Post-production advice

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This manual does not attempt to cover the technical details of post-harvest handling of horticultural crops. These are covered in a number of other publications by FAO and others. The main emphasis here is on the commercial implications of different harvest and post-harvest practices.

Harvesting. The timing, technique and conditions at harvesting can significantly affect prices.

Harvesting prices. With some crops, harvesting can be undertaken early to take advantage of high-priced opportunities, e.g. cabbage harvested as spring greens, young carrots sold in bunches, green plums and new potatoes. Exploiting these short-term market opportunities requires a close link with the market.

Harvesting and crop maturity. Shelf life and long-term storage is affected by the maturity of the crop at harvest. The storage characteristics of root vegetables are generally improved by only harvesting fully mature crops. Examples are sweet potatoes, tannia, carrots, onions, garlic, potatoes and yams.

Harvesting of cassava tubers can start between seven to ten months after the planting of cuttings, depending on variety. Cassava does not store well and on small family farms the largest tubers are harvested first, without cutting the stems. The small tubers are allowed to grow on. The production of cassava roots is at its highest 18 to 20 months after planting.

Long-term storage of cabbages depends on the cultivar and growing conditions. Cabbages suitable for storage are normally slow growing, large types grown under cool conditions and harvested at the correct stage of maturity. Harvesting should take place after the head has formed tightly and before the outer leaves start to die or the head shows any sign of splitting. At the correct stage of maturity freshly harvested cabbage heads should squeak when rubbed together.

Melons are another example of a crop where the timing of harvest is crucial for quality-too early and the full sugar content is not developed, too late and they lose sugar and become soft. Cantaloupe melons should be harvested when the fruit separates easily from the plant. If only part of the stem pulls off then the fruit is not ripe and will never ripen to a full flavour. Honeydew melons and watermelons do not separate from the stem when mature. A honeydew melon is ready for harvest when the fruit is well filled out, there is just a hint of green and the surface is covered with fine hairs. Watermelons should be harvested when the ground spot is pale yellow and the fruit gives a hollow sound when hit with the knuckle.

Some fruit have to be harvested when they are not completely ripe in order to transport them to distant markets This is particularly true of fruits which are not suitable for long-term storage but need their shelf life maximized. Examples are bananas, pineapples, mangoes and avocados.

In the case of bananas the fruit is generally harvested when still green and only at between three-quarters full (80 days from shooting) to high three quarters (90 days from shooting). The longer the period of transport the thinner the fingers at the time of cutting. Maximum storage of two to three weeks is achieved by keeping the fruit at 12-14Cin 85 to 95 percent relative humidity. On arrival at the market bananas are ripened either in special ripening rooms or by allowing the ambient heat to trigger the release of the ripening gas, ethylene. For local marketing the fruit is harvested when fully mature but before ripening has started.

Avocados and mangoes will generally ripen during transport. Pineapples for local consumption or canning are normally harvested when the fruit has yellowed up by 25 to 50 percent. For distant markets harvesting should take place when the first hint of colour change has been observed at the basal end.

The optimum harvesting stage for most crops will depend not only on the climate and distance to the market but also on variety and growing conditions. In individual cases, when new distant markets are being explored, experiments should be carried out to find the best stage to harvest fruits, by sending samples at different degrees of ripeness and assessing which is most favoured.

For fruits which are suitable for long-term storage, such as apples, pears, citrus and grapes, there are significant differences between cultivars, growing region and sometimes season in the optimum harvesting time. For example, maturity indices of citrus are based on juice content by volume, total soluble solids in the juices and the solids: acid ratio, according to variety and market. In general, fruits should contain at least 40 percent by volume of juice and the total soluble solids should be in excess of eight percent. Specialist skills and techniques are necessary which are beyond the scope of this book. Apples for long-term storage should be picked when fully mature but not fully ripe. The extension officer should call in the necessary expert assistance if long-term crop storage could significantly improve farmer incomes.

