Module 2. Post-harvest management of fruits and vegetables

Lesson 6

POST HARVEST MANAGEMENT OF FRUITS AND VEGETABLES

6.1  Introduction

Fruits and vegetables, fresh or processed, form an important component of our diet and there is an ever-increasing demand for these. India being the top producer of both fruits and vegetables in the world, more emphasis is needed to minimize past harvest losses. At present about 70-80% of our production goes waste mainly during transportation and storage. A clear understanding of biochemical and physiological changes in fruits and vegetables during post harvest operations will enable persons involved in handling, transportation and storage operation to regulate certain critical parameters.

6.2  Harvesting or Maturity Indices of Fruits and Vegetables

The stage at which the fruits and vegetables should be harvested is very important in determining the market life, storage, transport, eating and processing quality. Harvesting indices are defined in terms of either their “physiological maturity” or their “commercial maturity”. The former refers to a particular stage in the life of a plant organ and the latter is concerned with the time of harvest as related to a particular end-use that can be translated into market requirements. Physiological maturity refers to a stage in the development of the fruit or vegetable when maximum growth and maturation has occurred. It is usually associated with full-ripening in a fruit. It is followed by senescence. Clear distinction between the three stages of development of a plant organ is not always easy, since the transition between the stages is often quite slow and indistinct. Commercial maturity is the stage of a plant organ required by market. The marketing of fresh fruits and vegetables is aimed eventually at appealing to the consumers. There are two methods for determining the harvesting maturity as given in Table 6.1. They are (a) destructive and non-destructive methods and (b) physiological methods. Harvesting maturity should meet the following criteria:

i). Should be at a stage which will allow it to be at its peak condition when it reaches the consumer.

ii). Should be at a maturity that allows it to develop as acceptable flavour or appearance.

iii). Should be at a size required by the market.

iv). Should not be toxic.

v). Should have an adequate shelf-life.

Fruits are harvested at slightly immature or mature greens stages and yet their physiological activities continue. Harvesting of fruits and vegetables at appropriate maturity level is important and one of the basis of close observations, maturity indices is fixed for various commodities. These maturity indices are based on physico-chemical characteristics, like their weight, fullness of finger, total soluble solids, sugar to acid ratio and certain arbitrary units like colour, heat units, and period after blooming. Commonly following criteria have been utilized for fixing maturity standards:

· Computation of days from bloom to harvest

· Measurement of heat units

· Visual means- skin colour, persistence or drying of parts of plant, fullness of fruit

· Physical methods- ease of separation, pressure test, density, grading etc.

· Chemical methods- total solids, sugars, acid, sugar-to-acid ratio, starch content etc.

· Physiological methods- respiration methods etc.

6.3 Factors Affecting the Postharvest Quality of Fruits and Vegetables

Two types of factors are involved in the postharvest quality of fruits and vegetables. They are biological or internal factors and environmental or external factors.

6.3.1 Biological factors

a. Respiration rate

Even after harvesting fruits and vegetables behave as living commodity (entity) and continues to respire. However, the rate and pattern of respiration depends upon several factors like physiological maturity, injury, storage atmosphere. Respiration is the process by which stored organic materials (carbohydrates, proteins and fats) are broken down into simple end products with a release of energy. Oxygen (O₂) is used in this process and carbon dioxide (CO₂) is produced (Eq.6.1). The loss of stored food reserves in the commodity during respiration hastens senescence as the reserves that provide energy to maintain the commodity’s living status are exhausted. The energy released as heat, which is known as vital heat, affects post harvest technology considerations such as estimations of refrigeration and ventilation requirements.

