Study On The Processing Technology And Browning Mechanism Of Vacuum Fried Crispy Pleurotus Eryngii

In this study,the pleurotus eryngii was processed into a kind of ready-to-eat crispy product through the method of vacuum frying.The effects of pretreatments and process parameters on the quality of product were investigated.Then the vacuum frying parameters of pleurotus eryngii were optimized.In order to study the browning mechanism of the crispy pleurotus eryngii during storage,the enzyme activity of polyphenol oxidase（PPO）left over in crispy pleurotus eryngii were determined and the effects of storage time on the content of free amino acids,5-hydroxymethyl furfural（5–HMF）,reducing sugar,malondlaldehyde（MDA）and lipid peroxide value of crispy pleurotus eryngii were analysed.Then the effects of storage methods（airtight storage,low temperature storage and light-away storage）on the browning of crispy Pleurotus eryngii were studied.After that,the browning pigment of crispy Pleurotus eryngii was extracted through the method of alkali-solution and acid-isolation and the preliminary structural identification of this pigment was carried out.The principal research results were listed in following paragraphs.1.After immersed in maltose solution,the solid content of pleurotus eryngii improved and the oil content of vacuum fried crispy product decreased significantly（P<0.05）.The color protection trearment can significantly improve the color of the crispy pleurotus eryngii（P<0.05）.The suitable color protecting agent consist of 0.5%salt and0.3%citric acid.Both the temperature and time of vacuum frying affect the crispness,oil content and moisture content of crispy product significantly（P<0.05）.With the increase of the frying temperatureand time,the moisture content of product decreased while the oil content and crispness enhanced.The sequence of factors that have significant effects on the sensory quality of crispy pleurotus eryngii are as follows:vacuum frying temperature>vacuum frying time>the concentration of maltose solution>color protecting agent.The pleurotus eryngii was first treated with 0.5%salt and 0.3%citric acid for color protection and soaked in 24%（w/v）maltose solution for 1h.After,the pleurotus eryngii was fried at 90℃for 13min and the vacuum degree is 0.096MPa.Then the product was deoiled at the speed of 300 r/min for 8 min.The crispy pleurotus eryngii manufactured under these conditions has the yield of 11.08%,the moisture content of 5.27%,the oil content of 22.68%,the brightness（L value）of 79.17 and the crispness of 398g.2.The enzymes in vacuum fried products involoved in browning ractions have been inactivated.The residual enzyme activity of polyphenol oxidase is only 1.72%compared with that in fresh pleurotus eryngii.The possibility of enzymatic browning during storage can be ruled out.Besides,the browning degree has good correlations with the content of5–HMF,free amino acids,reducing sugar,MDA as well as the lipid peroxide value of sample.With the extension of storage time,the content of 5–HMF increased while the content of reducing sugar and free amino acid decreased,indicating that maillard reaction was involoved in the color deterioration of crispy pleurotus eryngii during storage.The content of MDA and the peroxide value of oil extracted from crispy sample increased with the extension of storage time,indicating that the color accumulation in crispy pleurotus eryngii is related to oil autoxidation.3.The browning pigment of crispy pleurotus eryngii was extracted through the method of alkali-solution and acid-isolation.On the basis of the four factors three levels orthogonal test,it is concluded that the sequence of factors’influence on the effect of extraction are as follows:extraction temperature>the solid-liquid ratio>Na OH concentration>extraction time.The optimal extraction conditions including a extraction temperature of 60℃,a solid-liquid ratio of 1:40（w/v）,a extractant concentration of 8%and a extraction time of 80 min.The yield and color value of pigment extracted under these conditions are 11.22%and 60.80 respectively.The yields and color values of browning pigment extracted from fresh crispy samples（stored under room temperature for less than 1 month）and browning crispy samples（stored under room temperature for 4months）were calculated.Before purification,there is no significant difference（P>0.05）between the yields of pigment extracted from fresh and browning crispy samples while the color value of pigment extracted from browning samples is much higher than that extracted from fresh crispy samples.After purification,the color values of pigment extracted from both fresh and browning crispy samples increase significantly（P<0.01）.The yield of purified pigment in browning samples is about 3 times of that of fresh samples.It is obvious that the storage time has positive correlation with the accumulation of browning pigment in crispy samples.4.The browning pigment of crispy pleurotus eryngii underwent long-term storage was purified and identified.The UV-visible spectrum of the pigment showed that the browning pigment has maximum absorption at 209 nm.There are also weak absorption peaks around 260-280nm.The FT-IR spectrum showed that there are long-chain compounds that contains more than 4 group of-CH₂-in the browning pigment.Besides,compounds with C=O and high conjugated C=C are also found in the browning pigment.The HPLC-MS spectrum showed that the browning pigment was a mixture that contains at least five compounds whose molecular weights were 554,562,582,600 and 642respectively.In these compounds,there are long chain fatty acid esters and derivatives produced by fatty acid oxidation.Besides,the existence of amides,aldehydes,ketones and carboxylic acids suggested that maillard reaction may also involved in this color deterioration during long-term storage.However,the complexity of chemical components in browning pigment demonstrated that the browning of crispy pleurotus eryngii during storage may caused by interactions among multiple reactions and further studies are needed to get fully understand of its mechanism.5.The effects of 3 diffierent storage methods（light-away storage,4℃storage and airtight storage）on the browning degree of crispy pleurotus eryngii during storage were determined.The results showed that all of the 3 storage methods inhibited the color deterioration of crispy sample significantly（P<0.05）.The 4℃storage has the best browning-inhibition effect,followed by the airtight storage and light-away storage.In the practical production of crispy pleurotus eryngii,the product can be packaged with nitrogen and opaque material,storaged at low temperature environment so as to prolong its shelf life.