Application Of Radiofrequency Heating Technology In The Low Temperature-Long Time(LTLT)Process Of Onion Powder

Onion(Allium cepa L.)is a biennial herb in Allium of Alliaceae,which consists of fleshy scales and buds.It is known as the "queen of vegetables" because of its rich nutritional value.As one of the deep-processing products of fresh onions,onion powder has low moisture content and aw is lower than 0.85.It belongs to low-moisture food,so it is generally considered as safe and storable food.However,in recent years,foodborne pathogenic bacteria such as Salmonella,Escherichia coli and Listeria monocytogenes in low-moisture food have caused many food safety incidents.Pasteurization of low-moisture food raw materials is an urgent problem to be solved.A large number of studies have shown that radio frequency heating technology has good effect and application potential on low moisture food sterilization.In this study,Salmonella enterica serova Eenteritidis Phage Tvpe 30(S.Eenteritidis PT 30),the most heat-resistant pathogenic bacteria in low-moisture foods,was taken as the target strain,and onion powder was taken as a representative low-moisture food to explore the heating characteristics of onion powder and the isothermal inactivation law of Salmonella enteritidis PT 30 in onion powder;To optimize the sterilization process of Salmonella enteritidis PT 30 in onion powder under radio frequency heating and carry out microbiological verification;Evaluate the quality change of onion powder after radio frequency heating treatment.The main contents and results of this study are as follows:1.Explore the thermal stability of different aw onion powder and the inactivation rule of Salmonella enteritidis PT 30 in onion powder under different temperature conditions.The law of onion powder with different moisture content changing with temperature was analyzed by differential scanning calorimeter.The results showed that different aw onion powder showed endothermic peaks in a wide temperature range(from 20 to 240 ℃),and different aw had a great impact on the phase change temperature and characteristic peak properties of onion powder.The larger the aw,the smaller the peak width,and the lower the peak temperature.The higher the aw,the lower the peak temperature,and the melting point is concentrated between 118～148 ℃.The specific heat capacity of onion powder increases with the increase of temperature and water activity,and the higher the temperature,the smaller the particle size and the greater the specific heat capacity.The isothermal inactivation rule of Salmonella enteritidis PT 30 in onion powder at 60 ℃,65 ℃ and 70 ℃ was obtained by TDT experiment.The results showed that the four orders of magnitude reduction of Salmonella enteritidis when heated to 60 ℃,65 ℃ and 70 ℃ required heat preservation for about 40,24 and 6 hours respectively.The fitness of Weibull model for the dynamics of Salmonella enteritidis inactivation of onion powder(aw,25 ℃=0.32 ± 0.02,water content 7.74 ± 0.1%,w.b)at 60 ℃,65 ℃ and 70 ℃ was higher than that of Log-linear model.2.The low temperature-long time(LTLT)process of onion powder in packaging by RF heating was optimized by orthogonal test.The radio frequency heating process was optimized by selecting the electrode plate spacing,the radio frequency processing time and the material thickness as single factors,combining with the orthogonal test and taking the heating uniformity as the evaluation index.The results showed that the optimum technological conditions for radio frequency heating were the electrode plate spacing of 800 mm,the heating time of 210 s,and the material thickness of 51 mm.Under these conditions,the hot spot temperature of onion powder heated by radio frequency is 88.7 ℃,the cold point temperature is 66.1 ℃,and the heating uniformity index is 0.107;Select non-woven fabrics as packaging materials,inoculate Salmonella enteritidis PT 30,and verify the germicidal effect under the optimal process conditions.The results show that the bacterial count of Salmonella enteritidis in the HA 66 ℃ group decreased slowly during hot air heating,and the bacterial reduction was 2.23 ± 0.02 log CFU/g after 48 hours of heating,and the bacterial reduction of Salmonella enteritidis in the RF-HA 66 ℃ group reached 3.59 ± 0.02 log CFU/g after 40 hours of radiofrequency treatment and heat preservation,and the same 40 hours of heat treatment,HA 66 ℃ bacteria reduction was only 2.14 ± 0.07 log CFU/g.By fitting the RF sterilization kinetic curve,Weibull model is better than Log-linear model.3.Evaluate the effect of RF heating on the quality of onion powder.By measuring the changes of water content,water activity,color difference and volatile matter content of onion powder before and after RF sterilization,and comparing with traditional hot-air sterilization,the effects of RF treatment on the quality of onion powder were analyzed.The results showed that the changes of water content,water activity and color difference showed a downward trend,but the impact on the finished product was not significant;However,RF-HA has better germicidal efficacy than HA,and has certain advantages in controlling microbial carrier.A total of 36 volatile compounds(matching coefficient>80%)were identified,mainly including sulfur compounds,aldehydes,alcohols and alkanes,no serious quality loss occurred.