Development Of Maturity Indices

A maturity index should relate consistently from year to year to quality of the marketed product. The many physiological and biochemical changes that occur during maturation and ripening of apples, pears, peaches, nectarines, plums, and cherries have led to testing of an extensive range of potential maturity indices. These maturity indices have been based on different criteria, depending on the industry involved, and include development of correlations between maturity-related attributes, the progression of these attributes with advancing maturity, and the relationships between these attributes and edible quality and/or the occurrence of physiological disorders.

Because of the differences in maturation and ripening physiology within each fruit type, there can be wide variations in the "best" index, or set of indices, that is regarded as suitable for any given cultivar. In addition, adoption of certain maturity indices is affected by regional differences in technologies available for maturity assessment, the size and sophistication of the specific industry, and market

FIGURE F4.1. Schematic illustration of the increase in fruit quality during maturation and ripening, and concomittant loss of storage potential. Autocatalytic ethylene production is generally associated with these changes in climacteric fruit.

requirements. In general, maturity indices should be simple; readily performed by growers, field staff, or industry personnel; and objective rather than subjective. Ideally, they require inexpensive equipment, but depending on the size of the industry, more expensive equipment may be used. For example, gas chromatographs for assessment of internal ethylene concentrations (IEC) are used in some apple maturity programs, where samples are consolidated across a region for evaluation in a single laboratory.

Several "maturity indices" are indicators of quality rather than maturity per se, and, in addition, harvest decisions have to be based, not only on physiological maturity, but also on market requirements. Thus, a fruit may obtain physiological maturity but, unless it meets market requirements, such as blush and background color, will not be acceptable in many markets. The term "harvest indices" is more ac

Maturation and Ripening Period

Maturation and Ripening Period curate for the factors used in making harvest decisions. Of the harvest indices available for temperate fruit, several of the more commonly used ones are discussed here:

1. The production of ethylene, an important plant hormone, is often associated with initiation of ripening and, therefore, is sometimes used as a major determinant in harvest decisions, especially for apples. However, the importance of ethylene in making harvest decisions is not straightforward; relationships between ethylene production and optimum harvest dates can be poor, and the timing, or presence, of increased ethylene production is affected by cultivar. Moreover, within a cultivar, ethyl-ene production is greatly affected by factors such as growing region, orchard within a region, cultivar strain, growing season conditions, and nutrition. Ethylene production may be a better indicator of when to complete the harvest, especially in cultivars where autocatalytic ethylene production precedes preharvest drop.

2. The starch test, in which the hydrolysis of starch to sugars as fruit ripen is estimated by staining starch with iodine solution, has become popular for assessment of apple fruit maturity. The resulting patterns, which reflect the extent of starch hydrolysis, are rated numerically using starch charts, either specific to cultivar or generic (Figure F4.2). Optimum starch indices are available for many cultivars, and because the change of indices is linear, the test can be used to predict optimum harvest dates.

3. Flesh firmness has been used as a maturity index, but it is affected by many preharvest factors, including season, orchard location, nutrition, and exposure to sunlight, that are independent of fruit maturity. It is the primary method for assessing maturity of pears. For other fruit, it is an important indicator of internal quality and can provide information that is important to fruit performance in storage. It can directly affect consumer satisfaction with many fruit. For apple, firmness is used as a quality criterion by wholesalers, especially in England.

4. The soluble solids concentration (SSC) of fruit generally increases as fruit mature and ripen, either directly by import of sugars or by the conversion of starch to sugars. It is also a quality index, rather than a maturity index, being affected by many preharvest factors, and concentrations do not necessarily reflect fruit maturity. As with firmness, SSC is increasingly being used as a quality criterion by wholesalers.

5. Titratable acidity (TA) primarily estimates the amount of the predominant acid, usually malic in most temperate fruit. TA decreases during maturation and ripening, but optimum values vary by cultivar and season.

6. The background, or ground, color change from green to yellow reflects the loss of chlorophyll. Preharvest factors, especially those which affect nitrogen content, can markedly influence chlorophyll concentrations, independent of maturity changes.

7. Full-bloom dates and days afterfull bloom, with and without incorporation of temperature records, have been established, but usefulness varies greatly by cultivar and growing region. Calendar dates alone have limited value in regions where temperature variations result in wide differences in bloom dates, but in more consistent growing regions, days from full bloom can be the most reliable harvest index for some cultivars.

Even when certain maturity indices are considered as imperfect harvest indicators, they may be useful in combination. For example, Crisosto (1994) reports that flesh firmness in combination with background color is an excellent indicator of maximum peach maturity.

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