A THEORETICAL APPROACH ON THE ROLE OF FERMENTATION IN HARVESTED PLANT PRODUCTS (1998)

Most research on controlled atmosphere storage (CA) and modified atmosphere packaging (MA) places emphasis on optimal gas concentrations, defined as the concentrations where quality change of a product almost ceases without introduction of tissue disorders. They depend on the tolerance of products to low O2 concentrations [O2], often, in the range of 1–4␘2. Tolerance to these conditions appears to depend on both morphological and metabolic adaptations that are both species and tissue specific (Ratcliffe, 1995). During O2 limitation, energy metabolism switches from respiration to fermentation leading to disorders like necrotic and discoloured tissues, off odours and off tastes (Kader et al., 1989), suggesting that they have a direct relationship. Kader (1986) stated that decarboxylation of pyruvate to acetaldehyde through to ethanol results in the development of off-flavours and tissue breakdown. In fact [O2] is often considered to be optimal when respiration rates are reduced without development of fermentation (Banks et al., 1993). Therefore, traditionally, research on CA has been focused on avoiding fermentation. There have been three concepts; the Extinction Point (EP, Blackman, 1928), the Anaerobic Compensation point (ACP, Boersig et al., 1988) and the Respiration Quotient Breakpoint (RQB, Gran and Beaudry, 1993). All are related to fermentation rates. The EP is the highest [O2] with no anaerobic metabolism, measured as ethanol or acetaldehyde production. However, ethanol is now detected as a normal constituent of apples and many other fruits held under aerobic conditions (Boersig et al., 1988; Ke et al., 1990, 1993; Colelli et al., 1991, Nanos et al., 1992), so that the EP is therefore an untenable concept (Boersig et al., 1988). The ACP, the [O2] at which CO2 production is minimal, can be explained as the [O2] where an increase in anaerobic CO2 production compensates for the decrease in aerobic CO2 production. This implies anaerobic metabolism at higher [O2] than the ACP. The RQB is the [O2] where the RQ increases when [O2] is further lowered. The ACP and the RQB are not directly related to fermentation rates but are derived from gas exchange rates. Both the ACP and the RQB accept a certain increase in fermentation rate, measured as an increased CO2 production or RQ. With fermentation active at supposed optimal concentrations, and even in normal air, it is unclear whether there is an [O2] which precludes fermentation. Therefore occurrence of fermentation itself cannot be a criterion for selecting optimal storage concentrations. Throughout the years two possible explanations arose for the correlation between increased fermentation and tissue disorders: (1), a toxic effect of fermentative metabolites; (2), insufficient energy production to cover energy demands. We will discuss the physiological relevance and implications for storage procedures. Toxic effect of fermentative metabolites: Fermentative metabolites are often considered to be the cause of storage disorders with which they are often associated. Many metabolic studies of the survival of plant tissues in the absence of oxygen are focused on the possible toxicity of the main fermentation end-products, lactic acid, acetaldehyde and ethanol (Perata and Alpi, 1993; Ricard et al., 1994). Lactic acid - In many plants, formation of lactate precedes ethanol production. Davies et al. (1974) therefore proposed a regulatory role for lactate during the shift from oxidative to fermentative pathways, through decreasing cell pH and thereby increasing ADH activity. Different researchers, however, have found that the production of lactate is not well matched with the initial fall in the cytoplasmatic pH (Ratcliffe, 1995). Also the occurrence of lactic fermentation prior to the induction of alcoholic fermentation is not universally present in plants (Perata and Alpi, 1993). Nonetheless, the regulation of cytosolic pH is considered to be the