Product Quality

There is little or no product degradation. For example, the color value for paprika differed from the control by only 2 ASTA units after treatment (reduced from 95 to 93). Volatile oils for rosemary did not change (0.7ml/100 g). In addition, the process inactivates enzymes.

21.3.4 Vacuum-Steam-Vacuum (VSV) Process

Thermal damage is a major problem for steam surface-pasteurized fruits and vegetables destined for the fresh market. Conventional wisdom seems to dictate that if the steam exposure time is sufficient to kill the bacteria, the produce is thermally damaged. The treated produce may be suitable for the processed fruit or vegetable market but not for the fresh market. If fresh quality is to be maintained by using a shorter exposure time, the bacterial population will not be sufficiently reduced. One solution to this problem is the U.S. Department of Agriculture's (USDA) novel VSV process [37].

To circumvent the problem of thermal damage, the film of air and moisture on the commodity surface is removed so that steam can rapidly contact the bacteria directly. It is a simple concept but difficult to achieve in practice. One approach was the concept proposed by Morgan et al. [24,38,39]. In this method, the food is exposed to vacuum to remove air and moisture. Next, saturated steam is applied to the surface. When the saturated steam contacts the product, it condenses to form a water film on the fruit or vegetable surface which impedes further bacteria reduction. Therefore, the food is exposed to a vacuum again to remove the condensate and to evaporatively cool the surface. Kozempel et al. [40] showed that cycling between vacuum and steam to remove the condensate enhanced the population reduction of Listeria innocua on hot dogs. This concept of alternating vacuum and steam is the basis of the VSV process.

Initial research used a stainless steel device consisting of a rotor and stator. The 150 mm long and 150 mm in diameter [24,38] rotor was turned rapidly around its horizontal axis, stopping at precisely determined angular positions, exposing the sample alternately to vacuum or steam. A 25 mm x 75 mm x 75 mm deep treatment chamber was milled into the surface of the rotor.

The treatment consisted of four steps: (1) air was removed by exposure to vacuum; (2) the sample was flushed with low-temperature saturated steam (this flush was later abandoned); (3) the sample was exposed to pressurized saturated steam; and (4) the sample was evaporatively cooled with vacuum. Bacterial reductions on chicken meat inoculated with nonpathogenic L. innocua were about 2 to 2.5 logs. Steam exposure time was 0.1 to 0.2 seconds [24,38].

This prototype proved the concept, but was not practical with actual fruits and vegetables such as cantaloupes. For mechanical reasons it was preferable to move the machinery and not the food sample. Therefore, a new prototype pilot plant unit was designed and fabricated. The surface intervention processor was designed to process chicken carcasses, specifically broilers. However, the design is also suitable for many fruits and vegetables, especially cantaloupes. The performance requirements of a surface intervention processor are to accept the individual food sample and enclose it in a chamber within a rotor; to evacuate that chamber; to pressurize the chamber with steam; to vacuum cool it; and, finally, to eject the sample into a clean environment. The simplest execution of this prototype, one chamber in one rotor, was designed and constructed [41]. Figure 21.1 shows the processor, and Figure 21.2 shows details of the product treatment section. The chamber is cylindrical, about 200 mm in diameter and 240 mm deep, and is provided with an 8-inch ball valve.

To admit vacuum or steam into the closed chamber, two opposing 200 mm holes were bored through the stator at right angles to both the axis of rotation of the ball and to the centerline of the open chamber. Two platter valves, consisting of a flat disk rotating against an inlet header that holds poly-etheretherketone (PEEK) seals, were close-coupled to the 200 mm ports. Each disk contained two holes, which when stopped at one of the ports in the inlet header permitted steam flow into or vacuum evacuation from the treatment chamber. Multiple holes reduced the rotor angular movement necessary for valve action and increased the cross-sectional area for gas flow. Each disk was programmed independently and moved by its own servomotor. To expose all

FIGURE 21.1 Vacuum-steam-vacuum processor.
FIGURE 21.2 Schematic of the product treatment section of the Vacuum-steam-vacuum processor.

exterior surfaces of the test sample to treatment, a screen was installed at the midpoint of the treatment chamber to hold the sample.

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