Effects of vacuum rate on the vacuum cooling of lettuce.

摘要

The deterioration of freshly harvested horticultural crops can be minimised by precooling prior to storage. This technique of quickly cooling the produce does not only increases the shelf life, but it also reduces the size of the refrigeration system needed for storage facility. Vacuum cooling is an effective method of precooling leafy vegetables, but has a major drawback of requiring substantial initial capital investment. Thus, vacuum coolers are generally limited to large-scale or co-op operations where the initial investment can be spread across a large quantity of produce.;The conventional philosophy behind precooling design is to establish systems to cool produce as quickly as possible; this concept is more so with vacuum coolers. By changing certain design criteria of a vacuum cooler, it may be possible to reduce the capital cost of vacuum coolers by reducing the rate of vacuum. Though the time to cool the produce may be increased, the reduction in the size of the vacuum pump and the refrigeration system, and hence the capital cost of the cooler, may be beneficial to small-scale producers who can not justify the large expenses incurred when purchasing a conventional system.;Experiments were performed on a modified vacuum cooler in which the rate of vacuum could be controlled. The cooling characteristics, including the temperature distribution and mass loss, and the lettuce quality were determined for different rates of vacuum. A relationship between the speed of the vacuum and the peak product refrigeration load was developed and tested with experimental data. The results suggest that slower vacuum coolers can be successfully designed and built for small-scale operations.