Comparison of unsteady- and steady-state methods for produce respiration rate determination - 1. Model development and validation

Closed respiration chambers are commonly used to determine respiration rates of produce as a function of pO(2) and pCO(2). We have observed that when the respiration behavior of cut rutabaga (Brassica napobrassica) is determined using this unsteady-state method, large swings in the ratio of CO2 production rate over O-2 uptake rate (also known as the measured respiratory quotient RQ(measured)) are observed. Typically, initial RQ(measured) values range from 0.8 to 0.9, but within hours can rise to values >1.0. To better understand this phenomenon, a model was developed that takes into account not only the CO2 found in the surrounding atmosphere but also the various species dissolved within the plant tissue. As predicted by the resulting equations, the initial RQ(measured) varied with the amount of produce loading (the ratio of produce liquid to container gas void volume). At all loadings examined RQmeasured increased as the respiration experiment progressed, and was shown to substantially exceed unity at high loadings. Internal pH initially dropped, particularly during the experiments conducted at low loading, but recovered to near normal by the end of the experiment. It is believed that this pH rebalancing was a result of malic acid metabolism, which produces OH-. RQ(measured) values exceeding unity were a result of the CO2 released by malate breakdown. (C) 2003 Elsevier B.V. All rights reserved.