Pasteurization Technology Of Wheat Products With Different Physical Structures Using Radio Frequency Energy

Salmonella and Escherichia coli in wheat and wheat-based products can survive for a long time,and multiply rapidly under certain suitable conditions.In order to control the quality of wheat-based food,prolong the shelf life of the products and reduce the risk of people infecting with pathogenic bacteria in wheat products,it is necessary to pasteurize wheat products using non-chemical methods.As a novel thermal treatment method,radio frequency(RF)heating technology has been widely used in the pasteurization of agricultural products and foods.At the same time,it can meet the pasteurization requirements while maintaining product quality.However,there is a lack of in-depth and systematic study about the influence of the corresponding food physical structural changes during the processing of wheat-based products on the heat resistance of microorganisms,and the RF pasteurization methods of wheat products with different physical structures.In this study,we chose wheat in granule and powder structures as the representative foods to study the RF pasteurization technology of wheat products.The specific research contents include:(1)the influence of different physical structures of wheat on heat resistance of surrogate bacteria was studied by thermal death time(TDT)heating block system(HBS);(2)the dielectric properties of wheat flour with different moisture content and wheat bran content were measured by an impedance analyzer;(3)the effect of moisture content and wheat bran content on the heating rate and uniformity of wheat flour was determined in the RF heating system;(4)a novel method on improving RF heating uniformity by adding aluminum foil sheet to the container was established;(5)the protocol of RF pasteurization for wheat grains and flour was developed and validated,and the influence of RF heating on enzyme activity and physical and chemical quality of whole wheat flour was investigated;(6)the influence of RF pasteurization on the quality of various wheat products was determined during storage.The main results are as follows:(1)The heat resistance of three representative strains in wheat grains and whole wheat flour was compared by using the TDT HBS,and the most heat-resistant strain was selected to study the thermal-lethal kinetics in wheat samples with different physical structures.The results showed that Enterococcus faecium NRRL-2354(E.faecium)had the strongest heat resistance among the three strains,and could be used as a non-pathogenic heat-resistant surrogate microorganism for Salmonella typhimurium ATCC 50115 and Salmonella braederup H9812,and Escherichia coli ATCC 25922.The heat resistance of E.faecium was affected by the physical structural changes of wheat materials,and showed the strongest heat resistance in granule structure among wheat grain,flour,and dough.Therefore,an effective technology for pasteurizing the wheat products during production process should consider the moistening stage of wheat grains to ensure the quality and safety of wheat products.(2)Dielectric properties of wheat flour at different moisture content and different wheat bran content in the frequency range of 10-3000 MHz and the temperature range of 25-85 °C were measured by the impedance analyzer.The results showed that the ε’ and ε"of wheat flour increased with the increasing moisture content and temperature,and had abrupt changes under the fixed temperature,which is related to moisture content and gelatinization.The ε’of wheat flour gradually decreased with the increase of the frequency,and showed a rapid downward trend after reaching the gelatinization temperature;the ε"of wheat flour had little change with the increase of the frequency under low temperature conditions.Then it decreased rapidly within 30-300 MHz and showed a slight increasing trend as the frequency continued to increase when the corresponding gelatinization temperature was reached.The ε’and ε"of wheat flour decreased gradually with the increase of wheat bran content.A polynomial regression equation among ε’and ε"of wheat flour,moisture content/wheat bran content and temperature at four given frequencies(27,40,915 and 2450 MHz)was established,and the cubic/quadratic model was well used to predict the different dielectric properties of wheat flour at moisture content or bran content(R~2>0.936/0.997,p<0.0001).(3)The RF heating rate and uniformity of wheat flour with different moisture content and wheat bran content were analyzed.The results showed that the heating rate of wheat flour first decreased and then increased with the increase of moisture content and increased with the increase of the wheat bran content between 0 %-15 %.The variation in the RF heating rates of wheat flour with changes of moisture contents and wheat bran contents can be explained by the difference between ε’and ? ε".The RF heating uniformity of wheat flour gradually became better with the decrease of moisture content and wheat bran content.The information obtained in this study is helpful to provide effective RF pasteurization technology for wheat flour and its bran mixture,as well as wheat products such as wheat grains and wheat kernels.