1 Postharvest handling and management of fruits and vegetables-2: Maturity Indices

Mr. Wridhama Prasad

1.0 INTRODUCTION:

Harvesting should be done when the produce is at a stage that will allow it to be at its peak quality (including sensory attributes) when it reaches consumer, at the same time it must also be not toxic and have an adequate shelf life.

Maturation of fruits and vegetables results alterations in the bio-chemical characteristics of the commodity, which are reflected in the changes of chemical components:

• Starch initially rises followed by decrease in the level; this decrease is accompanied with rise in the soluble solid content (starch levels are easily assessed by the starch-iodine test).
• Acid level decreases, which results in increased perception of sweetness as the sugar: acid ratio increases.

Maturity at the time of harvest is the most important factor that determines post harvest life and final quality of the produce. Fruits picked at less than mature stages are subject to grater shriveling and mechanical damage, and are of inferior flavor quality on the other hand over-ripe fruits are likely to become soft and/or mealy in texture soon after their harvest.

Depending upon maturity at the time of harvest, fruit-vegetables can be categorized into two groups:

1. Immature fruit-vegetables (cucumber): whose optimum eating quality is reached before complete ripening and delayed harvesting results in faster deterioration after harvest and inferior product quality.

2. Mature fruit-vegetables (muskmelons): includes those produce which reaches their peak eating quality after complete ripening at the plant and are incapable of continuing their ripening process once removed from the plant (except tomato).

Furthermore, it has also been found that:

•  If the fruit is harvested immature, it is possible to develop the color after harvest but not all the flavor characteristics.
•  If the fruit is harvested just as the maturity signs has begin to appear (appearance of slight patches of matured fruit color), it will reach the consumer with good quality.
• If the fruit ripens on the tree (full flavor and color development in fruit), it becomes soft, thus requires sophisticated handling, further its ripening process needs to be slowed down to maintain flavor and color.

2.0 MATURITY AND MATURITY INDICES:

Maturity could be defined as the stage of crop growth at which it can be used by the consumers for direct consumption or for products manufacturing based on them and it can be of two types: physiological maturity and commercial maturity.

Physiological maturity refers to the stage in the development of the fruit or vegetable, when maximum growth and maturation has occurred, i.e., the crop is fully ripened and is followed by senescence. Measurement of ethylene production and various chemical determinations, such as sugar: acid ratio can be used to estimate the maturity stage of the particular crop.

Commercial maturity refers to the stage of a plant organ required by a market. The terms immaturity, optimum maturity and over maturity relates to these requirements, and it has little relation with physiological maturity.

Commercial and physiological maturity needs to be related for every crop in order to have their long storage life and quality.

Maturity indices are objective standards dealing with the maturity of the crop. In practice, any chemical or physical characteristic of the crop that changes with physiological development and relates to consumer acceptance has the potential as a maturity index, and application of sensory evaluation to relate these changes with consumer acceptance validates the maturity index for the particular crop. For example, in tomatoes, red color perception in sensory evaluation relates with higher consumer acceptance and also with their physiological maturity.

Commonly selected factors reflecting horticultural maturity includes firmness, skin color, flesh color, sugar content, soluble solids, pigments, etc. Due to wide varieties of crops whose growth, maturation and ripening behavior vary widely with respect to each other, thus these are available for very few horticultural commodities and most regulations and many research publications rely on subjective measures of maturity.

Many criteria for judging maturity have been used or suggested and includes skin color, etc. But, to be practical, maturity tests should be simple, rapid and readily carried out in the field. For example: Climacteric crops (apple, melon) have a tendency of sudden increase in respiration rate during their storage from the increased rate of ethylene gas production; the produced ethylene could be measured by an instrument (gas chromatograph) and used as a measure of maturity. However, due to non-availability of rapid ethylene measurement instrument, presently this technique is not of much practical use. Hand operated instruments preferred over others. Also, if they can be non-destructive, higher will be the preference. But, these objective standards can not always be complied for maturity determination in many commodities (maturity determination in shelled crops).

3.0 DETERMINATION OF COMMERCIAL MATURITY:

This involves determination of the stage of development or maturation of fruit or vegetable by measuring characteristics known to change as they mature. Thus may include making decisions about levels of market and consumer acceptability along with objective measurements or subjective judgments or both.

