N.A. Michael Eskin∗ and Ernst Hoehn†

∗Department of Human Nutritional Sciences, Faculty of Human Ecology, University of Manitoba, Winnipeg, Manitoba, Canada

†Swiss Federal Research Station, Switzerland

I. Introduction

II. Respiration

A. Fruits

B. Vegetables

1. Control of the Climacteric Rise

2. Enzymatic Control

III. Initiation of Ripening

IV. Biosynthesis of Ethylene

A. Methionine as Precursor of Ethylene

1. The Yang Cycle, Recycling of Methionine

2. Methionine and Ethylene Biosynthesis

B. Regulation of Ethylene in Ripening Fruits

1. ACC Synthase and ACC Oxidase

2. Cyanide

3. Organic Acids

4. Lipid Peroxidation: Lipoxygenase

5. Galactose

V. Color Changes

A. Chlorophyll Changes during Ripening

B. Chlorophyll Biosynthesis

1. Phase 1: Glutamate to Chlorophyll a

2. Phase 2: The Chlorophyll Cycle

C. Regulation of Chlorophyll Biosynthesis

D. Mechanism of Chlorophyll Degradation

1. Chlorophyll Degradation: Processing and Storage

E. Carotenoids

1. Carotenoid Changes during Ripening

2. Carotenoid Degradation: Processing and Storage

F. Anthocyanins

1. Biosynthesis of Anthocyanins

2. Anthocyanins: Effect of Processing

VI. Texture

A. Cell Wall

1. Cell Wall Constituents

2. Cell Wall Degradation

VII. Flavor

A. Aroma

1. Aldehydes, Alcohols, and Esters

B. Taste

1. Starch–Sugar Conversion

2. Organic Acids

VIII. Storage

A. Cold Storage

B. Controlled Atmosphere Storage

C. New Developments in Postharvest Storage

References

I Introduction

Characteristics of fruits and vegetables such as flavor, color, size, shape, and absence of external defects ultimately determine their acceptance by consumers. The development of these characteristics is the result of many chemical and biochemical changes that occur following harvesting and storage. Since harvesting fruits and vegetables at their correct stage of maturity is critical for the development of a highly acceptable product for the fresh market, or for processing, it is important to understand more fully what changes are taking place. This chapter will highlight those changes occurring within fruits and vegetables during the postharvest period. It is during this period that fruits and vegetables show a gradual reduction in quality concurrent with transpiration and respiration, as well as with other biochemical and physiological changes. Ultimately the plant material deteriorates because of the undesirable enzyme activity and spoilage microorganisms.

The growth and maturation of fruits and vegetables are dependent on photosynthesis and absorption of water and minerals by the parent plant. Once detached, however, they are independent units in which respiratory processes play a major role. This chapter will focus on those changes in postharvest fruits and vegetables that affect quality.

II Respiration

Respiration is the fundamental process whereby living organisms carry out the exothermic conversion of potential energy into kinetic energy. In higher plants the major storage products are sucrose and starch. These are completely oxidized in the presence of oxygen to carbon dioxide and water, with the production of adenosine triphosphate (ATP):

The latter is the form in which energy is stored within the cell. The contribution of proteins and lipids to plant respiration is difficult to assess but can occur via the formation of acetyl-coenzyme A (CoA). In the absence of oxygen, anaerobic respiration occurs, resulting in only a partial degradation of carbohydrates and a lower ATP production.

The metabolic pathways involved in the respiration of plant tissue result in the conversion of starch or sucrose to glucose-6-P. The latter is then oxidized by glycolysis (Embden–Meyerhoff pathway) or the pentose phosphate pathway to triose phosphate, which enters the tricarboxylic acid cycle by way of pyruvate (Scheme 2.1) (ap Rees, 1977). Finally, in a third stage, oxidative phosphorylation converts NADH and FADH2 into chemical energy in the form of ATP (Browse et al., 2006).

The contribution of these two major pathways of carbohydrate oxidation to plant respiration remains unresolved. Difficulties were encountered with the experimental techniques used in assessing the relative roles of these pathways based on the production of 14CO2 or labeled intermediates from labeled hexoses (ap Rees, 1980). Evidence shows that both pathways exist in plant tissues (ap Rees, 1974) and that they change considerably during plant development (ap Rees, 1977). Current evidence supports the glycolytic pathway as the predominant one operating, while the maximum contribution of the pentose phosphate pathway may not exceed 30% of the total (ap Rees, 1980). The relative importance of these pathways probably depends on the particular plant, the organ, and the state of maturity.

Respiration rates of fruits and vegetables are affected by many environmental factors. In cases where this leads to negative effects on plant tissue it is defined as stress. During the storage of fruits and vegetables (Section VIII) effects of low temperatures, reduction in oxygen (O2) concentration and increase in carbon dioxide (CO2) concentration in the storage atmosphere are utilized to extend the storage life of produce. However, maintaining an adequate energy status is required to prevent browning or senescence of harvested fruits and vegetables (Saquet et al., 2000, 2003a; Xuan et al., 2005; Song et al., 2006; Jiang et al., 2007). It is well established that lowering of O2 concentrations during controlled atmosphere storage reduces respiration rates and energy supply and that severe limitations of O2 induce fermentative (anaerobic) respiration and metabolism in stored produce (Scheme 2.1). The net yield of ATP during anaerobic respiration is only 2 moles of ATP of hexose sugar compared with 36 moles of ATP per mole of hexose in aerobic respiration. Hence, energy status may be insufficient and provoke storage disorders (Jiang et al., 2007).