(4)A method of improving the RF heating uniformity of wheat grains was proposed by changing the placement position,size and thickness of the aluminum foil sheet on the outer wall of the rectangular container.The results showed that RF heating behavior of wheat grains varied by changing the orientation of the foil sheet.When the foil sheet was added to the vertically oriented wall of the rectangular box,a new cold spot appeared in the samples near the foil sheet,while the hot spots appeared on the opposite side of the foil sheet.The RF heating uniformity of the wheat grains was reduced with the presence of the foil sheet on the vertically oriented wall.When the foil sheet was added to the horizontally oriented wall,the cold spot was located in the center of the sample,and the hot spots were located in the corners and edges.The foil sheet at the container bottom can concentrate more RF energy in the cold spot area of the sample,thus an increasing heating rate and improving RF uniformity of the sample were observed.A better temperature distribution of treated wheat grains could be obtained by increasing the foil sheet size and thickness.The information obtained in this study is helpful to develop an effective RF pasteurizing technology in wheat grains at middle and high moisture contents with providing better heating uniformity.(5)A RF pasteurization process for wheat grains and whole wheat flour based on the optimized heating uniformity was established and verified.The results showed that the target temperature/time required to inactivate 4 log CFU/g E.faecium in wheat grains was lower/shorter than that required for E.faecium in whole wheat flour.As the target temperature increased,the required holding times of both wheat grains and whole wheat flour samples became shorter.When the target temperature reached 85 °C,both wheat grains and whole wheat flour could reach a target bacterial reduction of more than 4 log CFU/g,and the enzyme activities decreased by 70.4 % and 90.7 %,respectively.After the wheat grains were heat-treated,the brightness of the whole wheat flour increased,and the color difference ΔE was less than 2,which still met the requirement of the industry standard;when the whole wheat flour went partial gelatinization,the gelatinization characteristics were improved,and the change in the rheological properties was small.All quality changes were within the acceptable range,indicating that a quality improvement of the whole wheat flour to a certain extent was observed.Therefore,the RF treatment for wheat sample with the granular physical structure can improve the pasteurization efficiency of wheat products and ensure product quality.(6)The wheat grains and kernels were treated by RF heating combined hot air treatment,and the qualities of their recombinant samples(whole wheat flour with 30 % wheat bran content,whole wheat flour with 15 % wheat bran content,wheat flour with 0 % and wheat kernel flour)and processing samples(whole wheat flour,wheat flour,wheat bran flour,and wheat kernel flour)during storage stage were studied.The results showed that the heating rate of wheat grains was faster than that of wheat kernels,and an increasing heating rate and better heating uniformity were observed in middle-layer sample with the foil sheet at the container bottom.When the cold spot temperature of the sample reached 85 °C,the E.faecium in wheat grains and wheat kernels decreased by 9.09 and 4.68 log CFU/g,respectively,indicating that both of them reached the reduction of target bactericidal populations.After heat treatment,wheat flour with 30 % wheat bran content,wheat flour with 15 % wheat bran content,wheat flour with 0 % and wheat kernel flour were partially gelatinized,and the gelatinization characteristics were improved.Among them,the peak viscosity,trough viscosity,disintegration value,final viscosity and retrogradation value all increased with the increase of bran content.During the storage period,the moisture content of the whole wheat flour,wheat flour,wheat bran flour and wheat kernel flour changed little after heat treatment,but the water activity increased.Both moisture content and water activity had steady increase trends over the storage time.There was no significant difference in ash,fat,protein and fiber in four wheat processed products with the extension of storage time(p>0.05).The ash and fat contents in the four samples increased with the increase of wheat bran content.The color of each control wheat samples gradually darkened over a month,possibly due to the Maillard reaction.After RF combined with hot air treatment,the color brightness of each sample increased,but did not change significantly with the increase of storage time.The small color difference( ΔE<2)of the four processed samples during storage time indicated that the color of samples still met the requirement of the industry standard.The fatty acids of all samples became lower,and increased with the increase of storage time,and all of them were kept below the industry standard.Therefore,the RF heating technology provides a practical,effective and environmentally friendly pasteurization method for wheat products,while ensuring the product quality.