PostHarvest Technology - Topics 4

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Topics 4

Maturation and maturity indices

Learning objectives

The learner should:
• Explain the importance of maturity indices and their impact on shelf-life and quality.
• Determine maturity stage of fruits and vegetables using subjective and objective maturity indices.
• Gain skills in using the equipments for maturity indices.


The first step in the postharvest life of the product is the moment of harvest. For most fresh produce, harvest is manual, so the picker is responsible for deciding whether the produce has reached the correct maturity for harvest. The maturity of harvested perishable commodi ties has an important bearing on their storage life and quality and
may affect the way they are handled, transported, and marketed. An understanding of the meaning and measurement of maturity is therefore central to postharvest technology.

To most people mature and ripe mean the same thing when describing fruit. In postharvest physiology we consider mature and ripe to be distinct terms for different
stages of fruit development. Mature is best defined by as "having completed natural growth and development"; Most postharvest technologists consider that the definition should be "that stage at which a commodity has reached a sufficient stage of development that after harvesting and postharvest handling (including ripening, where required), its quality will be at least the minimum acceptable to the ultimate consumer."

Physiological and commercial maturity
Fruit quality is greatly impacted by maturity at harvest
Physiological maturity – refers to a particular stage in the development of a plant or plant organ. A fruit is physiologically mature when its development is over. A physiologically mature fruit may not necessarily be commercially mature. For example, papayas are harvested for domestic markets at physiological maturity, i.e. when three-quarters of the fruit assumes a yellow-to-green colour.

Commercial maturity – pertains to the timing of harvest to meet specific market and consumer requirements. A fruit is commercially mature when it reaches a developmental stage at which it can be marketed for a specific purpose, e.g. for consumption in the fresh state, or for processing. Papayas, for example, are harvested for export at the mature stage, i.e. when the fruit is firm and easy to handle. On arrival at the destination, the fruit is ripened in ripening rooms. Commercial maturity has little impact on physiological maturity.

Figure 4.1: Organoleptic quality of a fruit in relation to its ripening stage

Maturity indices
The maturity index of a fruit provides an indication of its stage of development or maturation. Maturity indices are based on characteristics that are known to change as the fruit matures. Maturity indices for harvest can be either subjective or objective (see table 1).

Table 4.1. Established methods for the evaluation of maturity in horticultural crops

Subjective maturity indices
Objective maturity indices

Subjective criteria for evaluating fruit maturity

Fruit shape and size

Fruit shape may in some cases be used to evaluate maturity. Some cultivars, for example, become less angular in cross section with development and maturation. The size and shape of stone fruit such as peaches, in particular is affected by variety, seasonal conditions, crop load and orchard variability. Constant measurement of these fruit is, therefore, vital in order to determine when they are of a marketable size. A Cranston fruit-sizing loop (see figure VI.1.2) is used for measuring the size of stone fruits.

Figure 4.2:  A Cranston fruit-sizing loop used to measure fruit size

Stone fruit are considered mature when fruit shoulders and sutures are well developed and filled out. Similarly, the fullness of the cheeks adjacent to the pedicel in mangoes (see Figure 4.3) provides an indication of maturity.

Figure 4.3: Physical cheek development of mangoes

In the case of bananas, the width of individual fingers can be used to determine harvest maturity. The maximum width of a finger located in the middle of the bunch is measured with the use of callipers. This measurement is referred to as the calliper grade.

Number of days after full bloom
‘Days after full bloom’ (DAFB) can provide an approximate harvest date or a ‘ballpark guess’. This approach relies on a reproducible date for the time of flowering and a relatively constant growth period from flowering through to maturity. The major problem with this type of measurement is that there is little consistency from year to year and a wide range in suggested DAFB values.

Fruit aroma
Volatile compounds synthesized during ripening give fruit their characteristic odour and provide an indication of the level of maturity. Fruit odour is generally detectable by humans when the fruit is completely ripe and is of limited use in commercial situations.

Fruit colour
As fruit mature and ripen they undergo a colour change from green to red or yellow (see figure. 4, below). The appearance of the colour for certain cultivars, e.g. tomatoes and litchis (see table 2) can, in some cases, signal an appropriate time to harvest. Skin colour is not, however, considered the most accurate index of maturity.

