Organisation:Massey University, Private Bag 11-222, Palmerston North, New Zealand Massey University
Author: Linus U. Opara
Edited by AGST/FAO: Danilo Mejía, PhD, FAO (Technical), Emanuela Parrucci (HTML transfer)

CHAPTER XXIV YAMS: Post-Harvest Operation

2 Post-Production Operations

2.1 Pre-harvest Operations

2.2 Harvesting

2.3 Transport & Packaging

2.4 Curing of Yam Tuber

2.5 Cleaning 

2.6 Storage

2.7 Processing

2.8 Dormancy in Yams

2. Post-Production Operations

2.1      Pre-harvest Operations

Maturity assessment is critical to achieving good quality yam. In the field, mature crop is generally distinguishable by cessation of vegetative growth and yellowing of leaves. The period from planting or field emergence to maturity is variable depending on the species (Table 4), and there is no standard reliable and objective index of yam tuber maturity. Some crude indices have been reported based on percentage of tuber length that was whitish at harvest, non-friable after cooking, or bitter after cooking (Onwueme, 1977). The most frequently reported measure is the period from planting to harvest (growing period), but it has been suggested that the time from emergence to maturity provides a better measure of growing period since planted tuber can remain dormant for some time (Onwueme and Charles, 1994).

Most edible yams reach maturity in 8-11 months after planting. Techniques such as using physiologically aged planting material, pre-sprouting of setts, application of sprout-promoting substances (e.g. ethephon and 2-chloroethanol and harvesting before complete shoot senescence can decrease the during of field dormancy and thereby reduce the length period from emergence to maturity (Onwueme, 1977; Gregory, 1968; Martin et al., 1974). In many parts of West African yam zone, mature yams are harvested at the end of the rainy season or early part of the dry season, which coincides with the end of vegetative growth. Yams for long-term storage (for marketing or seed) are usually harvested during the harmattan period (Dec-Jan) in many parts of southeastern Nigeria when the crops has attained maximum growth and maturity. During this period, the soil is generally hard and tuber breakage during harvesting can be an economical problem.

Average yield of tubers is variable amongst the major producing areas, and is influenced by the species, seed piece, and growing environment (Table 4). Yields range between 8-50 Mt.ha^-1 in 6-10 months. Yields of 8-30 Mt.ha^-1 in commercial yam production has also been reported, the exact value depending on the location, variety, and cultivation practices (Onwueme and Charles, 1994. Many yam cultivars produce only a single large tuber, and the approximate multiplication ratio (fresh-weight yield:weight of planting material) for yam is about 5. Between 1975-1990, there were yield increases in all major producing countries except Ghana. During this period, the average world yield increased by nearly 11%.

2.2      Harvesting

Harvesting is done by hand using sticks, spades or diggers. Sticks and spades made of wood are preferred to metallic tools as they are less likely to damage the fragile tubers; however, tools need regular replacement. Yam harvesting is a labour-intensive operation that involves standing, bending, squatting, and sometimes sitting on the ground depending the size of mound, size of tuber or depth of tuber penetration. In rainforest areas, tubers growing into areas where there are roots of trees can pose a problem during harvesting and often receive considerable physical damage. Many also get deformed during growth as a result of the obstacles they encounter. These tubers are usually downgraded. Aerial tubers or bulbils are harvested by manual plucking from the vine.

Although some success in mechanical yam harvesting has been reported, especially for D. composita tubers for pharmaceutical uses (Nystrom et al., 1983), these machines are still limited to research and demonstration purposes. The use of a potato spinner has been suggested for harvesting species which produce a number of small tubers (Onwueme, 1997). Current crop production practices and species used pose considerable hurdles to successful mechanisation of yam production, particularly for small-scale rural farmers. Extensive changes in current traditional cultivation practices, including staking and mixed cropping, and possibly tuber architecture and physical properties will be required.

Yams can be harvested once (single harvesting) or twice (double harvesting) during the season to obtain a first (early) and second (late) harvest. The first harvest has also been referred to by the terms ‘topping’, ‘beheading’, and ‘milking’, all of which have been considered inadequate and obsolete. In single harvesting, each plant is harvested once and this occurs at the end of the season when crop is mature. The harvesting processes involves digging around the tuber to loosen it from the soil, lifting it, and cutting from the vine with the corm attached to the tuber. The time of harvest is critical in terms of tuber maturity, yield and postharvest quality. Depending on the cultivar, the period from planting or emergence to maturity varies from about 6-7 months or even 6-10 months.

Periods of  8-10 months and 4-5 months from planting or emergence to maturity have been recommended for double-harvesting (Martin, 1984; Onwueme, 1977); harvest first at 5-6 months after planting and then 3-4 months later has also been reported (Bencini, 1991). First harvest is carried out by removing the soil around the tuber carefully and cutting the lower portion, leaving the upper part of the tuber or the “head” to heal and continue to grow. The soil is returned and the plant is left to grow to the end of the season for the second harvest. Some yam cultivars produce several small tubers in the second growth following the early harvest. Double harvesting is most applicable to short-term varieties such as D. rotundata, and to lesser extents D. Cayenensis and D. alata. Similar yields have been reported for single and double harvesting; however, single-harvested tubers had better eating quality than the double-harvested tubers (Onwueme and Charles, 1994).

