Study On The Effects Of Thermal Treatments On Antioxidant Activity,Phenolics And Volatiles Of Honey

The quality of honey has always been of great international concern.Thermal treatments are common processing units in the industrial production of honey,which are used to delay the crystallization,facilitate the filtration of impurities and prevent fermentation.However,thermal treatments may affect the quality,chemical composition and bioactivity of honey.The effects of thermal treatments at different temperatures（45℃,65℃and 80℃）on the physicochemical properties,antioxidant activity in vitro,phenolic compounds and volatile compounds of honey were investigated.This study focused on the search for chemical markers of overheated honey and the development of a method for the identification of overheated honey to provide a theoretical basis and industrial guidance for honey quality control.This study was divided into four chapters,the main contents of which are as follows:1.The effects of thermal treatments on the physicochemical properties and antioxidant activity in vitro of acacia honey were analyzed by chemical methods.The results showed that the content of 5-hydroxymethylfurfural（5-HMF）and total phenols increased significantly with the increasing temperature and time,while diastase activity and proline content decreased significantly.Besides,thermal treatments significantly increased the 1,1-Diphenyl-2-picrylhydrazyl（DPPH）radical scavenging power and Fe³⁺reducing power of honey.The chemometric results showed that the total phenolic content of honey was positively correlated with Fe³⁺reducing power of honey and DPPH radical scavenging power.2.The effects of thermal treatments on phenolic standards was analyzed by high performance liquid chromatography（HPLC）,and variation of 2,4-dihydroxybenzoic acid under acacia honey system during thermal treatments was investigated by liquid chromatography-mass spectrometry（LC-MS/MS）.The results showed that 2,4-dihydroxybenzoic acid was degraded to resorcinol after thermal treatments.The degradation mechanism of 2,4-dihydroxybenzoic acid in thermally processed honey was analyzed by chemical kinetic and thermodynamic methods.The results showed that the degradation of 2,4-dihydroxybenzoic acid accorded with the zero-order kinetic reactions at 45°C and the second-order kinetic equation at 65°C and 80°C.The degradation rate was the fastest at 80°C,with rate constants and half-lives of 8.4×10^-3mg³?kg^-1?h^-1 and 20.71 h.The thermodynamic parameters indicated that the degradation of 2,4-dihydroxybenzoic acid in the honey system was a non-spontaneous,endothermic reaction.3.A total of 56 volatile compounds were identified from acacia honey and jujube honey samples before and after thermal treatments by headspace solid-phase microextraction gas chromatography mass spectrometry（HS-SPME-GC-MS）.The contents of linalool oxide,limonene,maltol and methyl nonanoate increased,while the contents of heptanal,2,3-dihydro-3,5-dihydroxy--6-methyl-4H-pyran-4-one,1-nonanol and some easter compounds decreased.In addition,new volatile compounds such as nerol oxide,α,4-Dimethyl-3-cyclohexene-1-acetaldehyde and 2,4,6-trimethylbenzaldehyde may be generated during thermal treatments of acacia honey.Jujube honey may generate new compounds such as dimethyl trisulfide,2,4-Dihydroxy-2,5-dimethylfuran-3（2H）-one and 2,5-dimethylbenzaldehyde after thermal treatments.After heat treatment at 80°C,the major aroma components of acacia honey decreased from 16 to 13,while jujube honey increased from 17 to 18.The floral and green aromas of jujube honey were significantly reduced after thermal treatments,and the new compounds（dimethyl trisulfide）as the major aroma components contributed to unpleasant aroma（sulfur）.