Radio frequency heating to inactivate microorganisms in broccoli powder

Recently, the safety of low-water-activity (aw) foods, including dried vegetables, has become a major concern. It has been realized that microorganisms may not grow in low-aw foods but can survive for rather long period of time. But it represents significant risk especially when the low-aw foods are added to high-aw foods, because the inhibited microorganisms in low-aw foods will grow in high-aw environment. In this study, broccoli powder (aw = 0.586) was pasteurized by radio frequency (RF) treatment using a 6 kW, 27.12 MHz pilot-scale RF system. Heating patterns and temperature profiles in broccoli powder package in a polypropylene plastic pouch (17 × 12 × 5 cm) during RF heating were studied. The non-uniform heating pattern was validated, characterized by much higher temperatures (about 17–32°C) in the interior and centre of the food powder package than that in the exterior surface. Rolling over and rotation during RF treatment were proven effective to improve the heating uniformity. Microbial reduction and change of colour of broccoli powder after RF heating for different time periods were studied, and the results showed that the level of microbial inactivation was greatly reduced by 4.2 log colony-forming units (CFU)/g with insignificant colour degradation after RF heating for 5 min. The study also indicated that cold-shock treatment [kept the treated sample at freezing temperature (−18°C) for 48 h] following RF treatment further effectively reduced the microorganisms in broccoli powder from 3.0 log CFU/g immediately after RF treatment to less than 30 CFU/g, which indicated that cold shock in conjunction with RF heating is a promising technology with a potential to reduce the strength of applied RF and thus contributing to better retention of quality of low-moisture foods.