Studies On The Effects Of Low-frequency Ultrasound On The Quality And Efficiency Of Freezing And Frozen Storage Of Broccoli

Broccoli (Brassica oleracea L. var. botrytis L.) belong to cruciferous family and arepopular for their appealing color, flavor and texture. Broccoli are rich in health-promotingcompounds such as L-ascorbic acid, vitamin E, ?-carotene, polyphenols, glucosinolate,flavonoids and selenium. These compounds are known to provide different degree ofprotection against cancer and cardiovascular diseases. However, fresh broccoli have a limitedshelf-life of less than three days at20°C. It is well known that, freezing is one of the mostsuccessful methods for long term preservation of perishable fruit and vegetable products. Thismethod is also capable of preserving initial sensory characteristics of fruit and vegetableproducts and ensures that the products remain suited for further processing. But the freezingprocess and frozen storage can have negative impact on the quality of fruits and vegetables.The main physical processes occurring during freezing and frozen storage are moisturemigration, ice crystal formation and ice re-crystallization. The crystallization andre-crystallization phenomena negatively affect the texture of fruits and vegetables, greatlyincrease the loss of nutrients and weight if drip loss during thawing becomes significant.Moreover, chemical changes occurring in frozen fruits and vegetables are closely linked withthe onset of pigment and color degradation and chemical oxidation. In order to improve thoseproblems, the effects and mechanisms of low-frequency ultrasound on the quality andefficiency of freezingand frozen storage of broccoli were studied in this paper.The application of ultrasound during immersion freezing of broccoli was studied andparticular attention was given to the effects on freezing time, microstructure, firmness anddrip loss of broccoli. Broccoli florets were immersion frozen in an ultrasound-assisted freezerat two frequencies and four different power levels. The results showed that the total freezingtime and times required for pre-cooling, phase change and sub-cooling stages of broccoli weresignificantly reduced by the application of ultrasound-assisted freezing (UAF) at150(30kHz)or175W (20kHz) power level and with judicious combination of process parameters(exposure time, ultrasound irradiation temperature and pulse mode). The microstructure andthe firmness of broccoli tissue were better preserved and the drip loss was significantlyreduced by the application of optimized UAF compared to the normal immersion freezing.In order to further explore the regularity of low-frequency ultrasound impact on thefrozen broccoli, the effect of different intensity of ultrasound on the physical and chemicalquality of frozen broccoli were studied.The comparative advantage of using UAF over normalfreezing on the freezing time, cell-wall bound calcium to total calcium ratio, texturalproperties, color, drip loss and L-ascorbic acid contents was evaluated. Results showed thatthe application of UAF at selected power levels such as0.250-0.412W/cm2decreased thefreezing time and the loss of cell-wall bound calcium content. Compared to normal freezing, the values of textural properties, color, L-ascorbic acid content were better preserved and thedrip loss was significantly minimized by the application of UAF. However, the quality of thefrozen broccoli at some other power levels such as0.209,0.498,0.514and0.630W/cm2wasinferior compared to that of the normally frozen samples.In order to explore the positive effect of low-frequency ultrasound on the heat transferduring broccoli freezing, the three dimensional unsteady heat conduction model (convectiveheat transfer boundary conditions, variable physical properties, no inner heat source) weredeveloped.The results showed that the heat transfer model has good predictive ability topredict the freeze time and calculate the heat transfer coefficient and thermal conductivity ofbroccoli. During0.250-0.412W/cm2, low-frequency ultrasound can improve the heat transfercoefficient and thermal conductivity of broccoli. Those results were provide a theoreticalsupport for ultrasound-assisted frozen broccoli.In order to research the effects of low-frequency ultrasound on the frozen storage qualityof broccoli.The effects ofultrasound-assisted osmotic dehydration on the efficiency of osmoticdehydration and glass transition temperature (T‘g) of broccoli were analyzed byusingdifferential scanning calorimetry (DSC), low field nuclear magnetic resonance(LF-NMR) and magnetic resonance imaging (MRI). The results showed thatultrasound-assisted osmotic dehydration using trehalose can affect the T‘g of broccoli bychanging the state of water within the sample structure. The values of T‘g of the osmoticallydehydrated broccoli samples ranged between-27.63°C and-23.58°C, which constituted anincrease of about2-6°C compared with that of the untreated sample. Compared with thenormal osmotic dehydration with2h, ultrasound-assisted dehydration with shorter time (30min) could get the higher value of T‘g by increased the water loss and accumulation oftrehalose and decrease the mobility of water in the broccoli cell tissue. However, when theultrasound treatment time was40min, it resulted into decrease in T‘g indicating the importantrole of treatment time in ultrasound-assisted osmotic dehydration process.In order to explore the effect of low-frequency ultrasound on the stability of broccoliduring frozen storage, the quality parameters (drip loss, color, firmness and L-ascorbic acidcontent) of ultrasound-assisted osmodehydrofrozen broccoli during six months frozen storagewere investigated. The results showed thatcompared to osmotic dehydration, theultrasound-assisted osmotic dehydration shortened the needed dehydration time and betterpreserved the firmness and L-ascorbic acid content after osmotic dehydration pre-treatment.In addition, the ultrasound-assisted osmotic dehydration minimized the drip loss, loss ofL-ascorbic acid content and better maintained the color and firmnesswhen stored at-25°C forsix months.