2. Fresh produce

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2.1. The living food

In most societies, starchy staple foods, particularly cereal grains, provide the main energy requirement of human diet. In certain areas, especially the humid tropics, root and tuber crops together with plantains and other cooking bananas are the staple diet, or are used to supplement the cereal staples.

Fruit and vegetables are important sources of minerals and vitamins and when eaten together with some of the root and leguminous crops (pigeon pea, beans etc.) can provide a proportion of human requirements of protein. In addition fruits and vegetables add variety and colour to what might otherwise be a monotonous diet.

Fruits and vegetables are living plant organs which when growing exhibit all the features indicative of plant life, such as respiration, transpiration, synthesis and degradation of chemical constituents, and possibly also photosynthesis as well. When harvested, the produce is at once removed from a source of water, mineral and organic nutrients, but it remains living as shown in Figure 2.1 (see Figure 2.1. Fresh produce remains living after harvest) below. Greening and sprouting of stored onions and root tubers, the sweating of produce in polythene bags as a result of transpiration and water loss, are just a few examples of this retention of living processes.

 

2.1.1. Energy Requirements

Starches and sugars, formed within the plant for its own use are used as energy foods by people. Starch is the main component of root and tuber crops, also of plantains and green bananas. Fruits and some vegetables contain sugars as an energy source. Oils and fats are also energy foods. Fresh produce contains only very small amounts of these, except avocadoes, which may contain 15-25% oil.

 

2.1.2. Food for Body Growth and Repair

Proteins are essential to the building and repair of muscles and other organs. They are required in large amounts by growing children.

Fresh produce is low in protein content, although on a dry weight basis some root crops such as sweet potato and Irish potato and leaves of several crops, have protein contents approaching that of animal products. Cassava has a very low protein content.

Minerals - Many minerals are required for healthy living but only in small amounts compared with energy foods and proteins. Sodium, potassium, iron, calcium, phosphorous and many trace elements are essential. Vegetables contain significant amounts of calcium and iron, and of some other minerals.

Vitamins - are essential for the control of chemical reactions in the body. Fruits and vegetables and to a lesser extent root crops, are important sources of Vitamin C and other essential vitamins.

Fibre or "roughage" - Fresh produce contains large amounts of indigestible fibres, known as "roughage". Medical research has shown that a high fibre content in the diet may reduce susceptibility to several diseases in man, notably heart disease. Thus a high level of fresh produce in the diet is considered to have a beneficial effect.

 

2.1.3. Post-Harvest Effects on Nutritional Value

The preparation of fresh produce after harvest will affect its nutritional value, for example:

  1. The dry matter content (energy supply) is reduced with time as the continuation of living processes within the produce uses up stored food reserves.
  2. Vitamin C content decreases with time after harvest, little may be left in green leaves after two or three days.
  3. Cooking partially destroys vitamins C and B1. Raw fruit and vegetables are particularly valuable provided they are grown and handled hygienically.
  4. Peeling may cause significant loss of food value, especially in Irish potatoes and many other root crops where the protein and vitamin content is located immediately under the skin.
  5. Minerals, including trade elements are dissolved out in cooking water, which is often thrown away.

Further information on nutritional value of fresh produce can be obtained from your local Nutritional Council.

2.2. Types of fruits and vegetables

Compared with all other foodstuffs, fruits and vegetables are characterized by an extreme diversity of size, form, structure and physiology (see Figure 2.2. diversity of fresh produce types and structures, over page). This diversity is a result of evolution and natural selection but some is of course due to breeding programmes in which the edible parts have been accentuated.

Fruits and vegetables are grown all over the world under many different conditions and thus have inherent structural and physiological features which enable them to function normally under the growing conditions for which they are adapted.

This diversity means that different types of fresh produce will react differently to the environment and its changes. Produce harvested from a farmers plot may endure many changes in temperature, humidity, and physical handling before the consumer prepares it for consumption. Therefore when we think about marketing of fresh produce we should also think about the physical and environmental aspects of marketing as well and remember what might be perfectly alright for one type of produce may be disastrous for another and different type of produce.

