S. Wang1, M. Gao2, J. Tang1, Y. Wang3, R. Villa-Rojas4
1 Dept. of Biological Systems Eng., Washington State University
2 College of Life Science, Yangtze University
3 College of Food Sci. and Eng., Northwest A&F University
4 Dept. de Ingeniería Química, Alimentos y Ambiental, Univ. de las Américas

California is responsible for most of nut production in the United States and almond is one of the most representative nuts for food safety concerns. Over 508,000 metric tons of almonds were produced in 2006, worth more than US$2.2 billion. Almonds are the top agricultural export for California, and the leading horticultural export for the United States, with about 70% of the total crop shipped for export to more than 80 countries. A major problem of raw almonds is cross contamination of Salmonella enterica Serovar Enteritidis during harvesting, drying, hulling and shelling practices. Recent Salmonella Enteritidis incidents and increased regulations on fumigants have forced California nut industry to seek alternatives for postharvest control of Salmonella in dry nuts. Radio frequency (RF) treatment holds potential as a pasteurization method to control Salmonella in almonds without causing a substantial loss of product quality. Thermal resistance of Salmonella can be reduced by increasing water activity, thus a soaking process was designed prior to RF treatments. A pilot-scale 27 MHz, 6 kW RF heating system was used to rapidly heat 1.7 kg washed in-shell almonds with hot air heating at 55°C. To achieve appropriate heating rate, constant drying temperature and short time cooling, the RF treatment protocol was obtained using an electrode gap of 13 cm for heating, 14 cm for drying, and followed by forced room air cooling of 5-cm thick samples. The results showed that almond temperatures above 75°C at 23% moisture contents for 2-4 min RF heating could meet the requirements to achieve 5-log reduction of Salmonella. The RF treatment process for 20 min reduced the moisture content to 5.7% w.b. The RF treated almond quality was acceptable since peroxide value, fatty acids values and kernel colors met good quality standard used by nut industry. This study may address critical challenges and enhance sustainability and competitiveness of the nut industry in international markets.