Chapter 2 - Experimental Errors in Hypobaric Storage Research

Abstract

Academic skepticism regarding low-pressure storage originated from experimental errors. Researchers underestimated the difficulty of constructing a leak-tight laboratory vacuum apparatus, and the effect that leakage has on commodity water loss during hypobaric storage. The lower the pressure the more in-leaking air expands, increases in volume and decreases in humidity as it enters an evacuated storage chamber, and at a lower pressure leakage provides a greater part of the air change flowing through the system. It is not obvious that leakage is occurring because the low-pressure system’s pressure controller automatically compensates for leakage and maintains the selected vacuum, while increased commodity water loss elevates the storage humidity close to saturation. An experimental error caused by humidifying air at 1atm pressure instead of at the low storage pressure is evident in many academic experiments. The humidity declines in direct proportion to the decrease in pressure because the humid atmospheric air expands as it passes through the pressure controller. In other studies so few air changes per hour were flowed that the stored commodity consumed all available [O2] and suffered anaerobic damage. Laboratory low-pressure chambers sometimes were installed in cold-rooms that had a nonuniform air distribution pattern that produced a “cold spot” on the vacuum chamber’s surface. This created an evaporation/condensation cycle between the commodity and chamber cold spot. In other studies the commodity was stored in a sealed system that accumulated an active ethylene concentration between occasional ventings. Several researchers claimed that hypobaric storage cannot displace “active-bound [C2H4]” from within plant tissues, and only influences ripening or senescence by lowering [O2]. This opinion was disproved by studies showing that ethylene’s measured dissociation constant from its receptor site has the same value as the applied [C2H4] concentration causing a half-maximal biological response. “Bound” and intercellular C2H4 equilibrated within 15min after the elongation growth of plants that do not produce autocatalytic ethylene was inhibited by applied ethylene and the plants were transferred to fresh atmospheric air. Tomatoes and apples do not develop normal flavor and aroma when they are stored for an extended period in a hypobaric system, causing academics to conclude that a vacuum “out-gasses” flavor and aroma volatiles. When it was demonstrated that the same response occurs in CA, it became obvious that it is caused by low O2, rather than low pressure. Other researchers claimed that pump-down and venting damaged plant matter, but damage does not occur when these processes take place during 30min periods.