2.3. How and why spoilage occurs

2.3.1. What are the Causes of Losses?

All fruit, vegetables and root crops are still alive after harvest. They contain from 65 to 95 percent water, depending on the type of produce. For example, watermelons contain about 95% water, while potatoes, yams and other starchy root crops are from 65 to 70 percent water. They also contain food materials which enable living processes to continue. After harvest the continuation of living processes in the produce uses up both the water and stored food.

  1. Water is lost from all produce. When the amount lost is from 3 to 10 percent (depending on the plant parts involved), the produce begins to wilt or shrivel up and cannot be restored to its original condition.
  2. At the same time water is being lost the food materials are also being used up.
  3. Loss of water and food materials means a loss in weight of the produce.
  4. Eventually the food and water reserves are completely used up and the produce breaks down and decays.
AS SOON AS PRODUCE IS HARVESTED THE PROCESSES LEADING TO BREAKDOWN BEGIN, AND CANNOT THEN BE STOPPED: THE RATE AT WHICH BREAKDOWN OCCURS CAN, HOWEVER, BE SLOWED UP AND LOSSES MINIMISED BY EMPLOYING THE CORRECT HANDLING METHODS AFTER HARVEST.

 

2.3.2. What Affects the Rate of Breakdown ant Loss?

The conditions to which produce is exposed after harvest govern its rate of deterioration. These are:

  1. The temperature of the produce, which is related to the temperature of the environment, and the heat of respiration of the produce.
  2. The extent of damage inflicted during market operations, including physical and physiological damage.
  3. The moisture content of the environment.
  4. The effect of infection by decay organisms (e.g. fungi or bacteria).

 

2.3.3. Temperature Effects

HIGHER TEMPERATURE AFTER HARVEST MEANS QUICKER SPOILAGE.

An increase in temperature increases the:

The post-harvest life of produce is approximately halved for each 10øC rise in its temperature.

  1. How does the produce become hot?
  1. Because it generates heat itself as a result of its own living processes. A mass of produce without ventilation will quickly produce a very big rise in temperature at the centre of the mass.
  2. Exposure to the sun's heat causes a large temperature rise in produce at any stage after harvest.
  3. Through exposure to artificial heat sources, often during transport,. (e.g. heat from ships engines, especially in inter-island vessels like wooden sloops - see Section 6.4.1.).
  1. How can produce be kept cools
  1. Harvesting should be carried out during a cool part of the day, and the produce kept cool thereafter.
  2. Harvested produce should be protected from the sun at all times. In the field natural shade of trees or simple pole and thatch structures without walls should be used to protect harvested produce.
    Transport should have a canopy to protect produce from the sun - but ventilation must not be obstructed.
  3. Produce must not be stacked in compact piles or masses. Ventilation must be provided to disperse heat, taking advantage of prevailing winds where possible.
  4. Use ventilated field and marketing containers, making sure that the ventilation holes are not blocked either by produce or due to an incorrect stacking pattern.
  5. Forced air ventilation or refrigeration can be used to lengthen post-harvest life of produce but it is very costly and not recommended for general use.
  6. Minimise time between harvest and consumption. It is always better wherever possible to move produce quickly from grower to consumer, avoiding the additional cost of storage.
  1. Chilling injury
    Refrigeration is a potent method of keeping fresh produce cool but under excessive refrigeration, fresh produce will freeze at around -2øC and a return to higher temperatures leads to a breakdown of tissues and off-flavours and the produce is not usually marketable. Most tropical fruits and certain vegetables can also be irreversibly damaged by cool temperatures above freezing. This damage, known as 'chilling injury', is very important to the marketing of many fruits which if held below 14øC for any significant period will lead to tissue breakdown, unsightly blackening and off-flavours. See Table 2.1, below for details