Table 4.2: Established methods for evaluation of maturity in selected horticultural crops

Figure 4.4: Standard colour charts showing fruit at various stages of maturation and ripening

Objective criteria for evaluating fruit maturity
Measurement of soluble solids or total soluble solids content
The sugar content of fruit, measured as total soluble solids (TSS) or soluble solids content (SSC) is influenced by factors such as irrigation, the nutritional status of the plant, weather conditions and the position of the fruit on the tree. It is only partially influenced by fruit maturity. While TSS data does not help in deciding when to harvest, it provides and indication of when the fruit has attained an appropriate level of sweetness. Apples, for example, should not be presented for sale if they have a TSS of less than 10 per cent. The TSS can be measured with the use of a refractometer (see Figures 4.5). Minimum SSC measurements for selected produce items are listed in Table 3)

Figure 4.5: Refractometer (handheld)

Table 4.3: Minimum soluble solids content (SSC) measurements for selected fresh produce items

Measurement of fruit firmness
Fruit firmness is affected by seasonal and orchard variability, tree vigour, fruit size, the nitrogen and calcium levels of the fruit and the pre-harvest use of growth regulators. Fruit firmness is measured with the use of a penetrometer (or pressure tester; see figure. 6). Penetrometer measurements provide an indication of the storage performance of the fruit. In many instances pack house operators refuse fruit for long-term storage if the penetrometer reading is below a specified level. In the case of apples, this reading is 14 lbf .

Topic 5   Harvesting

Learning outcomes

The learner should:
• Describe harvesting methods that are appropriate for small-scale and commercial farmers.
• Evaluate the impact of inappropriate harvesting methods on quality and safety.
• Identify the importance of using appropriate harvesting equipment and containers.

Careful and correct harvest techniques are essential in ensuring the integrity of harvested produce and preventing rejections at the pack house. Wounding during harvest can provide entry points for pathogens, so causing decay. Those involved in harvesting must be trained in efficient and careful handling of fresh produce.

Manual harvesting
Manual harvesting is one of the most popular methods of harvesting produce. Workers must, however, be properly trained if quality is to be assured (see Figure 5.1). Selective harvesting (see Figure 5.2) by well-trained and experienced harvesters ensures that fruit is harvested only at its peak ripeness. With the provision of incentives, such as bonuses for high-quality produce, field sorting as a first stage in selective harvesting can be encouraged.

Correct harvesting procedures with the use of the appropriate harvesting tools (see Figures 5.3 and 5.4) can also prevent damaged or contaminated produce from entering the pack house, thereby reducing wastage and possible contamination of sound fruit.

Figure 5.1: Training requirements to assure proper manual harvesting of produce

Figure 5.2: Selective harvesting for good quality and temporary storage of fresh produce

Figure 3. Manual harvesting equipment

Figure 4. Harvesting avocadoes using a picking pole

Harvesting containers
Rigid containers, such as wooden and plastic crates, and plastic buckets (see figure 5), can be used for the field collection of harvested produce. Containers must be smooth, with no sharp edges or projections as these could damage the produce. They must be clean and must not be overfilled.

Harvesting bags equipped with either shoulder slings around the neck, or waist slings, can be used for the collection of firm-skinned fruit such as citrus and avocados. They are easy to carry and leave both hands free to harvest. Harvesting bags must be designed to open at the base, so as to allow produce to be emptied easily into a field container without tipping the bag (see figure 6).

Figure 5. Collection of fruit in different containers to minimise damage

Figure 6. Harvesting bag

Mechanical harvesting
Mechanical harvesting results in a significant reduction in management and labour costs and can result in savings as high as 30-45 per cent. Mechanical harvesters are primarily designed for a shake-catch action, which helps to detach the fruits by shaking or vibrating trees or bushes. Given that they have no means of detecting quality, even carefully adjusted harvesters will harvest significant quantities of unacceptable fruits. Immature, overripe, diseased and damaged fruit, which would ordinarily be discarded by a human picker, are often harvested by mechanical harvesters. Elimination of unacceptable fruit and foreign matter is difficult and costly. Mechanical harvesting is, therefore, recommended for large-scale operations where labour is not available.

Good practice during harvesting
Containers used for field collection:
o must be smooth, with no sharp edges or projections to damage the produce;
o must be clean; and
o must not be overfilled.

• Harvested produce:
o must not come into contact with the soil or contaminated surfaces, e.g. surfaces that are visibly contaminated with dirt, oil or chemicals;
o must not be dropped; and
o must be gently transferred to collection bins and protected from sun or rain until such time it can be transported to the pack house.

