Surface Pasteurization With Steam

Steam is a gas — gaseous water. Because water vapor molecules are many orders of magnitude smaller (about 2 x 10_4 pm) than bacterial cells such as salmonella (4 ^ long and 0.7 ^ thick), and the mean free path length of water vapor molecules (0.4 is smaller than bacterial cells, steam should be able to enter any crevices or pores that bacteria can enter [24]. Steam is a unique fluid for pasteurizing food surfaces. It is sufficiently hot to kill virtually all bacterial vegetative cells on contact. However, much like hot water, treatment with steam may damage heat-sensitive foods like fruits and vegetables.

Much of the research on the use of steam for surface pasteurization has been on meat rather than fruits and vegetables. Of course, the meat-related research can be relevant to fruits and vegetables, but meats, except poultry, are generally more thermally resistant and forgiving than fruits and vegetables. Unfortunately, most of the information found on steam treatment of fruits and vegetables is not in the peer-reviewed literature but on web sites and company brochures.

In 1970 Klose and Bayne [25] experimented with steam to kill bacteria on the surface of chicken. Chicken samples were hung inside a three-necked flask, and steam was introduced under vacuum at 70 to 75°C. They obtained a 3 log reduction of naturally present bacteria with a 2-minute exposure, and a 5 log reduction after 16 minutes. Unfortunately, treatment above 60°C resulted in partial cooking of the outer layers of the samples.

In a follow-up study, Klose et al. [26] developed a cylindrical metal vacuum chamber to treat whole chicken carcasses with steam. Reductions of 3 logs of inoculated S. Typhimurium were achieved by application of subatmospheric pressure steam at 75°C for 4 minutes. However, ''the cooked breast meat was almost twice as tough for steam treated as for controls (5.4 versus 3.0 kg shear) and was similarly judged by a trained taste panel,'' presumably because the surface was cooked.

Davidson et al. [27] used a double-walled steel plate steam chamber to treat whole chicken carcasses and chicken parts with 180 to 200°C steam for 20 seconds. They realized a 1 to 2 log reduction of the aerobic plate count (APC) on whole carcasses and breasts. The kill on legs and wings was 2 logs. They reported ''evidence of fat separation in the skin and a lightly cooked appearance of skin and exposed muscles.''

Steam has been used commercially as a surface treatment for meats [28]. Nutsch et al. [29] reported that the bacterial reduction in a commercial beef processing plant using atmospheric pressure steam for 6 or 8 seconds was 1.35 logs.

When steam is brought into contact with food surfaces, it displaces the air while compressing a very thin film of air against the food surface. This film of air insulates the food surface against direct contact by the steam. The steam is hot enough to kill bacteria instantly, but to do so it must transfer its thermal energy to the bacterial cell. With a film of air present, the steam cannot contact the bacteria directly and must transfer the energy across the compressed air film to the bacteria. This is a relatively slow process compared to condensation of steam directly onto the bacteria cell walls. The process is so slow, in fact, that the steam will cook the surface before killing the bacteria, which is detrimental to the quality of thin-skinned and heat-sensitive commodities. However, for some thick-skinned fruits and vegetables which are destined for subsequent processing, such as for production of juice or fresh-cuts, this might not be a problem since the thermal injury would not extend into the edible portion of the commodity.

In the following sections, new steam surface pasteurization technologies applicable to fresh produce are described.

21.3.1 Thermosafe Process

Thermosafe is a patented [30,31] process of Biosteam Technologies, Inc. that uses condensing steam to kill bacteria on the surface of fruits and vegetables. Steam raises the surface temperature of fruits and vegetables to a preset value for a preset hold time. Chilled water follows the steam treatment to quench cooking. Bacterial reductions of 5 logs or greater can be realized with this process. The resultant product is acceptable for produce destined for further processing. It is not acceptable for the fresh food market because the steam cosmetically degrades the surface.

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