Chapter 1

Harvest

1.1 Harvest systems

Harvesting is the gathering of plant parts that are of commercial interest. These include: Fruits - e.g. tomatoes, peppers, apples, kiwifruits, etc.; root crops - e.g. beets, carrots etc; Leafy vegetables - spinach and Swiss chard; Bulbs - onions or garlic; Tubers - potatoes; stems - asparagus; petioles - celery and Inflorescences - broccoli, cauliflower etc. Harvest marks the end of the growing period and the commencement of market preparation or conditioning for fresh products.

Harvesting can be performed by hand or mechanically. However, for some crops - e.g. onions, potatoes, carrots and others - it is possible to use a combination of both systems. In such cases, the mechanical loosening of soil facilitates hand harvesting. The choice of one or other harvest system depends on the type of crop, destination and acreage to be harvested. Fruits and vegetables for the fresh market are hand harvested while vegetables for processing or other crops grown on a large scale are mainly harvested mechanically.

The main advantages of mechanized harvesting are speed and the reduced costs per ton harvested. However, because of the risk of mechanical damage, it can only be used on crops that require a single harvest. A decision to purchase equipment requires careful evaluation of: the initial investment required, maintenance costs and the long period in which equipment may have to stand idle. In addition to this, the entire operation needs to be designed specifically for mechanized harvesting - distances between rows, field leveling, pesticide spraying, cultural practices to varieties which can be adapted to rough handling. Market preparation (grading, cleaning, packing, etc.) and the trade should also be able to handle large volumes of produce.

Hand harvesting is particularly suitable for crops with an extended harvest period. The rate of harvesting can be increased by hiring more workers if, for example, due to climate, there is accelerated ripening and a need to harvest the crop quickly. The main benefit of hand harvesting over mechanized harvesting is that humans are able to select the produce at its correct stage of ripening and handle it carefully. The result is a higher quality product with minimum damage. This is important for tender crops. However, adequate training, including supervision of the harvest crew, is required. In Figure 1 apples harvested by an unsupervised crew had more bruises than those in which close supervision was available.

Contractual arrangements with harvest labour also influences the final quality of product harvested. When wages are paid per week, fortnight or month, harvesting is undertaken carefully. However, when payment is per box, meters of row or number of harvested plants, harvesting can be careless. Establishing teams and division of labour also influences quality. Long working days and/or few breaks as well as extremely adverse conditions (excessive heat or cold) can result in unnecessary rough handling of produce. Harvest labour needs to be adequately trained to give them the necessary skills to select produce at the correct stage of ripeness or degree of maturity as well as sorting techniques to minimize damage.

1.2 Harvest ripeness and readiness for harvest

In many cases harvest ripeness and readiness for harvest are used synonymously. However, it is more technically accurate to use "ripeness" for fruits such as tomato, peach, pepper, etc. Here, the consumption stage continues after certain changes in colour, texture and flavor. On the other hand, in species where these changes do not occur such as asparagus, lettuce, and beets, the term "readiness for harvest" is preferable.

Maturity is the harvest index most widely used in fruits. However, physiological maturity needs to be distinguished from commercial maturity. The former is reached when development is over. It may or may not be followed by the ripening process to achieve the commercial maturity required by the market. Every fruit shows one or more apparent signs when it reaches physiological maturity. For example, in tomato, the gelatinous mass fills the internal locules and seeds cannot be cut when fruits are sectioned with a sharp knife. In peppers, seeds become hard and the internal surface of the fruit starts colouring (Figure 2).

Over maturity or over ripening is the stage that follows commercial maturity and is when the fruit softens and loses part of its characteristic taste and flavor. However, it is the ideal condition for preparing jams or sauces (Figure 3). Commercial maturity may or may not coincide with physiological maturity. For cucumbers, zucchinis, snapbeans, peas, baby vegetables, and many others, commercial maturity is reached well before the end of development.

Figure 1: Number of slight and severe bruises per 100 apples according to the degree of supervision of the picking crew. A: closely supervised and B not supervised.(Adapted from Smith, et al., 1949).

At this point, it is necessary to differentiate between two types of fruits: climacteric and non-climacteric. Climacteric include for example tomatoes, peaches etc. They are capable of generating ethylene, the hormone required for ripening even when detached from the mother plant. Non-climacteric include for example peppers, citrus etc. Commercial maturity is only obtained on the plant (Table 1). Climacteric fruits are autonomous from the ripening point of view and changes in taste, aroma, colour and texture are associated with a transitory respiratory peak and closely related to autocatalytic ethylene production. Figures 4 and 5 illustrate this point: Climacteric fruits such as tomato reach full red colour even when harvested green (Figure 4, left). On the other hand, in non- climacteric fruits such as bell peppers, slight changes in colour take place after harvest. Full red colour is only obtained while fruit is attached to the plant (Figure 5). As a general rule, the more mature the product, the shorter its post-harvest life. For distant markets, this means climacteric fruits need to be harvested as early as possible, but always after reaching their physiological maturity.