Harvesting and quality. What is often not understood by growers is the effect of their harvesting and handling on the quality of the produce in the market. Once a fruit is plucked from a plant or a root or leaf vegetable is harvested, it is cut off from its source of food and, particularly, water. The effects of poor treatment normally show themselves some days later, when the produce is being presented for sale or is in storage. Poor treatment has two effects; firstly the price is reduced and secondly, in the long term, the reputation of the production area is diminished (again tending to result in lower prices).

An improved system of harvesting and handling produce will result in a product with better appearance and shelf life. In general, prices for the produce will be improved but sometimes the system has to be changed to ensure that the price rises are passed back to the grower.

Timing of harvesting during the day. Ideally harvesting should take place when the crop and the climate is coolest and the plant is most turgid, i.e. has the highest moisture content. This is in the early morning. In practice other criteria also have to be taken into account. For example, the dew should be dry on citrus end the latex flow of mangoes is at a minimum a/mid-morning. Harvesting also has to take into account labour availability and when collection will take place, to minimize the time produce is left standing in the field.

Harvesting techniques. On high trees fruit can be harvested with a hook and a catching bag on a pole or similar harvesting aid. This prevents fruit falling to the ground.

For other crops knives and clippers can improve harvesting practices because they can cut through fibrous tissue, stems and leaves can be trimmed and clean cuts reduce the likelihood of infection. Tools like this are used for harvesting lettuce, cabbage, sweet pepper, egg-plant, honeydew melons and banana. In the case of bananas a slight cut is made in the upper pseudostem to allow the bunch to ease down gently. Individual hands are cut off from the bottom hand upwards.

Leafy vegetables are harvested by cutting the plant with a sharp knife as close to the root as possible. Uprooting results in soil coming into contact with the produce.

Bulb crops such as garlic and onions are harvested by pulling the leaves at the neck and then cutting the leaves about 3 cm from the bulb.

Occasionally diseases can be transmitted from plant to plant. Tools should be cleaned often and, when virus diseases are a problem, knives should only be used for trimming not for cutting the fruit from the plant.

Many fruits are harvested by hand, e.g. apples, citrus, papaya, peppers, tomatoes. The fruit should beheld by the palm of the hand not by the fingers. Whenever possible the harvesting should be carried out by plucking the stem, e.g. with strawberries, fine beans, peas.

Tuber and root crops are normally harvested with forks or hoes. The digging should start some 15 cm (6 inches) away from the tease of the plant. In general, it is preferable to lever and pull the roots rather than attempt to dig the roots out. Harvesting is easiest when the soil is relatively dry as both damage and the need for washing is reduced.

Field containers. Picking bags or baskets attached to the waist of the picker enable both hands to remain free. The crop damage associated with moving sacks of produce through the field is reduced. With picking bags it is preferable to be able to release the bottom so that the produce can be let out gently, rather than upending the bag.

Baskets or boxes with sharp or rough edges should either be avoided or lined with paper or leaves. Damage is often caused by transferring produce from one container to another. If possible, produce should be harvested into the container in which it will be stored or transported.

Harvesting system. With highly perishable produce damp cloths can be used to give protection against the sun's heat. Field containers should be removed to a shaded area as soon as possible. Some leafy vegetables may be sprinkled with water at intervals to maintain leaf turgidity. Field assembly points, such as a shadehouse made out of natural materials or a canvas tent, should be used in order to keep the produce cool and allow ventilation.

In general, the quality of fruit and vegetables cannot be improved after harvest. However, the more careful the handling the slower is the deterioration in quality. Containers must be emptied carefully to minimize drop heights and fruit-to-fruit damage. Containers should be periodically cleaned.

Curing and drying. Bulb crops such as onions and garlic can be dried in the field by being spread one layer thick over about six dry days. Alternatively drying can take place in stacked shallow trays under cover. The aim is to harden the outer scales and remove moisture from the neck in order to extend storage and marketing life.