C₆H₁₂O₆ + 6O₂   6CO₂ + 6H₂O + 673 K.cal .........(Eq. 6.1)

Table 6.1 Methods for determining the harvesting maturity

Destructive & Non-destructive methods

Physiological methods

Field methods Skin colour Shape Size Aroma Time between flowering and fruit bearing ready for harvesting Leaf changes Abscission Firmness Post harvest methods Firmness Juice content Oil content Sugar content Starch content °Brix-acid ratio Specific gravity Heat units Acoustic and vibration tests Electrical properties Colour difference tests Optical properties Near infra red reflectance Nuclear magnetic resonance technique

Rate of respiration Ethylene production

Respiration rate is expressed as ml of O₂ consumed or ml of CO₂ evolved per kg of fruit per hour. Gas analyzers are placed to measure the level of gases. Respiration rate indicates the storage life of the commodity. A high rate of respiration usually associated with a short life. It would also indicate the rate at which the fruit is deteriorating in quality and in food value. Moreover, respiration is a rather complex process that is affected by a number of factors. Knowledge of these factors is of immense importance from the handling and storage point of view. Fruits, on the basis of their respiration pattern during ripening, can be classified as either climacteric or non-climacteric. Non-climacteric fruits are not capable of continuing their ripening process once removed from the plant e.g. dates, grapes, pineapple, lemon, lime, pomegranate, etc. while climacteric fruits can be harvested mature and ripened off the plant e.g. apple, papaya, banana, mango, guava, sapota (chikoo), etc. Respiration pattern of climacteric and non-climacteric fruits is given in Fig.6.1 and a generalized respiration rate of a climacteric fruit during different stages of growth is given in Fig.6.2.

Fig. 6.1 Respiration pattern in climacteric (e.g. tomato) and non-climacteric (orange) fruits

Fig. 6.2 Respiration rate of a climacteric fruit during different stages of growth

Most of the physico-chemical changes occurring in harvested fruit are related to oxidative metabolism including respiration. There are three phases of respiration:

I. Breakdown of storage macromolecules like polysaccharides, fats or proteins

Breakdown process is carried out by enzymes such as carbohydrases (pectic enzymes, celluloses, hemicelluloses, and amylases), proteinases and lipases. In many cases it is also apparent that metabolism of organic acids can account for a significant proportion of respiration.

II. Oxidation of sugars to pyruvic acid

Respiratory pathways namely utilized glycolytic and oxidative Pentose-Phosphate pathways by fruits are common to all plant (OPP) tissues.

III. Aerobic transformation

Pyruvate and other organic acids are aerobically transformed into carbon dioxide, water and energy. This involves TCA cycle and electron transport chain.

b. Ethylene production

Ethylene, the simplest of the organic compounds affecting the physiological processes of plants, is a natural product of plant metabolism and is produced by all tissues of higher plants and by some microorganisms. As a plant hormone, ethylene regulates many aspects of growth, development and senescence and is physiologically active in trace amounts (less than 0.1 ppm). Ethylene biosynthesis starts with the amino acid methionine, which is energized by ATP to produce S-adenosyl methionine (SAM). The key enzyme in the pathway, ACC synthase, converts SAM to 1-aminocyclopropane-1-carboxylic acid (ACC), which is converted to ethylene by the action of ACC oxidase. Ethylene production rates, which depend on the fruit, generally increase with maturity at harvest, physical injuries, disease incidence, increased temperatures up to 30°C, and water stress. On the other hand, ethylene production rates by fresh fruits are reduced by storage at low temperature and by reduced O₂ (< 8%) and elevated CO₂ (> 1%) levels in the storage environment around the commodity.

c. Transpiration or water loss

Water loss is the main cause of deterioration because it results not only in indirect quantitative losses (loss of salable weight) but also in losses in appearance (wilting and shriveling), textural quality (softening, flaccidity, limpness and loss of crispness and juiciness), and nutritional quality. The outer protective coverings (dermal system) govern the regulation of water loss by the commodity. Transpiration (evaporation of water from the plant tissues) is a physical process that can be controlled by applying treatments to the commodity (e.g. waxes and other surface coatings or wrapping with plastic films) or manipulation of the environment (e.g. maintenance of high relative humidity and control of air circulation).

