and then we defined the?t of the frying oil during the frying of French fries.The difference of the total polar component content(?TPC)of the frying oil can be used to characterize the?t.The main effect factors of?t were further investigated.This study provided a theoretical basis and technical means for ensuring the consistency and optimum of the flavor of fried foods such as French fries.