Most root crops (but not cassava) respond to warm moist conditions after harvest by thickening and hardening their skins. This provides protection against dehydration and infection. Wound healing occurs. This is called curing and it significantly improves storage life. Curing can be carried out in tropical areas at little cost by stacking the produce in conditions where temperature and humidity are allowed to rise to 25 to 35C with a relative humidity of 85 to I 00 percent for one to seven days, depending on crop and variety.

Trimming and sorting. Cabbages, cauliflower, chinese cabbage and lettuce will have their outer leaves trimmed, except for three or four wrapper leaves, to give some protection to the head. Long stalks attached to the fruit, as in citrus, should be cut as close to the fruit as possible to prevent damage to other fruit.

Provided the market wants graded produce and is prepared to pay for it then selection and grading are justified. The additional prices must cover the additional costs. Buyers may specify grading standards, particularly in the export market where international standards may be enforced. Produce for longterm storage should be disease and blemish free and therefore needs to be sorted. When transport is expensive it is often only justifiable to send top quality crops. Produce is generally separated according to quality criteria, it may also be graded according to ripeness or colour and size. The crop is then normally packed into different containers. This facilitates marketing into different markets.

Grading and packing is often carried out on the ground under the shade of a tree. This is both unhygienic and inefficient. Specialist grading areas or sheds are generally open-sided, with tin or preferably thatched roofs. Grading while standing or sitting at tables enables people to work faster Tables covered with polythene sheeting are easy to clean and the sheeting can be replaced cheaply. Lighting should be good. Tin roofs can be painted white to reflect heat while water trickled down the outside of a shed helps reduce the heat inside the building.

Packaging and presentation. The two main functions of packaging are to help prevent mechanical damage and to sort the produce into an acceptable size for the market and for handling. Good packaging can also enhance the attractiveness of the produce.

The four main types of mechanical damage are cuts, compressions, impacts and vibration rubbing.

Care in harvesting and handling will help eliminate cuts and wounds. Lining of packaging material with paper or leaves can also prevent damage.

Compression bruises can be restricted by using containers which are strong enough to withstand multiple stacking. The packaging materials need to be particularly strong at the vertical corners. The packaging should also be shallow enough to prevent the bottom layers of produce being damaged by the weight of produce above. Cartons must not be overfilled. Damage is caused by the full weight of the pile of produce pushing down on the top layer of fruit or vegetables, causing the weight to be transmitted to the lower layers.

Impact damage and bruising can be the result of shocks in transport or dropping. This may occur either because each package is small enough to be thrown or too big to be easily handled.

Vibration damage generally occurs during transport; vibration being transmitted through the produce. This kind of damage can be significantly reduced by achieving a balance between preventing the produce from moving within the packaging and forcing the produce together. Fruits are prevented from rubbing against one another by the use of cellular trays, individual wraps or cushioning pads. An example is paper and straw used to separate layers of apples. Alternatively, the box is gently shaken to settle the produce and then the space created is filled.

Reference was made earlier to the size criteria of packaging (see Chapter 3). The largest size should not exceed 50 kg as this is the maximum weight which can be easily handled. Below that the size specification will depend on the customers' requirements-be they the retailers or consumers.

As the calculation in Table 11 indicated, packaging can be the single most expensive cost, particularly with non-returnable containers made of wood or cardboard. The benefits must be shown to justify the investment. In the next example, where $3 000 is invested in packaging for 6 000 kg of cucumbers worth $14 850, the costs can be said to be worthwhile if:

TABLE 13. Cost-benefit analysis of packaging

  $
Net income with packaging for sales of 6 000 kg of produce 14850
Less cost of packaging (or $ 0.5/kg) 3000
Income to obtain the same return without packaging 11850
Therefore: Average sale price can tall to $1.975/kg, i.e. by $ 0.5/kg

Or: 1212 kgs can be wasted, a factor of 20 percent.

In practice the situation is usually more complex. Without packaging both wastage and damage levels will increase. The question is whether the cost of reducing losses results in a lower or greater profit for the grower.