Commonly, following criteria have been utilized for fixing maturity standards:

(a)   Visual means (skin color)

(b)   Physical methods (pressure test, density)

(c)    Chemical methods (total solids, sugars, acid, sugar-to-acid ratio, starch content)

(d)   Physiological methods- (respiration rate estimation)

(e)   Computation and Heat units

(f)     Optical methods

(g)   Electrical methods

3.1 Visual Methods:

3.1.1 Skin color: As the fruit skin color changes as the fruit ripens and thus external color and appearance of the crop indicates about its maturity.

Color charts are used for some cultivars of apple, tomato, peach, chili, pepper, etc for their harvesting at appropriate maturity.

3.1.2 Shape: fruit shape changes during maturation and thus is used as a characteristic to determine harvest maturity. For example:

• Banana: gets more round (cross section) in shape.
• Mango: immature mangoes have shoulders below their point of attachment with the stalk, mature mangoes have similar level of the two, while over mature mangoes have shoulders above the position of the stalk attachment.

Reactions Size: Fruit size increases with maturity. Thus, larger the fruit, more the maturity. Also, higher the maturity and higher the market demand but, higher will be the post harvest losses. In addition, the standard size for a produce is mainly decided by the market demand for a particular size, especially those marketed early in their development, for example zucchini.

3.2 Physical methods:

3.2.1 Fruit opening: sometimes, fruit upon maturity accompanies its opening, indicating that only short period is available for its distribution and consumption (example cauliflower).

3.2.2 Leaf changes: the condition of the leaves indicates the crop condition on it (specially for root crops, which grows below the ground)

• If potatoes are to be stored then the optimum harvest time is after the stem and the leaves have died down. If they are harvested earlier, the skin is less resistant to harvesting and handling damage and is more prone to storage diseases. In certain cases where the leaves do not senescence naturally, they me be cut off or removed mechanically to produce tubers firmer and stronger skins before they are harvested.
• For onions, for storage they must be ripened before harvesting indicated by bending of leaves just above the top of the bulbs and fall either.
• For melons, a leaf, in whose axis a fruit is borne, dies indicates the fruit ready for harvesting.

3.2.3 Abscission: Abscission layer naturally develops on the fruit. Complete abscission layer development indicates complete ripeness of the fruit and its short marketing life (example melon).

3.2.4 Acoustic and vibration tests: the sound from a fruit, as it is tapped sharply with a finger knuckle, changes with maturation and ripening. In equipment for these tests, vibration energy is put into the fruit and fruit’s response to this is measured. Good relation has been found using this technique for maturity estimation of tomatoes, apple, avocado, etc.

3.2.5 Firmness: fruit softens or tenderizes upon ripening and this can be measured using destructive and non-destructive methods.

3.2.5.1 Destructive method: Measurement of texture (by tenderometer, etc,) indicates about the firmness of the fruit and when the value reaches a pre-determined value, it is harvested. Furthermore, different instruments for texture measurement do not necessarily give the same numerical value if used on the same produce, although each instrument will give a reproducible value. It is, therefore, necessary to specify the instrument when reporting pressure test values or attempting to set standards.

3.2.5.2 Non destructive method: This method employs a small amount of force on the fruit surface and measures the deformation accurately. Also, the deformation is not damage and is not judged by consumers. Force application can be by air, steel balls on opposite side of the fruit or a narrow cylinder.

3.2.6 Specific gravity: As the fruit matures, its specific gravity increases. This is mostly used for grading the crop and rarely for estimating maturity of the crop.

3.2.7 Nuclear Magnetic Resonance (NMR): NMR correlates well with the sugar content of banana, apples and oil content of avocado and this can be used for high speed sorting of the produce based upon their maturity, but it has high cost.

3.3  Chemical Methods:

3.3.1 Aroma: As the fruit ripens, volatile compounds are synthesized in them, which gives the fruit its characteristic odor and thus can be used to determine weather it is ripe or not. Further, these odors may only be detectable to human senses when the fruit is completely ripe, and therefore, has limited commercial applications without the use of gas chromatographs, which has the limitation of hiving high cost and complex in nature.

3.3.2 Juice content: As the ripening progresses, juice content increases in fruit. Relating juice volume (extracted by employing a standard method from the fruit) to original mass of the fruit can be used to set standard for fruit harvesting (especially for citrus fruits, example: lemon, minimum 25% juice).

3.3.3  Oil content and dry matter percentage: In avocado, oil content increases with ripening.

Rate of dry matter accumulation can also be used to predict optimum harvest time.

3.3.4 Sugars: In some fruits, during ripening the accumulated carbohydrate (starch) gets converted to simple sugars, while in others simple sugars are converted to starch, thus, in both the cases a certain level of sugar is optimum for harvesting.