• Cuts and bruises must be avoided during harvesting operations
Time of harvest

The time of day and prevailing weather conditions can influence produce quality and shelf life:
• Produce must be harvested during the coolest time of the day, i.e. early morning or late evening when the physiological activity of the fruit is low
• Produce must not be harvested when wet (from rain or dew), because wet produce generates heat and decays quickly
• Harvested produce must be maintained in a cool and shady location with adequate ventilation

Temperature management at harvest
At harvest, the temperature of the fruit is close to that of the ambient air, which varies according to the location and time of year. In order to ensure the lowest possible temperature at harvest, it is generally recommended that fruits and vegetables be harvested during the coolest part of the day, which is usually early morning. Citrus fruit are one exception to this recommendation, given that they are damaged if handled in the morning when they are turgid.

Harvested produce must be retained under shade or in a cool temporary storage area, and must be pre-cooled within the shortest period of time in order to remove the field heat.

Good practice for the maintenance of harvesting equipment
All field equipment used in harvesting produce must be cleaned and repaired on a regular basis. It is important that farmers ensure that all equipment (i.e. knives, pliers and so on) issued to harvesters is accounted for at the end of the picking day. Farmers must also check whether all equipment is still sound and unbroken. If a knife blade is broken, the harvester’s batch numbers must be traced and the produce put on hold to prevent physical contamination. If produce has been delivered to a pack house, the manager should be informed and the batch put on hold. If a pack house is HACCP (Hazard Analysis and Critical Control Point)-certified, it is essential that pack house management be notified. Under such circumstances, pack houses might implement online metal detectors for the identification of contaminated batches.

Torn bags, broken boxes and other containers used in harvesting must be repaired if produce damage is to be avoided. Wooden splinters from containers, for example, could cause wounding of produce, ultimately resulting in infection and decay. Broken equipment is also difficult to maintain in a hygienic condition since small cracks provide the ideal niche for microbes, which may cause decay or present a food-safety concern.

Regular cleaning of all harvesting equipment is essential. All harvesting tools must be washed daily in a soap solution and in certain cases, as occurs with heavy soil or sticky substances, a disinfectant such as bleach should be used at recommended concentrations. The intervals for cleaning larger items of harvesting equipment, such as large collection containers, will vary in accordance with the type of produce harvested. It is generally recommended that harvesting bags be washed at the end of the harvesting season, while harvesting crates should be washed daily to remove dirt and debris. Crates used for mangoes should be washed on a daily basis to remove the latex secretions from the stem-end of the fruit; if not removed regularly, the mango crates will become stained and soiled, which could pose a food-safety risk.

All harvesting equipment should be stored overnight in a closed facility, protected from rats and birds. The most important reason for protecting the equipment at night, or over weekends or during non-harvesting periods, is to ensure that the equipment stays intact and is not contaminated with animal faeces unnecessarily, which could introduce a food-safety risk further down the chain. Maintaining harvesting equipment makes sound economic sense, since such equipment often reflects considerable investment by the farmer.

Figure 6. Firmness testers used in the fruit industry

Measurement of starch content
The storage life of fruit is greatly influenced by the starch content. As fruit ripen, starches are converted to sugars. This starch to sugar conversion is initiated in the core of the fruit, moves to the flesh and ultimately to the skin.

An iodine test provides an indication of the distribution of starch in the fruit and helps to estimate the extent to which starch has been converted to sugars. This test can be performed by cutting the fruit in half and dipping it in an iodine solution. Iodine reacts with the starch, resulting in a navy blue/black colour. The stained pattern is compared with the use of a rating scale and the average rating of the sample, known as the starch index, is calculated. This is a reliable method. Iodine solution (2 per cent) is readily available from a local chemist. The starch test should be performed on a weekly basis or every three to four days for varieties that undergo rapid starch breakdown.

Figure 7. Starch measurement and rating system

Measurement of juice content
The juice content of many fruits increases as the fruit matures on the tree. The juice content is determined by extracting the juice from a representative sample of fruit in a standard and specified manner. The volume of juice extracted is related to the original mass of fruit, which is proportional to its maturity. Minimum values for citrus juices are presented in table .4.

Table VI.1.4. Minimum juice percentages required for good marketability of different citrus varieties
Citrus fruit Minimum juice content (%)
Clementines 40
Grapefruit 35
Lemons 25
Mandarins 33
Naval oranges 30
Other oranges 35

Measurement of oil content and dry matter
Oil content can be used to determine the maturity of fruit such as avocados. Oil content can be determined by weighing 5-10g of avocado pulp and then extracting the oil using a solvent (such as benzene or petroleum ether) in a distillation column. This method has been used successfully for cultivars that are naturally of a high oil content.