 

Table 2.1. Lowest safe temperatures and chilling injury symptoms in fruits and melons
Commodity Lowest safe temperature Type of injury incurred below safe temperature
°F °C
Avocado 40 - 55 4.5 - 13 Blackening of pulp and peel.
Banana 55 - 60 13 - 15 Dull peel, brown streaking of peel, hardened placenta, off-flavour
Grapefruit 50 - 60 10 - 15.5 Scald, surface pitting, water-logging.
Lime 45 - 50 7 - 10 Pitting
Mango 50 - 55 10 - 13 Pulp and peel blackening, uneven ripening, off-flavour.
Melon 35 - 50 2 - 10 Pitting, decay, failure to ripen.
Orange 35 - 45 2 - 7 Pitting, surface browing.
Pawpaw 40 - 45 4.5 - 7 Pitting, off-flavour, failure to ripen.
Pineapple 45 - 55 7 - 13 Irregular ripening, "glassy spoilage, tendency to Endogenous Brown Spot.

 

2.3.4. The Effects of Injuries

INJURY TO PRODUCE AFTER HARVEST WILL HASTEN ITS DETERIORATION .

Injuries take many forms, including cuts, punctures, scraping of outer surfaces, internal and surface bruising, sunburn, heat damage and cold damage.

Their effect on harvested produce is to:

  1. How are injuries caused?
  1. Careless harvesting practices, such as knocking fruit to the ground from trees, damaging stem-end areas when harvesting, cuts from long finger nails or the jewellery of harvesters.
  2. Through rough field handling of produce, such as dropping or throwing items into field boxes, dropping or throwing packed field and market containers themselves. (See Figure 2.3)
  3. By using unsuitable containers with rough or sharp edges to ventilation holes, made from rough and splintered wood, carelessly made with protruding nails or staples, and containers too large to be handled easily.
  4. From overpacking containers causing crushing of contents when they are closed or stacked, or underpacking so that damage is caused by excessive movement within the package. (See Figure 2.3. Causes of injury to fresh produce)
  5. By people walking or sitting on produce in containers, or in bulk.
  6. From exposure to the sun after harvest, resulting in sunburn.
  7. From exposure to excessive artificial heat or cold (chilling and freezing damage).
  1. How can injuries be avoided?
    The fragile nature of most fruit and vegetables products makes them very susceptible to injury, and the complete avoidance of such injury is not possible. However, damage may be reduced to a minimum by giving attention to:
  1. Care in harvesting, especially with tree fruit, which are severely damaged if they fall or are thrown to the ground.
  2. Being careful not to harvest wet produce, especially citrus fruit - because it is more easily damaged in this condition.
  3. The selection of suitable field and marketing containers, which should not be too large for careful handling. They should be strong enough to protect produce but should not cause damage to produce due to sharp edges, poor manufacture or assembly.
  4. The avoidance of overpacking or underpacking containers, which should be filled to an extent that will exert a slight pressure on the contents when closed. This will prevent movement of produce within the container.
  5. The careful handling of produce at all stages, especially when in containers, which must not be rolled, dropped or thrown.
  6. Transport conditions. Loads should be stacked in a manner which will prevent either the movement of individual containers or the collapse of the stack during transport. Riders should not be permitted on top of the load, especially when it consists of produce in bulk or in sacks. Vehicles should have a canopy to protect the load from the direct heat of the sun, but it should not restrict ventilation.

 

2.3.5. The Effect of Surface Water on Harvested Produce

The effect of the loss of internal water from the produce due to natural causes, excessive heat and injuries has been explained, but often the presence of free water on the surface of produce will also lead to problems, such as:

- Increase in post-harvest decay

This often occurs where produce is washed before packing. Most moulds and bacteria causing decay require free water to establish infection, particularly where injuries, even though small, are present on washed produce and the washing water is stagnant or recirculated. It may also be a problem where condensation occurs on the surface of produce when it is moved from cold stores to high ambient temperatures, or when produce is exposed to rain after harvest.