Sugar content in fruit can be estimated by placing the fruit in a beam of Infra red light in a dark chamber and measuring the light transmittance or by titrating the extract against appropriate chemicals.

3.3.5 Starch content: Dipping the cut crop sample into iodine solution could give an indication of the starch content in them. By the aid of Perspex templates, even the starch concentration can be estimated. Pear fruits usually are harvested at 60-70% starch content.

3.3.6 Acidity: with an increase in maturity, acidity of the fruit increases. Also, acidity (measured by titration) should be used and not by pH measurement, due to buffering capacity of the crop. Further, acidity is also expressed as Brix: Acid ratio (which is a more comprehensive test for maturity status estimation of the crop).

3.4 Physiological Methods: Depending upon the crop (climacteric or non-climacteric), respiration rate curve can be related to the appropriate time of harvest. Respiration rate is expressed in terms of amount carbon dioxide evolved or oxygen in-hailed by the crop per unit time per unit weight of it and the respiration curve can be obtained by plotting the respiration rate against complete life span of the crop. The appropriate point on the respiration curve can be related to some characteristics which can be readily determined at the field and thus aiding as maturity index.

The concept of green life is based on respiration rate and indicates the time from harvest of the fruit to the onset of ripening. It has been developed to indicate the physiological immaturity at the time of harvest and also a useful expression of post-harvest life.

3.5 Computation and Heat Units: A characteristic number of degree days are required to mature a crop (heat units) and this and vary very less with varying climatic conditions. Example: for banana, 8-9 week is the optimum time for harvest after flowering.

The number of degree days to maturity is determined over a period of several years by obtaining the algebraic sum of the differences, plus or minus between the daily mean temperature (commonly the minimum temperature at which the growth occurs). The average or characteristic number of degree days is then used to forecast the probable date of maturity for the current year. Further, it is also helpful in planning planting, harvesting, and factory program for crops, such as corn, pea and tomato for processing.

These harvesting criteria are often the product of the grower’s experience with his crop in his particular environment.

3.6 Optical Methods: Electromagnetic properties of crop are affected by the level and type of components present in it, which changes along with growth of it. Measurement of changes in these properties is used to predict the maturity status of the crop.

3.6.1 Diffuse reflectance: this technique measures the reflected light from just below of the crop surface. As the crop ripens its diffuse reflectance changes. Further, it can also be used to calculate soluble solids, pigments and firmness of the crops.

3.6.2 Delayed light transmission: the fruit is exposed to a bright light which is then switched off so the fruit is in the dark. A sensor then measures the amount of light emitted from the fruit, which is proportional to its chlorophyll content and thus its maturity.

3.6.3 Near infra-red reflectance (NIR): NIR is used for measuring moisture content and internal qualities (calcium content, firmness, acidity) using light emitting diodes. This operates at water absorbing wavelengths.

3.6.4 Radiation: X-rays are passed through the fruits and transmission rates are measured.

Transmission rate has inverse relationship with density; also internal disorders can be detected.

Objective measurement of color is possible using various optical based instruments, but the high capital cost associated with them usually restricts them to packing house with a high throughput of produce. Also, as the labor cost is increasing, it may be expected that such mechanization will increase, for example electronic color sorting is now common for lemons and for processing of tomatoes.

3.7 Electrical Characteristics:

These methods are based on the fact that due to change in the concentration of dissolved electrolytes (in the flesh) during maturation, change occur in its electrical properties (resistance, etc.) and measuring these changes indicates the maturity status of the crop.

Di-electric constant decreases in peaches with increasing ripening while in the case of water melons, it increases with ripening due to increase in sugar content, which indicates different phenomenon for changes in electrical properties with maturity for different crops.

Further, these measurements requires application of sophisticated instruments which makes them useful at laboratory but of little value at the field.

4.0 CONCLUSION:

Delivering high quality fruit-vegetables to the consumers and reducing post harvest losses can be achieved by picking them at optimum maturity along with other measures. Much information is available about growth, maturity and respiration; however, more research is needed to develop indices based on them which are economic, easy to learn and apply at the field.

Suggested Readings

• Kader AA (2002). Postharvest Technology of Horticultural Crops, 3rd Edn, University of California, Davis publication, USA.

• Mishra, V. K. and Gamage, T. V. (2007). Postharvest physiology of fruits and vegetables, In Rehamn, M. S. (ed.) Handbook of Food Preservation, 2nd edition, CRC Press, Boca Raton.