Effect Of Ultrasonic Treatment On Quality Of Pumpkin Juice During Storage

In recent years,ultrasound is considered as a potential non-hot fruit and vegetable juice processing technology.Existing studies have shown that ultrasonic treatment can improve the color properties,nutrition and sensory quality of fruit and vegetable juice,but the impact of ultrasonic on the quality of freshly pressed pumpkin juice and prolong the shelf life is uncertain.In this study,fresh squeezed pumpkin juice(Cucurbita Moschata D.)was treated by ultrasound,a constant frequency of 25 k Hz with different power(200,400,600 W)for 10 min,the samples were stored for 0,4,8,and 12 days under 4 dark conditions,and subsequently analyzed.The combined effects of power level and storage period on color parameters(L~*,a~*,b~*,C~*,h~0,ΔE,BI,YI),carotenoid content(β-caroteneα-carotene,β-cryotxanthin,zeaxanthin,lycopene),total phenol content,total antioxidant activity,enzyme activity(polyphenol oxidase and peroxidase),texture(turbidity,particle size,microstructure change and rheological properties)were investigated.This study provides a theoretical basis for further optimizing the conditions of ultrasonic processing of freshly squeezed pumpkin juice,so that ultrasonic processing could be applied production of pumpkin juice on an industrial scale successfully.The main results of this study are as follows:1.The effect of ultrasonic treatment on sterilization of pumpkin juice during storage period:The results showed that the levels of microorganisms in pumpkin juice decreased especially at higher ultrasound power ranging from 200 to 600W.Ultrasound treatment with higher power was sufficient to inactivate total aerobic bacteria,molds and yeasts in pumpkin juice.The total number of yeast and mold are at a lower level at storage 12days.Therefore,it was concluded that the higher ultrasound power can effectively maintain microbiological safety,and extend the shelf life of pumpkin juice.2.Influence of ultrasonic treatment on color and antioxidant activity of pumpkin juice during storage period:1)Ultrasound treatment(US200,US400,US600)did not induce changes(P>0.05)in the L~*,a~*,b~*,C~*,h~0,ΔE,BI,and YI values of pumpkin juice.Fr om 8 to12 days of storage,the US0 values of L~*,a~*and b~*were significantly decreased,which significantly increased on US400(ΔE>2).However,h0 and YI values did not significantly change(P>0.05)either by ultrasonic treatment o r during storage.During storage,the unstable change of BI was closely related to the change of enzyme activity in pumpkin juice.2)A significant increase(P<0.05)in carotenoid content(β-caroteneα-carotene,β-cryotxanthin,zeaxanthin,lycopene)was observed in the pumpkin juice processed by ultrasound when compared to that at US0.This is closely related to the mechanical rupture of pumpkin juice cells caused by ultrasound.Moreover,the carotenoid content of US400 was always the highest after ultrasonic treatment and during the storage period.3)Ultrasonic processing increased the content of total phenol content and the total antioxidant activity in freshly squeezed pumpkin juice significantly(P<0.05).The variation trend between the total antioxidant activity and the content of total phenol in pumpkin juice was similarly,which enhanced with the increase of ultrasonic power and decreased with the extension of storage time.The content of total phenolic and total antioxidant activity of samples treated by ultrasound decreased more slowly during storage compared with US0.4)There was a significant decrease in the activity of PPO and POD.On 12 days of storage,PPO and POD residual activity of all treated samples was higher than that on day 0.Obviously,under the experimental conditions of this study,ultrasound alone could not completely inactivate PPO and POD in pumpkin juice.3.Influence of ultrasonic treatment on texture of pumpkin juice during storage period:1)Ultrasonic processing(200,400,600 W)significantly reduced the particle size of pumpkin juice,the particle size decreased 48.82%,56.03%,56.53%when compared to that at US0.During the whole storage period from 0 to 12 days,for US200,the particle size of pumpkin juice increased by 21.95%during storage,followed by US400(18.46%),US600(14.85%),and US0(2.02%).2)Among all the treatments,the cloud values of US200,US400,and US600 were significantly different,and the cloud values of pumpkin juice increased with increasing ultrasonic power.On days 4,8 and 12 of storage,the cloud value of the US0 samplesignificantly decreased,while the cloud values of US200,US400,and US600decreased slowly and remained more stable with the increase in storage time.The results showed that the stability of pumpkin juice treated by ultrasonic wave was improved.Furthermore,the destruction of tissue cells caused by cavitation leads to the release of intracellular compounds(such as carotenoids)during ultrasound treatment,which may also contribute to the increase in the cloud value of the juice.3)This study confirmed that pumpkin juice cells were sensitive to ultrasonic power.When the ultrasonic power was 600 W,the cell wall of pumpkin juice was broken and no intact cell structure was observed under an optical microscope.After 4–12 days of storage,no significant changes were observed in the cells of all treated pumpkin juice samples compared to that on day 0.4)The shear stress,apparent viscosity,storage modulus and loss modulus of pumpkin juice changed significantly and complicated during ultrasonic treatment and storage.At the 12th day of storage,the shear stress and apparent viscosity of US200were significantly higher than those of US0,US400 and US600,and US600 had the lowest shear stress and apparent viscosity during the whole storage period.Similarly,at the 12th day of storage,both storage modulus and loss modulus show an increasing trend with the increase of frequency.US200 has the highest value at the same frequency,followed by US0,US400 and US600.Given the sum of the results of this study,pumpkin juice is suitable for low temperature(25℃)and high-power treatment(400 W/10 min).Pumpkin juice has better product quality,bactericidal effect,effective inhibition of enzyme activity,carotenoid content,total antioxidant capacity,cloud value and rheological properties.Therefore,ultrasonic is a new technology that can improve the quality of fresh squeezed pumpkin juice and ensure its quality and safety.