- Increased susceptibility to surface Injury

Produce saturated with water, from rain or other causes may become 'soft' and more easily damaged than when dry. This damage not only provides opportunity for infection by decay agents but may in itself leave unsightly surface damage, leading to down-grading and lower prices. This is often seen in citrus fruits, where fruit harvested when wet develop the skin blemish known as "oleocellosis".

It may not always be possible to keep produce dry but field crews should avoid harvesting freshly wet produce. Do not wash produce after harvest unless it is essential If it has to be washed it is usual to apply an anti-fungal dip immediately afterwards. The produce should then be dried in the shade, preferably on a mesh or slatted rack - this will help to cool it, especially if it is exposed to a breeze.

DO NOT PUT WET PRODUCE DIRECTLY ON TO BARE SOIL.

DO NOT PILE UP WET PRODUCE IN THE SUN TO DRY.

If produce has been in a cold room try not to remove it into a warm, humid and unventilated atmosphere. It should be held under ventilated ambient conditions in the shade to prevent the accumulation of excessive condensation on its surface.

 

2.3.6. Ripening of Fruits and the Ethylene Factor

Fruits undergo a natural process of ripening and although this is an attractive and beneficial aspect as far as the consumer is concerned, the ripening process adds several complications to the marketing and distribution process. Many vegetables, such as tomatoes, melons, green peppers and hot peppers, are also fruits in that they undergo a ripening process as part of their development leading to senescence and death of the tissues.

The rate and nature of the ripening process differs significantly between species of fruits, cultivars of the same species, different maturities of the same cultivar (eg.'3/4 full' banana compared with 'full' banana), and also between production areas. Fruits may also differ in their ripening responses to postharvest conditions, Nevertheless, some general features are recognizable in the ripening behaviour of fruits:

  1. Changes associated with ripening - these are:
  1. Climacteric and non-climacteric - Basically there are two distinct patterns of ripening which can be identified and these are termed climacteric and non-climacteric types. (Specialists seeking detailed information are referred to the literature and references in Section 12, since only a brief coverage will be given here.)
    In non-climacteric fruits the process of maturation and ripening is a continuous but gradual process. In contrast, the climacteric fruits undergo a rapid ripening phase when triggered by changes in hormonal composition. The onset of climacteric ripening is thus a well defined event marked by rapid increase in the rate of respiration and the natural evolution of ethylene gas by the fruit at a point in its development known as the respiratory climacteric. Figure 2.4 (see
    Figure 2.4. Respiration of climacteric and non climateric fruits) shows the differences between the two ripening patterns in a graphical form, and Table 2.2 gives listings of common climacteric and non-climacteric fruits and vegetable fruits.

The importance of the respiratory climacteric is that fruits such bananas may be held at a reasonable temperature when in the green state, but as they begin to ripen they will rapidly increase their respiration and generate much more heat. The consequence may be that this heating cannot be controlled and even more respiration will occur in an inflationary spiral rapidly leading to spoilage of the fruit in a very short time. Once climacteric fruits start to ripen, there is very little that can be done except to market them for immediate consumption.

Table 2.2. Classification of fruits and fruit vegetables based on climacteric and non-climacteric ripening patterns
  CLIMACTERIC NON-CLIMACTERIC
TEMPERATE
FRUIT
Apple
Pear
Peach
Apricot
Plum
Cherry
Grape
Strawberry
'VEGETABLE'
FRUIT
Melon
Tomato
Watermelon
Cucumber
COMMON
TROPICAL
FRUIT
Avocado
Banana
Mango
Papaya
Fig
Guava
Passion fruit
Persimmon
Orange
Grapefruit
Lemon
Lime
Olive
Pineapple
Litchi
LESS
COMMON
TROPICAL
FRUIT
Cherimoya
Soursop
Breadfruit
Jackfruit
Mamey apple
Sapote
Cashew apple
Java plum
Other Eugenia sp

Ethylene is present in all fruit and is recognized as the central fruit ripening hormone which, in climacteric fruits, can actually initiate ripening when present at concentrations as low as 0.1 to 10 parts per million (ppm). Non-climacteric fruits also respond to ethylene application by increasing their respiration rate but the actual ripening process is only triggered by the fruit itself.

