Cheyney, Kasmire & Morris - UC Davis
The effect of temperature on shelf life and decay of lettuce is dramatic. As known, reduction temperature from 10ºC to 5ºC approximately doubles shelf life. Further reduction to 2.5ºC will generally result in another doubling of shelf life. The incidence and severity of decay, the most important factor in reduced salability, is even more sensitive to control through low storage temperatures.
“Vacuum cooling wrapped lettuce”, a classic paper, 1979, by Charles C. Cheyne, Robert F. Kasmire, and Leonard L. Morris, University of California, Davis, supported by the California Iceberg Lettuce Research Advisory Board, explains the effect of the amount and characteristics of film applied to the head, the head solidity, and the vacuum cooling technique, on the cooling results.
All film wraps and even wrapper leaves interfere somewhat with vacuum cooling by restricting movement of water vapor away from the lettuce heads during cooling. Perforated films should only be used because these restrict cooling the least. Only the minimum amount of film needed should be used to wrap each head. Overlapping the film restricts ventilation by blocking vents, and also wastes material. The temperature reading to know the average load and unload temperature is important.
Shippers should consider the effects of the lettuce-film-tube combinations and adjust vacuum cooling procedures to compensate for variations in different films, initial product temperature, and different lettuce solidities. Poor cooling results in warmer temperatures during transit, which, depending on the transit time and physiological status of the lettuce, can result in poor arrivals.
The complete work, published by California Agriculture, October 1979, is available on the internet, and as pdf below.
The picture is an equipment by Moelco, a main provider of vacuum cooling equipment in Spain and internationally. The company is a main provider for Campo de Cartagena, the largest lettuce growing area in Murcia, Spain.