Pasteurization Technologies Of Postharvest Almonds Using Radio Frequency Heating

Almonds,walnut,pistachio and hazelnut are the most four famous tree nuts in the world.Almond kernels are rich in free fat acidity,vitamins,protein and minerals with good health beneficial and pharmacological effects.In recent years,due to increasing domestic consumption demand and encouraging national policies,the production volume and output of China’s almonds are increasing year by year,but it still can’t meet the domestic market demand,and a large amount of almonds also needs to be imported from United States every year.However,the food safety problems caused by Salmonella contamination in post-harvested almonds restrict the development of the almond processing industry in the world.Therefore,reducing the Salmonella population in almonds to an acceptable level,and ensuring the safety and quality of almonds become major concerns,which need to be studied and solved in nut industry.Nonpareil almond,which is the major consumption variety,was chosen as the representative product sample to develop radio frequency(RF)pasteurization technology.Then the mechanism and thermal resistance of non-pathogenic Escherichia coli ATCC 25922to replace Salmonella under thermal stress using test cell methods and the pasteurization process for in-shell almonds and almond kernels were studied.The detail research objectives include:(1)the thermal tolerance of Salmonella PT 30 and Escherichia coli ATCC 25922 in almonds was compared,and the thermal resistance and heat shock protein of Escherichia coli ATCC 25922 were studied with test cell methods;(2)thermal and dielectric properties of the almond kernels were measured using thermal property analyzer,and open-ended coaxial-line probe with impedance analyzer at different temperatures and moisture contents;(3)the protocol for RF treatments to control surrogate microorganism Escherichia coli ATCC 25922in in-shell almonds was developed and validated;(4)the protocol for RF pasteurization process for almond kernels was also studied and verified;(5)the influence of RF heating and storage time on the quality of almonds was evaluated.The main contents and results of this dissertation were as follows.(1)The thermal tolerance of pathogenic Salmonella PT 30 and non-pathogenic alternative Escherichia coli ATCC 25922 in different moisture content of almonds was compared,and the thermal resistance and heat shock protein of Escherichia coli ATCC25922 were studied with test cell methods.The results showed that the thermal resistance of Escherichia coli ATCC 25922 in almonds was higher than that of Salmonella PT 30.The D-value of Escherichia coli ATCC 25922 decreased with the temperature increase.The preheating conditions at three sub-lethal temperatures resulted in increased heat resistances of E.coli with maximum D-values after heat shock at 45℃ for holding 5 min.The trend of relative mRNA level after heat shock treatment was similar to that of D-values.The increased heat resistance could be eliminated by adding one-day storage of the contaminated products at room or cold temperatures.Avoiding heat shock proteins or increasing temperatures or treatment times must be considered to ensure food safety while designing effective thermal processes to control E.coli ATCC 25922 in postharvest agricultural products.(2)The thermal properties,such as specifc heat capacity and thermal conductivity of almonds with different moisture content and temperature were measured using thermal property analyzer.The dielectic properties,such as dielectric constant and loss factor of almond kernels were measured using an open-ended coaxial-line probe with impedance analyzer at frequencies between 10 and 3000 MHz,moisture contents between 4.2%to 19.6%w.b.and temperatures between 20℃ and 90℃.The temperature profiles of RF heated almond kernels under three moisture levels were made using experiment and computer simulation based on measured thermal and dielectric properties.The results showed that specific heat capacity and thermal conductivity of the almond kernels increased with increasing temperature and moisture content.Both dielectric constant and loss factor of the almond kernels decreased sharply with increasing frequency over the RF range(10~300MHz),but gradually over the measured MW range(300~3000 MHz).Both dielectric constant and loss factor of almond kernels increased with increasing temperature and moisture content,and largely enhanced at higher temperature and moisture levels.Quadratic polynomial equations were developed to best fit the relationship between dielectric constant or loss factor at 27,40,915 or 2450 MHz and sample temperature/moisture content with R~2greater than 0.967.The heating rates of three different moisture contents of almond kernels in the process of RF heating were in agreement with the heating rates simulated by computer,which proves the reliability of data of thermal and dielectric properties of almond kernels.Penetration depth of electromagnetic wave into samples decreased with increasing frequency(27~2450 MHz),moisture content(4.2%~19.6%w.b.)and temperature(20℃~90℃).The penetration depth at 27 MHz RF heating with moisture content of 11.9%w.b.and temperature of 60℃ was 90.62 cm,while that for 2450 MHz heating with the same moisture and temperature was only 2.74 cm.The larger penetration depth indicated that RF heating might be more practical in developing industrial-scale pasteurization treatements for almonds with better heating uniformity.(3)An effective RF pasteurization process for in-shell almonds was developed.In-shell almonds were soaked for 75 s to make the moisture content of almond shell to achieve 41.6%w.b.,and equilibrated for 1 h before RF treatments.Then hot air(55℃)assisted RF treatments with the electrode gap of 10.5 cm for 1.5 min were used for pasteurization to achieve 5-log reduction,and with the electrode gap of 12 cm for 8.5 min used for drying.After that,the RF treated almonds were cooled by forced room air for 24min in 3 cm deep layer.(4)The effective protocol of RF treatment for almond kernels was obtained and validated.The moisture content of almond kernel was adjusted to 10.01%w.b.by directly adding pre-calculated distilled water to reduce the thermal resistance of E.coli ATCC 25922.The preconditioned almond kernels were equilibrated for 7 days before RF treatments.Then the inoculated almond kernels were pasteurized with the electrode gap of 11 cm,72℃ hot air,intermittently rearranging the five-layers in a container from L1,L2,L3,L4,and L5 to L5,L4,L3,L2,and L1,and holding for 15 min.This protocol was used for pasteurization to achieve more than 4-log reductions of E.coli ATCC 25922,drying to reduce the moisture content to 7.06%w.b.,and cooling by forced ambient air in the single layer for 8 min.(5)Effects of RF treatments and storage time on quality of in-shell almonds and almond kernels were evaluated.The moisture contents of in-shell almonds and almond kernels were both reduced after RF treatment and storage,but no significant difference was found between treated in-shell almonds and controls,and significant difference was observed between treated almond kernels and controls.The mean peroxide value(PV)and free fat acids(FFA)values of in-shell almonds and almond kernels increased with RF treatment and storage time,however,the values remained within the marketable range(PV<1.0 meq/kg and FFA<0.6%)according to the nut industry standard.The color values of kernel skin,inner kernel and ground kernel showed that there were no significant differences(P>0.05)between control and RF treatments during the storage.The fatty acid(FA)composition showed that there was no significant difference(P>0.05)in the five major FA compositions between control and treated samples after RF treatment and during the storage.