Sometimes, instead of crates, a cheaper but possibly more cost-effective method for packaging and transporting may be tried. For example, for loose produce, e.g. melons, it may be possible to introduce a combination of sacks and straw cushioning in the truck together with shelving. This will again affect not only the quality and quantity of produce sold but also the transport costs.

When attempting to introduce new types of packaging, the extension officer's first task is to compare its cost with the existing system of packaging. Subsequently he should monitor trial shipments and then carry out a cost/benefit analysis using actual, rather than theoretical, figures. The key is to select costeffective packaging which is appropriate to the demands of the market.

Produce packaging materials can be conveniently divided into six classes.

Locally available natural materials, for example baskets woven from bamboo, willow or cartons made from thin strips of wood or rushes. Typical problems associated with these materials are:

There are, however, a number of advantages with using locally available materials. Material costs are low. Both jobs and incomes are created for local businesses who make the packaging. Local sources of packaging also make it easier to ensure its timely arrival.

The benefits are such that in the first instance the extension officer should try to develop and improve on the use of existing local packaging. This may be achieved by new designs or through improvements in the handling system.

Wooden boxes and trays are widely used throughout both the developing and developed world. They are strong, rigid and can be manufactured locally as well as recycled. They can also withstand refrigeration. However:

Improved design is particularly likely to result in both savings in wood and reduced crop damage. The European produce tray has been successfully introduced in a number of countries. Critical design features of this tray include:

Shallow trays are used for easily bruised crops such as tomatoes, peaches, grapes and mangoes. Deeper boxes are used for apples and citrus. Larger but flimsier boxes are often used for cabbages and cauliflowers. In these developments the extension officer needs to work closely with the local box manufacturers. Again, pricing of packaging per kg of produce needs to be compared with that of existing packaging and should be tested before commercial introduction.

Fibre board or corrugated cardboard are increasingly being used, particularly in developed countries. There are a number of very cleverly designed boxes which can be copied. The boxes are light and easily printed and can be made to look very attractive. However:

Recent design improvements include boxes that are made from a combination of wood, for structural strength, and cardboard. Plastic has also been incorporated in designs, particularly to increase strength at the corners.

Plastic containers are expensive and generally have to be imported. They are so expensive that they have to be recycled and are mainly used as field boxes or to supply a regular outlet such as a factory or supermarket. Some polystyrene packaging is now being used for non-recyclable containers.

Bags and nets are cheap but provide no protection from damage. They can be used to package suitable produce like onions and potatoes into convenient units for handling and marketing.

Plastic and paper is often used as lining or wrapping for produce.

Printing, packaging presentation and brand names can all add value to produce but only in markets where consumers are wealthy and appreciate aesthetics and image. For example, in the produce markets of the Arabian Gulf multi-coloured printing is common because it has been observed to increase returns. In contrast most of the African and Asian markets are insensitive to the quality of packaging material used, if indeed packaging is used at all.

Much emphasis has been given to improved handling, grading and packaging. Improved prices may be expected because the market will respond to the quality of the individual consignment. In the longer term, premium prices can be obtained by establishing an identity and a reputation as a consistent high quality supplier. An extension officer may help to achieve this by:

This top quality produce would be sold into high-priced, quality-conscious markets. This is called creating a brand image. There are many examples of areas which always obtain premium prices because of their reputation for supplying good quality produce.

It should be remembered that it can take years to establish a good reputation but it can be quickly destroyed by one or two poor consignments.

Storage. Produce can be stored for both short-term and long-term purposes.

Short-term storage is used to provide flexibility in marketing, for example when awaiting transport or because buyers are not immediately available. Most horticultural crops are perishable end can only be stored for a few days. Only rarely is it worthwhile storing crops of this nature to await higher prices. Storage will reduce quality and shelf life. It is costly and, in most instances, when the produce is withdrawn from storage it has to compete in the market against freshly arrived produce,

A few crops are adapted for long-term storage (see Table 14). These can be held in stores well beyond the normal harvesting period. In turn, higher prices can normally be obtained and greater volumes of produce sold. Often in the case of cold storage the successful stores are located in urban areas because:

Refrigerated storage is much emphasized in the literature but extended shelf life can be achieved without investment in expensive equipment. In practice the quality of the produce and humid, shady conditions are higher priorities. (See Table 15.)

