Technologies

There are three basic designs for administering hot water treatments: the batch system, the continuous system (hot water treatment, HWT) [22], and the hot water rinsing and brushing system (HWRB) [15].

Most of the hot water immersion treatment facilities that are in commercial use currently are of the batch system. In this system, baskets of produce are loaded onto a platform, which is then lowered into the hot water immersion tank, where the fruits or vegetables remain at the prescribed temperature for a certain time before being taken out, usually by means of an overhead hoist. In the continuous system the produce is submerged (either loosely or in a wire or plastic mesh basket) on a conveyor belt, which moves slowly from one end of the hot water tank to the other. The belt speed is set to ensure that the produce is submerged for the required length of time. This system requires an instrument to monitor the speed of the conveyor belt.

The main components in these two systems are an insulated treatment tank of several hundred liters, a heat exchange unit operated by gas, diesel, or electricity, a pump and water circulation system to provide uniform water temperatures throughout the treatment process and to avoid the formation of cool pockets during treatment, and temperature sensors to control and monitor water temperature during treatment [22,23]. An inexpensive hot water immersion system can be assembled easily; the machinery can even be made mobile with little difficulty [23,24].

In contrast to hot water immersion, a new technology based on a brief HWRB for simultaneous cleaning and disinfestation of fresh produce was first introduced commercially in 1996 [20]. The fourth generation of the HWRB machine (Figure 20.1) contains 18 to 22 parallel brushes, all of which are controlled by a single motor. All components in the machine, including the hot water tank (300 to 500 l), are made from stainless steel materials. The produce is prewashed by nonrecycled tap water (ambient temperature) for about 5 to 10 seconds while revolving on cylindrical brushes. A speed-adjustable conveyor belt is connected to the simultaneous cleaning and disinfecting stage, and controls the duration of exposure to hot water, which is heated with a thermostatically controlled gas or electric heating element. Fruit or vegetables are rinsed with the pressurized hot water, from nozzles that point down either vertically or at predetermined angles onto the produce, which rolls on brushes made from medium-soft synthetic bristles. The produce is exposed to water at temperatures between 48 and 63°C for 10 to 25 seconds, depending upon produce type and cultivar [25-34]. The water is filtered and then recycled, while being supplemented from time to time with new water to compensate for loss due to evaporation, adsorption, spilling, etc. At the end of the hot washing treatment, forced-air fans, or hot forced air is used to dry the produce inside a 4 to 6 m long tunnel for 1 to 2 minutes. The estimated cost of the HWRB system, including the drying tunnel, is $15,000 to $30,000 for units with washing capacities of 1 to 25 tons/h (further information can be obtained from the author).

FIGURE 20.1 Hot water rinsing and brushing machine, fourth generation, made of stainless steel: (1) conveyor; (2) tap water rinsing and brushing unit; (3) hot water rinsing and brushing unit with adjustable nozzles, with and without fruits. Water is recycled. (4) Drying tunnel equipped with forced air fans; (5) heat unit (120,000 kcal); (6) hot water container equipped with water pump (P) to pressurize and recycle the hot water and filter (F).

FIGURE 20.1 Hot water rinsing and brushing machine, fourth generation, made of stainless steel: (1) conveyor; (2) tap water rinsing and brushing unit; (3) hot water rinsing and brushing unit with adjustable nozzles, with and without fruits. Water is recycled. (4) Drying tunnel equipped with forced air fans; (5) heat unit (120,000 kcal); (6) hot water container equipped with water pump (P) to pressurize and recycle the hot water and filter (F).

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