Figure 2: Physiological maturity in bell pepper is reached when seeds become hardand the internal cavity of fruit starts colouring.

Figure 3: Organoleptic quality of a fruit in relationship to its ripening stage.

Table 1: Climacteric and non-climacteric fruits.

Source: Wills, et al., 1982; Kader, 1985

Changes in colour are the most apparent external symptoms of ripening. They are the result of chlorophyll degradation (disappearance of green colour) and the synthesis of specific pigments. In some fruits such as lemons, chlorophyll degradation allows yellow pigments that are already present to show. However, these are masked by the green colour. Other fruits such as peaches, nectarines and some varieties of apples have more of one type of colour - the ground one is associated with ripeness and the cover in many cases is specific to the variety (Figure 6). Maturity can be estimated by colour charts based on the percentage of desired colour (Figures 4 and 5) or by objective measurements with colourimeters (Figure 7).

Degree of development is the harvest index most widely used in vegetables and some fruits, in particular those harvested immature. In soybean, alfalfa and other legume sprouts, harvest is undertaken before expansion of the cotyledon; in asparagus, when the stems emerging from soil reach a certain length; in haricot beans and other snap-beans when they reach a certain diameter (Figure 8); in snow peas and other legumes before seed development becomes evident (Figure 9). In lettuce, cabbage and other "head" forming vegetables, harvest is based on compactness while "shoulder" width is used in beets, carrots and other roots. Plant size as a harvest index is used in many vegetables such as spinach. However, in the case of potato (Figure 10), sweet potato and other root vegetables, the percentage of tubers of a specific size is utilized.

Figure 4: Degree of ripening in tomato (from left to right): 1) Mature green; 2) Breaker; 3) Turning; 4) Pink; 5) Light red and 6) Red. Due to its climacteric ripening characteristics, the tomato fruit reaches stage 6 even when harvested at maturity stage 1.

Many crops show apparent external symptoms when ready for harvest. These include for example tops falling over in onions (Figure 11), development of abscission layers in the pedicel of some melons, hardness of the epidermis of certain pumpkins, or shell fragility in some nuts. Degree of filling is an index used in bananas and mangoes while sweet corn is harvested when kernels are plump and no longer "milky".

Colour, degree of development or both are the main criteria used for harvest in most fruits and vegetables. It is, however, common to combine these with other objective indices. These include for example, firmness (apple, pear, stone fruits) (Figure 12), tenderness (peas), starch content (apple, pear) (Figure 13), soluble solid content (melons, kiwifruit), oil content (avocado), juiciness (citrus), sugar content/acidity ratio (citrus), aroma (some melons), etc. For processing crops, it is important to keep a constant flow of raw material in the harvesting schedule. It is therefore normal practice to calculate the number of days from flowering and/or the accumulation of heat units.

Figure 5: Degree of ripening in bell pepper. As with other non-climacteric fruits, ripening does not follow after harvest.

Figure 6: Some cherry varieties have a ground colour that changes when fruit reachesmaximum development. (Photograph: A. Yommi, INTA E.E.A. Balcarce).

Figure 7: Objective colour measurement with a colourimeter.

Figure 8: Harvest maturity based on the bean diameter.

Figure 9: Readiness for harvest based on the degree of seed development.

Figure 10: Readiness for harvest based on the percentage of tubers with the desired size.

Figure 11: Tops bending and falling over indicate that onions are ready for harvest.

Figure 12: Objective firmness measurement.

Figure 13: Starch stains dark when fruit cross sections are dipped in an iodine solution. Percentage of starch disappearance can be used as a maturity index in apples (Reproduced from Ctifl, 1993).

1.3 Handling during harvest

Harvesting involves a number of other activities undertaken in the field. This includes those of commercial interest. Examples of operations to facilitate preparation for the market include pre-sorting, removal of foliage and other non-edible parts. In some cases, the product is completely prepared for the market in the field. However, the normal practice is to empty the harvest containers into larger ones for transportation to the packinghouse (Figure 14). Here, they are dry or water dumped onto grading lines. While these activities are being undertaken, bruising which has a cumulative effect can affect the final quality of product (Figure 15).