The level of acidity in many types of fruit, changes progressively during maturation and ripening. In the case of citrus and other fruits, the level of acidity is reduced progressively as the fruit matures on the tree. Titration of a juice sample against a standard alkaline solution gives a measure that can be related to the optimum time of harvest. The sugar to acid ratio or total soluble solids (TSS) to acid ratio (Brix: acid ratio) is often better related to the palatability of the fruit than either sugar or acid levels alone.

Specific gravity
Specific gravity is the relative gravity or weight of solids or liquids as compared to pure distilled water at ambient temperature. Distilled water has a specific gravity of one. The specific gravity is obtained by comparing the weights of equal bulks of other bodies with the weight of water. In practice, the item of fresh produce is weighed in air, then in pure water. The weight in air divided by the weight in water gives the specific gravity. As a fruit matures, its specific gravity increases.

Specific gravity is rarely used in practice to determine time of harvest, but could be used in cases where development of a suitable sampling technique is possible. However, it is used to grade crops according to different maturities post-harvest. This is done by placing the fruit such as mangoes in a tank of water; those that float are less mature than those that sink.

Sampling of fruit for evaluating maturity
In order to determine the optimum harvest date within an orchard, a random sample of 20 fruit must be collected from each orchard block and the soluble solids, firmness and starch content measured. The following steps must be followed for correct sampling:

• Select five trees at random within the orchard block
• Depending on the size of the fruit, pick fruit (about ten for small fruit such as litchi) of similar size from each tree at eye level, approximately 1.5m from the ground
• Select a mix of fruit from the inside and outside of the canopy and from a north, south, east and westerly direction; damaged fruit should be avoided
Sampling should be done a number of times leading up to harvest.

Maturity indices for selected crops
Maturity indices for selected fruits and vegetables are shown in Tables 5 and 6.

Table 5. Maturity indices of selected tropical fruits
Apple Banana
• Textural properties – firmness, tenderness, starch and sugar content
• Burst of internal ethylene production • Bunches are harvested when the top leaves start drying
• The colour of the axis of the fingers changes from dark to light green
• Brittleness of the floral ends should fall with slight touch
• Changes in the angularity of fingers from triangular to round or sharp
• Number of days from emergence of inflorescence: 95-110 days
• Pulp to skin ratio – 120:1.2
Citrus Mango
• Change in colour (green to orange)
• Ease of separation
• Starch content
• Rate of respiration
• Days from blooming
• Seed colour (green to brown)
• Change in organic acid
• Juice content (>50 per cent) • Slight colour development of the shoulder or fullness of the shoulders; change in colour of pedicel from green to brown
• Growth of the fibres on the stone/corrugations
• Flow of latex from the stalk, e.g. faster drying latex
• Summation of days taken from flowering to maturity by tagging flowers
• Appearance of bloom on the surface of the fruits
• Computation of heat units or cumulative degree days
• Change in lenticel morphology
Specific gravity of 1.0-1.02 for varieties Alphonso and Pairi
Papaya Pineapple
• 33% colour development for long-distance market and 85.5% colour development for local market
• Harvested when fruit show signs of yellow to purple colour
• When fruits show signs of yellowing
• High TSS and low acidity (TSS 13%; acidity 0.5– 0.6%)
• Tips of the bracts projecting as the eyes start drying
• Acid ratio 21-27 and specific gravity 0.98–1.02.

Table 6. Maturity indices for vegetables

Asparagus Carrots
• Spears grow above the ground
• Spears should be harvested when they are not too long, before tops begin to spread
• Size is the primary consideration and at least ¾ diameter
• Proper colour development, without zoning

Cauliflower Okra
• Head size and condition and before heads become discoloured, loose, ricy or blemished
• Over-mature flowers result in flower stocks that are too long; they elongate, becoming fuzzy and ricy
• Pods are still young and tender, exhibiting maximum growth
• The pods are readily shaped when picked
• When mature, pods are fibrous and tough

Peas Tomato
• Sugar content > 5-6 per cent;, sugar declines with maturity; ratio of starch to protein increases
• Tenderness and appearance of pods – should be well filled with young tender peas
• Changing in colour from dark to light green, with firmness of 5kg/cm2 • Mature green, pink or breaker (just starting to turn red) and red ripe
• Pulp surrounding the seeds is jelly-like, seeds slip away from the knife
• For long-distance shipment, is harvested at the mature, green stage
• The ripe stage indicates that most of the surface is pink or red, and firm