Ventilated stores in the right conditions with good management can be extremely cost-effective. Ideally they require cool night temperatures. The building should be protected from the sun's heat by such techniques as shady trees, painting the building white and double-skinned walls. The building should be positioned to intercept the prevailing night time winds. When the ambient air temperature falls below that of the produce, normally at night, the air is allowed to flow through the stored produce by the opening of louvres. This process can be automated and fans can be used to increase air flow rates.

Evaporative cooling from the incoming air assists in cooling and humidifying the store.

This kind of store can be used for holding potatoes through the winter (three to nine months) provided they have been cured and treated with sprout suppressant. Onions and garlic can also be stored using the same techniques but with lower humidities. Garlic in California can be held for three to four months. In onions there are great differences between varieties and production locations. Both crops will need to have been dried and cured in the field. Sweet potatoes need to be cured at 28 to 30C for a few days. Subsequently they can be stored for up to six months. Cabbages, carrots, pumpkins, apples, pears and lemons have all been successfully stored using this technique.

In Syria, unirrigated apples are stored in caves for nearly 10 months. Yam barns are common in West Africa where individual yams are tied to 2-m high vertical posts under a palm-thatch roof. In other parts of the world yams are stacked vertically on a raised platform under a straw roof. These structures are very effective, as the tubers receive adequate ventilation and are protected from both termite attack and flooding.

The extension officer can improve on-farm storage practices by training growers in correct techniques and by himself carrying out comparisons between the recommended practices and those that the farmers normally undertake. At an open day the difference between the two batches of crops can make a vivid demonstration of the validity of his recommendations. Photographs should be taken to be able to demonstrate the differences in future years.

TABLE 14. Storage life and recommended storage conditions of crops suitable for long term storage

Crop

Degrees (Celsius)

Relative humidity (Percentage)

Storage life (Months)

Fruits      
Apples ** +

0 to 4

90 to 95

2 to 6

Fresh date

0

85

1 to 2

Coconut

0 to 1

80 to 85

1 to 2

Grape ** +

-1 to 0

90 to 95

1 to 4

Persimmon

-1

90

3 to 4

Kiwifruit +

-05

90 to 95

2 to 35

Orange ** +

0 to 4

85 to 95

3 to 4

Pear ** +

0

90

2 to 5

Lemon (coloured) +

0 to 4 5

85 to 90

2 to 6

Mandarin

4 to 6

85 to 90

1 to 1 5

Mangosteen

4 to 5

85 to 90

1.5 to 2

Lemon (green)

