A combination treatment of ultrasound and ε-polylysine to improve microorganisms and storage quality of fresh-cut lettuce were investigated. Fresh-cut lettuce was treated with ultrasound (20kHz, 17-29 W/L), ε-polylysine (0.1-0.6 g/L) and their combination at 20 ºC for 10 min and then packaged as well as stored at 4 ºC for d.The results showed that 23 W/L ultrasound combined with 0.4 g/L ε-polylysine treatment was superior to 23 W/L ultrasound or 0.4 g/L ε-polylysine treatment alone in inhibiting the growth of microorganisms such as total number of colonies, mold and yeast, and total coliform counts of fresh-cut lettuce during storage. Ultrasound combined with ε-polylysine treatment reduced weight loss and total color difference (∆E), declined POD and PPO activities, maintained the higher level of total phenolics content, vitamin C and chlorophyll, as well as decreased water mobility and respiration rate in fresh-cut lettuce during storage compared to control. Therefore, these results demonstrated that ultrasound combined with ε-polylysine treatment was helpful for inhibiting microorganisms and improving storage quality, and can be an effective preservation method to extend shelf-life of fresh-cut lettuce.
Microwave heating has been applied in the drying of high-value solids as it affords a number of advantages, including shorter drying time and better product quality. Freeze-drying at cryogenic temperature and extremely low pressure provides the advantage of high product quality, but at very high capital and operating costs due partly to very long drying time. Freeze-drying coupled with a microwave heat source speeds up the drying rate and yields good quality products provided the operating unit is designed and operated to achieve the potential for an absence of hot spot developments. This review is a survey of recent developments in the modeling and experimental results on microwave-assisted freeze-drying (MFD) over the past decade. Owing to the high costs involved, so far all applications are limited to small-scale operations for the drying of high-value foods such as fruits and vegetables. In order to promote industrial-scale applications for a broader range of products further research and development efforts are needed to offset the current limitations of the process. The needs and opportunities for future research and developments are outlined.
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