Respiration climacteric

Flavor formation in fruit products has also extensively been reviewed (10), A distinction can be made between the primary aroma components, which are biosynthesized by the whole fruit and secondary aroma compounds (e.g. hexanal, 2-hexenal), formed after disruption of the cells during processing or chewing (11). The Importance of the peel for aroma formation has also been stressed by several authors (12). An extensive literature on the respiration climacteric (13), the role of ethylene (14) and the enzymes and substrates required for biosynthesis is available (15). 

Fig. 13.—Pattern of Post-harvest Respiration at 20 in Mangoes (After Krishnamurthy and Subramanyam384). [(a) Preclimacteric period, (b) climacteric rise, (c) climacteric peak, (d) over-ripeness (senescence). Solid line (1) shows the pattern obtained in a single fruit, and dotted line (2) shows the pattern obtained by averaging results from randomly selected fruits.]...

Laties and associates589-592 provided evidence for an alternative, cyanide-resistant path of respiration in avocado mitochondria. Uncouplers were considered to stimulate glycolysis to the point where the glycolytic flux exceeds the oxidative capacity of the cytochrome pathway, with the result that the alternative pathway is engaged. However, these authors concluded that the alternative pathway is not required in order to sustain the elevated rate of respiration that characterizes the climacteric. Clarification of the role, if any, of this alternative pathway in fruit ripening awaits further study. 

The tolerance limitation of fruit for irradiation establishes the maximum acceptable dose. If this dose controls decay organisms, the use of irradiation for a particular fruit may appear promising. Response to irradiation may be influenced by fruit maturity, variety, pre- and postharvest temperatures, handling, and extent of fungus growth. Climacteric fruits irradiated prior to the normal rapid increase in respiration usually show an immediate increase in respiration and the production of ethylene. These fruits are frequently retarded in ripening. 

Figure 1. Effect of ethylene on respiration of climacteric and nonclimacteric fruit. Ethylene causes greatest response in climacteric fruit when applied to mature fruit prior to the climacteric rise. In nonclimacteric fruit high concentrations of ethylene stimulate respiration for short time periods. This stimulation is observed at any time upon application of ethylene (3).

Figure 3 shows the hypothetical kinetics of growth, respiration and relative hormone levels in a climacteric fruit at different stages of its life cycle. Hypothetical hormone levels during development and ripening have been speculated on before (13). The rationale for this outline is based on the known influences of the various hormones on cell division,... 

Fruits ripen in two major ways climacteric and non-climacteric. The former type involves both autocatalytic evolution of ethylene and a rise in respiration whereas the latter shows no increase in ethylene formation and downward drifts in respiration [14]. In both types, fruit ripening is characterized by many biochemical and physiological changes such as chlorophyll d radation, pigment accumulation, textural modification and the production of volatile aromatic compounds. 

Post-harvest application of AA to non-climacteric fruits has been reported to cause induction of CO2 production (a climacteric-like respiration) in orange (Fidler 1968 Pesis and Avissar 1989), fig (Hirai et al. 1968), strawberry and blueberry (Janes et al. 1978) and grape (Pesis and Marinansky 1992). In fig and orange, AA application leads to reduced acidity (Hirai et al. 1968 Pesis and Avissar 1989). 

Studies on banana tissue slices have shown that valine and leucine concentrations increase about threefold following the climacteric rise in respiration [10]. Radioactive labeling studies have shown that valine and leucine are transformed into branched chain flavor compounds that are essential to banana flavor (2-methyl propyl esters and 3-methyl butyl esters, respectively). As can be seen in Figure 4.6, the initial step is deamination of the amino acid followed by decarboxylation. Various reductions and esterifications then lead to a number of volatiles that are significant to fruit flavor (acids, alcohols, and esters). Recent work has shown that amino acids play a role in apple flavor as well. For example, isoleucine is the precursor of 2-methyl butyl and 2-methyl butenyl esters in apples [24,25]. An unusual flavor compound, 2-isobutylthiazole, has been found to be important to the flavor of tomato. It is hypothesized that this compound is formed from the reaction of 3-methyl-l-butanal (from leucine) with cysteamine. 

The respiration rate is affected by the development stage of the fruit. A rise in respiration rate occurs with growth. This is followed by a slow decrease in respiration rate until the fruit is fully ripe. In a number of fruits ripening is associated with a renewed rise in respiration rate, which is often denoted as a climacteric rise. Maximal CO2 production occurs in the climacteric stage. De-... 

The climacteric respiration rise is so specific that fruits can be divided into ... 

Climacteric and nonclimacteric fruits respond differently to external ethylene (Fig. 18.10). Depending on the ethylene level, the respiratory increase sets in earlier in unripe climacteric fruits, but its height is not influenced. In contrast, in nonclimacteric fruits there is an increase in respiration rate at each ripening stage which is clearly dependent on ethylene concentration. 

Fig. 18.10. The effect of ethylene on fruit respiration, (a) climacteric, (b) nonclimacteric. Numerals on the curves ethylene in air, ppm (according to Biale, 1994)...

---