Study On Qualiyt And Efficiency Of The Whole Handling Process Of Frozen Strawberry Enhanced By Low-frequency Ultrasound

Freezing is one of the commonly used methods to preserve the quality of fruits andvegetables. It can not only prevent microbial growth and reproduction, also can reduceactivity of endogenous enzymes in fruits and vegetables, thus reducing the occurrence ofbiochemical reactions. The quality of frozen fruit and vegetable is closely related to each linkin the process. Therefore, in order to produce frozen fruit and vegetable products with highquality, it is necessary to strictly control each processing units, including raw material select,pretreatment, freezing process, frozen storage and thawing. In the thesis, strawberriy was usedas test material, through using low-frequency ultrasound in each operating units to improvethe quality of frozen strawberry and related products.The effect of thermal and low-frequency ultrasound treatments on the PPO activity,physicochemical properties and bioactive compounds of frozen strawberry puree wasinvestigated. Results showed that PPO activities in strawberry purees significantly decreasedduring low-frequency ultrasound or thermal treatments. No significant difference in pH, totalsoluble solids and titratable acidity was observed among the treated and control sampleswhether thermal or low-frequency ultrasound treatments were used. Low-frequencyultrasound treatments significantly increased the electrical conductivity, while significantlyreducing the viscosity of strawberry purees. Low-frequency ultrasound treatments were foundto better maintain or increase the bioactive compounds and the color attributes compared tothe thermally treated samples.The effect of ultrasound irradiation temperature and ultrasound intensity on the freezingparameters of strawberries was studied. The application of ultrasound irradiation at differenttemperatures was able to induce nucleation at lower degree of supercooling compared to thecontrol samples. The achieved degree of supercooling in the ultrasound irradiated strawberrieswas to be linearly correlated to the ultrasound irradiation temperature. The application oflow-frequency ultrasound at higher intensities was found to effectively shorten the CFT. Thedegree of supercooling in low-frequency ultrasound irradiated samples was found to be notlinearly related with ultrasound intensity. Low-frequency ultrasound assisted freezing wasfound to better maintain the microstructure and firmness, thus reducing the drip loss of frozensamples.The effect of low-frequency ultrasound and pulsed-vacuum treatments on thedehydration kinetics and the status of water during osmotic dehydration of strawberries wasinvestigated. Nuclear magnetic resonance (NMR) was used to determine the spatialdistribution and status of water within the cellular and intercellular spaces. Differentialscanning calorimetry (DSC) was used to determine the thermodynamic properities ofstrawberriey samples. The Peleg’s model fitted the experimental water loss and solid gainkinetics data well. The highest water loss occurred while using low-frequency ultrasoundtreatment, while the highest solid gain were achieved by pulsed-vacuum treatment. NMR results were able to quantify the effect of water-osmotic solute exchange on the cellcompartments (vacuole, cytoplasm plus intercellular space and cell wall). DSC results showedthat the decrease in water content and increase in the osmotic solutes depressed the initialfreezing point and the freezable water content in osmotically dehydrated strawberries.Comparative study on the effect of osmotic dehydro-freezing and immersion freezing on thequality of thawed strawberries. Results showed that osmotic dehydration pretreatmentsaccelerated freezing rate, reduced the drip loss and L-ascorbic acid loss, and improved thefrmness of samples.The effect of low-frequency ultrasound treatments on the shelf-life and quality of frozenstrawberry was investigated. The predictive model of shelf-life was estabished based on thedegradation of L-ascorbic acid in osmotic dehydro-frozen strawberry during storage, and thereliability of the model was verified by sensory evalution. The storage stability oflow-frequency ultrasound assisted frozen strawberry were studied. With the extension offrozen storage time, the bioactive components (total phenol, anthocyanins and ascorbic acid)and DPPH free radical scavenging activity gradually decreased, and there was a linearlyrelationship between bioactive components and antioxidant capacity. GC-MS results showedthat the relative content of esters in frozen strawberries after8months storage reduced, whilethe relative contents of aldehydes and alcohols increased. The frozen strawberries srored at-18oC and temperature fluctuation mode for8months produced irritating odor, which wasascribed to the appearance of isooctanol.The effects of different thawing methods on the thawing rate and nutritive value ofstrawberry purees were investigated. Results showed that microwave thawing andlow-frequency ultrasound assisted thawing significantly shorten thawing time, compared towater immersion thawing. Low-frequency ultrasound assisted thawing were found to bettermaintain titratable acid, reducing sugar and L-ascorbic acid content. With the increase ofultrasound intensity among3.20-7.03W/cm2, the electrical conductivity of thawed strawberrypuree gradually decreased. Compared with water thawed samples, low-frequency ultrasoundthawed samples had lower viscosity, and the higher the ultrasound intensity, the lower theviscosity. LC-MS analysis showed that there were9kinds of anthocyanins in thawedstrawberry puree, and four main kinds of anthocyanins were Pg-3-O-glu, Pg-3-O-rut,Cy-3-O-glu and Pg-3-O-mal-glu, respectively.