d. Physiological disorders

Physiological disorders that occur in fruits and vegetables are chilling injury, freezing injury, heat injury, disorders due to pre-harvest nutrient imbalances, breakdown of fruits and vegetables due to very low (< 1%) oxygen and elevated (> 20%) carbon dioxide concentrations. Freezing point of fruits and vegetables is slightly below the freezing point of water, for example apple has freezing point of – 1.5°C. Freezing point may vary among cultivars or even depends of crop production practices. The varied amount of soluble solids is also one of the reasons for variation in freezing point. Freezing injury occurs when fruits and vegetables are held below the freezing temperatures of cell sap, they get damaged, which is referred as “freezing injury”.

Chilling injury occurs when fruits and vegetables are held at temperatures above their freezing point and below 15°C depending on the commodity. Chilling injury is more common in fruits which are of tropical or sub-tropical in origin. These include mango, papaya, banana, citrus, tomato, pineapple, guava, cucumber, eggplant and pepper. Chilling injury is manifested in a variety of symptoms, which include surface and internal discoloration (internal/external), surface pitting, appearance of water-soaked areas, necrotic (black spots) areas, uneven ripening or failure to ripen, off-flavour development, and accelerated incidence of surface molds and decay. Fruits suffered with chilling injury sometimes fail to ripen when bring back at ambient conditions. Chilling injury is generally noticed after transferring to non-chilling temperature.

Heat injury results from exposure to direct sunlight or to excessively high temperatures. Symptoms include surface scalding, uneven ripening and excessive softening and desiccation.

e. Physical damage

Physical damage causes greatest amount of loss to fresh horticultural crops. Certain most prevalent physical damages include surface injuries, impact bursting and vibration bruising, during harvesting, transportation and storage. Mechanical injuries are not only unsightly but also accelerate water loss, stimulate higher respiration and ethylene production rates and favor decay incidence. They also render produce more susceptible to microbial invasion. Physical damage also leads to tissue discolouration.

f. Pathological breakdown

Decay is one of the most common or apparent causes of deterioration. However, attack by many microorganisms usually follows mechanical injury or physiological breakdown of the commodity, which allow entry to the microorganism. In a few cases, pathogens may infect healthy tissues and become the primary cause of deterioration.

6.3.2  Environmental factors

a. Temperature

Temperature is the most important environmental factor that influences the deterioration rate of harvested fruits and vegetables, for each increase of 10°C above the optimum temperature, the rate of deterioration increases by two- or three-fold. The term Q₁₀ if often used to denote the ratio of reaction rates with 10°C rise in temperature (Eq. 6.2). Temperature also influences how ethylene, reduced oxygen and elevated carbon dioxide levels affect the commodity. The growth rate of pathogens is greatly influenced by temperature and some pathogens are sensitive to low temperatures. Thus, cooling of commodities below 5°C immediately after harvest can greatly reduce bacterial and mold rot incidences.

………(Eq. 6.2)

b. Relative humidity (RH)

The rate of water loss from fruits depends upon the vapour pressure difference between the commodity and the surrounding ambient air, which is influenced by temperature and relative humidity.

c. Air movement

Air circulation rate and velocity can influence the uniformity of temperature and RH in a given environment and consequently rate of the water loss from the commodity.

d. Atmospheric composition

Reduction of oxygen and elevation of carbon dioxide, whether intentional such as in modified or controlled atmosphere storage or unintentional, can have a beneficial or harmful effect on deterioration. The magnitude of these effects depends upon commodity, variety, physiological age, O₂ and CO₂ level, temperature and duration of storage.

e. Ethylene

The significance of ethylene has already been dealt in previous sub-section. A concentration as low as 50 parts per billion (ppb) ethylene for example leads to kiwifruit softening at 0°C. Use of ethylene to ripen citrus fruits can accelerate their senescence and increase their susceptibility to decay-causing pathogens.