Process engineering variables in the spray washing of meat and produce

Recently, much attention has been focused on the safety of fruits and vegetables. Washing is a fundamental operation in the processing of produce. Aqueous spray energy can be, and often is, used to remove mineral, chemical, or biological contaminants from produce. A few advantages of spray washing over washing by dipping, soaking, or gravity rinse are increased energy directed to contaminants, reduced volume of water use and wastewater generation, and reduced water uptake by produce. The kinetic energy of the spray droplets produces the cleaning action. Increased spray pressure increases energy. If the energy is too great, produce may be physically damaged. If the energy is too little, the surface may not be cleaned. Indeed, studies on meat have shown that water pressures ranging from 1,379 to 2,070 kPa (200 to 300 psi) are effective in reducing microbial contamination, and a water flow rate of 7.5 liters/min is recommended. Water temperature >70 degrees C has been found to reduce bacterial counts in carcass tissue by 2 to 3 log CFU/cm2. These levels are likely too high for the fragile produce; hence, the main function of spray washing in produce applications will probably shift to being a delivery system for antimicrobial agents. Several other equipment, process, and product variables are relevant to the optimization of such a system. Qualities of the spray, such as droplet spectrum, droplet velocity, angle of droplet impingement, number and orientation of nozzles, spray rate, and resident time of the produce in the sprayer, also can be manipulated to adjust the amount of energy directed to the surface. There is a need to scientifically investigate the effects of these processes and equipment parameters on the removal of microbiological contaminants on meats and produce. Such empirical investigations guided by the results from fundamental studies about produce surface characteristics and the mechanism of bacterial attachment to plant tissue surfaces would allow for the efficient development of spray washers that effectively decontaminate produce.