Different types of lesions exist. Wounds (cuts and punctures) occur as a result of loss of tissue integrity. This type of damage is frequent during harvest and mainly produced by the harvesting tools used for the removal of plants. Other causes include the nails of pickers or peduncles from other fruits (Figure 16). Rotting fungi and bacteria penetrate produce in this way. This type of damage can be easily detected and is usually removed during grading and packing. Bruises are more common than wounds. They are less noticeable and symptoms show up several days later when the product is in the hands of the consumer. There are three main causes of bruises:

1. Impact: Injury caused either by dropping the fruit (or packed fruits) onto a hard surface or the impact of fruit rubbing against other fruit.These types of bruises are common during harvest and packing (Figure 17).
2. Compression: Deformation under pressure. This often occurs during storage and bulk transportation and is caused by the weight of the mass of fruits on bottom layers. It also happens when the packed mass exceeds the volume of the container (Figure 18) or by the collapse of weak boxes or packages unable to withstand the weight of those piled up high.
3. Abrasion: Superficial damage produced by any type of friction (other fruits, packaging materials, packing belts, etc.) against thin-skinned fruit such as pears. In onions and garlic abrasion results in the loss of protective scales (Figure 19).

Bruise symptoms depend on the affected tissue, maturity, type and severity of the bruise. They are cumulative and in addition to their traumatic effect, trigger a series of responses to stress, including the onset of healing mechanisms. This physiological reaction is as follows: a temporary increase in respiration which is associated with degradation; a transient production of ethylene, which accelerates maturation and contributes to softening. In some cases, mechanical disruption of membranes puts enzymes in contact with substrates which leads to the synthesis of secondary compounds that may affect texture, taste, appearance, aroma or nutritive value. Firmness on the site of impact decreases rapidly because of damage and cell death as well as the loss of tissue integrity. The more mature the product, the more severe the damage. Its effect is exacerbated by higher temperatures and longer storage periods. Ethylene removal or neutralization under controlled or modified atmosphere conditions reduces the speed of healing. However, atmospheric composition also reduces the rate of stress response mechanisms.

Figure 14: Harvested fruits ready to be transported to the packinghouse.

Figure 15: Cumulative impact of bruises on Bartlett pears during post-harvest handling (Adapted from Mitchel, 1985).

Figure 16: Injury caused by the peduncle of other fruit during transport.

Figure 17: Impact bruise in pear.

Figure 18: Compression injury in tomato.

Figure 19: Loss of scales in onion bulbs due to abrasions against rough surfaces.

1.4 Harvest recommendations

· If the time of day can be selected, it is recommended to harvest during the cool morning hours. This is because products are more turgid. Furthermore, less energy is required for refrigeration.

· Harvesting maturity is a function of the distance to the destination market: those within close proximity, allow ripening on the plant.

· Harvested product needs to be kept in the shade until the time of transportation

· Avoid product bruising. Harvesting scissors or knives should have rounded ends to prevent punctures and be sharp enough to prevent tearing off. Harvest containers should be cushioned, smooth and free of sharp edges. Do not overfill field containers and move them carefully (Figure 20). Minimize drop heights when transferring produce to other containers.

· Train harvest labour to handle produce gently and identify correct maturity for harvest. Wear gloves during harvest and handling to avoid damage to fruits.

Figure 20: Foliage can be used for cushioning and protecting cauliflower during transportation.

1.5 Curing

Curing complements harvesting in certain crops and is needed to achieve a quality product. It is a process involving rapid loss of superficial humidity. In addition to developing some tissue changes, it prevents further dehydration. It also acts as a barrier for penetration of pathogens. In onions and garlic, curing is the drying of external scales together with colour development and neck closure. For root crops such as sweet potato, yams and tubers - e.g. potato - skin hardening prevents skinning during harvest and handling and the development of the healing periderma on wounds (suberization). In pumpkins and other cucurbits, curing is the hardening of the skin while in citrus it is the natural formation of a layer of lignified cells. This prevents the formation and development of pathogens.

Curing is normally undertaken in the field. In garlic and onions it is performed by undercutting and windrowing plants to protect them from direct sun or in heaps or burlap bags (Figure 21) for a week or more. In potato, tubers must remain in the soil for 10-15 days after foliage is destroyed with herbicides. In sweet potato and other roots it is similar although it is normally carried out under shelters. If required, curing may be performed artificially in storage facilities by forced circulation of hot and humid air (Table 2). After curing, temperature and relative humidity conditions are set for long-term storage.

Figure 21: Curing onion bulbs in burlap sacks.

Table 2: Recommended temperature and relative humidity conditions for curing (Adapted from Kasmire, 1985).