10 to 14

85 to 90

1 to 4

Casaba melon

9 to 13

85 to 90

1 to 1 5

Honeydew melon

9 to 13

85 to 90

1

Vegetables      
Cabbage **

0

95

1 to 3

Carrot (topped) ** +

0

95

5 to 6

Celery

0

95

1 to 3

Garlic +

0

65 to 70

6 to 7

Leek

0

95

1 to 3

Onion (dry) ** +

0

65 to 70

6 to 8

Parsnip

0

90 to 95

2 to 6

Turnip

0

90 to 95

4 to 5

Potato +

4 to 6

90 to 95

4 to 8

Ginger +

13

65

6

Pumpkin

10

to 13 50 to 75

2 to 5

Sweet potatoes +

13

to 16 85 to 90

4 to 7

Yam +  

16 85 to 90

3 to 5

** Dependent on cultivar and origin

+ Commonly held in long-term storage

TABLE 15. Generalized storage advice for fresh produce

Advice

Reason

Harvest produce at the proper maturity stage Immature produce has thinner skin resulting in faster evaporation
Keep produce in shade Water losses are tour times quicker in sunlight
Store only crops which are clean Diseased produce may infect sound crops. Damaged produce is easily infected and loses water. Dirt is a source of disease
Remove leaves attached to fruit and root crops Leaves lose water rapidly
Apply approved sprout suppressant to potatoes, onions, garlic and ginger Helps reduce sprouting during storage
Wash fruit in chlorinated water (200 ppm), benlate or baristan for about half a minute, rinse and dry Controls fungus disease
A thin coating of petroleum jelly will reduce shrivelling Water loss is reduced
Line baskets with paper or leaves, and containers with polythene film with some ventilation holes All act as barriers to high moisture loss
Store as soon as possible The quicker produce is cooled, the slower the water evaporates and microbial activity is reduced
Storage rooms and containers should be clean Reduces chance of infection from previous crop
Allow air circulation Removes heat and ethylene given oft by produce
Separate ripe from unripe fruit Ripening gives oft ethylene which hastens ripening of unripe fruit
Avoid mixing produce in the same storeroom Odours and gases given off can damage other crops
Store leafy vegetables at a high relative humidity Dry air rapidly draws moisture out of leaves
Cool moist conditions can be created by dripping water through burlap or jute sacks which serve as the wall covering of the cooler The latent heat of evaporation cools the air. The high humidities lessen water loss
Keep root crops in moist and slightly warm environment for 10 days before storage This is called curing and hastens the healing of wounds
Bulb crops should be dried or cured until the neck is tight and the outer scales rustle Diseases, particularly neck rot, are controlled, and moisture loss is reduced
Store roots and bulbs in drier atmosphere than other produce Root crops sprout easily under moist conditions
Store tropical produce at 10 C or above, and temperate crops at below 10 C Tropical crops suffer chilling injury at low temperatures
Use containers which can withstand stacking Optimizes volume of store without injuring produce
Maintain high humidities in cold stores by preventing entry of warm air through using plastic strip curtains, keeping doors closed and wetting floors High humidities for most crops reduce shrinkage and weight loss
Do not store onions in sacks piled to more than six high To minimize compression damage
Keep potatoes stored in the dark In sunlight they become green and poisonous
Small produce grown without irrigation tends to store best Small cell size and high solid matter content restrict water loss
Clean, moist sawdust can be used to store fruit like tomatoes, rambutans and mangoes High humidities are maintained. Sawdust should be dried before re-use

Transport. Most growers who do not sell to traders will transport produce to market in hired lorries or pick-ups. The grower will be given a fixed price for individual use of the transport or the lorry owner will charge by the canon. In both systems there are possibly two major inefficiencies.

Firstly, if the lorry is not fully loaded the unit costs are higher. Cost savings can be achieved by improving the process of assembling produce and encouraging the sharing of transport. The extension officer may, for example, encourage growers to assemble produce on a particular day of the week at specified collection points. Larger volumes of produce can bring cost savings through both economies of scale and by attracting a number of lorry owners to encourage price competition between them.

Secondly, when transport is costed by the canon, transporters will generally overload lorries in order to maximize their income. By assembling produce at one point so as to guarantee full loads, a fixed price for the lorry can be negotiated and the growers can themselves ensure that the transport is not overloaded.

Generally the larger the individual load, i.e. the larger the truck used, the cheaper the unit/cost of transport. In Pakistan, for example, an 8-tonne lorry travelling from northern Punjab to Karachi costs 4 000 rupees, i.e. 500 rupees per tonne of produce. The recently introduced 20-tonne articulated lorries cost 7 000 rupees for the same trip, i.e. 350 rupees per tonne. Longer wheel-based vehicles reduce vibration and consequently crop damage.

Farmers who make investments in transport mainly buy small trucks. The unit costs of transport are therefore generally higher than when hiring space in a larger lorry. Pick-ups do, however, offer farmers the advantages of:

In Table 16, some general advice to reduce damage in the transport of fresh produce is provided.


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