As well as being involved in ripening and increased respiration in fruits, ethylene also plays an important role in all plant materials and is produced in response to stress from wounds and injuries. In other words, ethylene produced by wounding or stressing may also trigger ripening in the damaged fruit as well as the undamaged fruits around it. Damage one green fruit in a box and the the whole box load may ripen prematurely. For this reason, good ventilation of fresh produce with fresh air, refrigerated if necessary, is vital to ensure that ethylene levels do not build up to significant levels during storage and transport.

Ethylene can also adversely affect certain vegetables. Carrots for example develop bitter flavours, and parsley and other leafy herbs will rapidly wilt when exposed to ethylene in stores and during retail display. It is important therefore not to mix ripening fruits with such sensitive vegetables at any stage in the marketing process. Retailers in particular should be careful about displaying fruits next to carrots and parsley or the vegetables will either spoil rapidly or develop bitterness.

 

2.3.7. Pests, Diseases and Spoilage

  1. Pests - notably insect pests, are a serious problem during production of fresh produce which in the tropics must be controlled by the use of careful cultural practices and controlled application of insecticides provided that no harmful residues remain on the crop at harvest.
    Infested produce at harvest is relatively easy to spot and separate from clean produce. The rapid marketing of most fresh produce also means that there is little opportunity for insect pest infestation provided that reasonable precautions are taken and that produce infested prior to harvest is rejected and not mixed with clean produce.
    Fruit flies are particularly difficult to control and keep away from harvested produce and is the main reason why the USA operates such stringent quarantine regulations.
  2. Diseases - Post-harvest infection of fresh produce by fungi and bacteria may cause physical injury, increased water loss and increased respiration leading to rapid deterioration and spoilage of the produce.
    Bacteria multiply by rapid cell division and enter produce mainly through cut surface. or natural abscission points. Bacterial contamination of produce is most commonly by contact with infected water or by contact with soil harbouring the bacteria.
    Good phytosanitary practices will help prevent most postharvest infections. Use clean and sharp knives and clippers at harvest and only use clean water for washing, and do not put cut surfaces, of cabbage for example, onto the ground.
    Fungi multiply by cell extension and division and by forming spores for dispersal in air or water, or even by various animal and insect vectors. Fungal infection of produce may result from entry into cut surfaces or natural abscission points as with bacteria, or by pathogenic entry of produce. Pathogenic entry of intact healthy tissue is confined to a few organisms and generally entry is via cut surfaces, or damaged or weakened tissue.
    During storage, marketing and distribution, all produce will age and become weaker with a gradual breakdown of cell structure and integrity. The produce is thus stressed and less able to withstand invasion and infection by disease organisms. Alternatively, the produce being a fruit, ripening produces a sweet and readily utilizable food source for fungal growth. Ripening fruits are also weakened and easier to penetrate. Fungal spores present at harvest, but unseen, may thus germinate and spoil the fruit once ripening commences. This "latent infection" is typical of 'Anthracnose' and other similar fungal spoilage diseases.

2.4. Further information

The material as presented in this section is merely by way of an introduction to the various technical terms and considerations adopted by the fruit and vegetable marketing industry on an international basis. Far more information about the physiology of fresh produce, and how and why spoilage occurs, and how it can be avoided or controlled, can be obtained by reading the literature and references included in Section 12. In addition, readers of this manual are strongly urged to contact the postharvest specialists working in the Ministry of Agriculture in each island who have information and experience of the particular local postharvest problems.


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