2.3      Transport & Packaging

After harvest, yam tubers are traditionally placed into woven baskets made from parts of the palm tree or coconut fronds. These are ideal for transporting small quantity of tubers over short walking distances. The basket is carried on the head, shoulder, or tied to a bicycle and transported to the market or storage facility. Compression damage is reduced since the basket is able to bend and thereby reduce the amount of force acting on individual tubers. However, when large quantities of tuber are harvested, these baskets are not suitable because of their limited size. Packaging tubers in full telescopic fibreboard cartons with paper wrapping or excelsior reduces bruising and enables large quantity of tuber to be transported over long distances. Tubers can be contained in loose packs, or units of 11 kg and 23 kg (McGregor, 1987). The cartons are hand-loaded or unitised on pallets.

Storing yams in modified atmosphere packaging (MAP) has beneficial effects, particularly using appropriate packaging material with suitable size and number) of holes for gas permeation. Sealing yam tubers in polyethylene film bags reduced storage losses due to weight loss and development of necrotic tissue (Table 5). Coating tubers with Epolene E10 (a commercial vegetable wax improved the appearance quality but there was no effect on levels of fungal infection (Thompson et al., 1977). The effect of this treatment on weight loss of tuber was inconsistent.

2.4      Curing of Yam Tuber

Curing of root crops allows suberisation of surface injuries and reduces subsequent weight loss and rotting in root crops. Curing of yams is recommended before storage so as to “heal” any physical injury, which may have occurred during harvesting and handling. This can be accomplished under tropical ambient conditions or in a controlled environment. Traditionally, yams are cured by drying the tubers in the sun for a few days. The optimum conditions for curing are 29°-32°C at 90-96% rh for 4-8 days (McGregor, 1987). Tubers cured at higher temperature (40°C) for 24 hours or treated with gamma radiation at 12.5 krads were free of mold and had least losses during subsequent storage. Storing at 15°C with prompt removal of sprouts was found to improve the eating quality of tubers (Coursey, 1967), presumably due the waterloss associated with curing and the inhibition of the biochemical synthesis that accompany sprouting.

2.5      Cleaning 

Prior to long-term storage and marketing, yams are cleaned (without water) by scrapping off soil and other debris on the surface. A knife or piece of stick is usually used. The root ‘hairs’ are also removed to so that the tuber has a smooth surface. Water must not be used to clean tubers before storage because of increased susceptibility to microbial infection and growth under the ambient humid storage conditions.

2.6      Storage

The three main conditions are necessary for successful yam storage: aeration, reduction of temperature, and regular inspection of produce. Ventilation prevents moisture condensation on the tuber surface and assists in removing the heat of respiration. Low temperature is necessary to reduce losses from respiration, sprouting and rotting; however, cold storage must be maintained around 12-15°C below which physiological deterioration such as chilling injury occurs. Regular inspection of tubers is important to remove sprouts, rotted tubers, and to monitor the presence of rodents and other pests. In general, tubers should be protected from high temperatures and provided with good ventilation during storage. The storage environment must also inhibit the onset of sprouting (breakage of dormancy) which increases the rate of loss of dry matter and subsequent shrivel and rotting of tuber. Both ware yam and seed yam have similar storage requirements.

Notwithstanding cultivar differences, fresh yam tuber can be successfully stored in ambient and refrigerated conditions (Table 6). The recommended storage temperature is in the range 12°-16°C. Optimum conditions of 15°C or 16°C at 70-80% rh or 70% rh have been recommended for cured tubers (Martin, 1984; McGregor, 1987). Transit and storage life of 6-7 months can be achieved under these conditions. The onset of sprouting is enhanced at ambient conditions, especially if ventilation is inadequate. For example, during storage at ambient conditions (20°-29°C, 46-62% rh), D. trifida began to sprout within 3 weeks (Thompson, 1996). Yam tuber decay occurs at higher humidity, and like most tropical crops, they are susceptible to chilling injury (CI) at low storage temperatures. To avoid tuber damage, minimum storage temperatures of 10°C, 12°C and 13°C (Martin, 1984; McGregor, 1987) at or below which CI occurs have therefore been recommended. Storage of D. rotundata tubers at 12.5C resulted in CI (Coursey, 1968), and storage of D. alata at either 3° or 12°C resulted in total physiological breakdown within 3-4 weeks (Czyhrinciw and Jaffe, 1951). Storage of D. alata at 5°C for 6 weeks gave good results but CI symptoms developed rapidly when tubers were subsequently put in ambient (25°C) conditions (Coursey, 1961). There is no reliable data on beneficial effects on CA technology on the commercial storage is important yam cultivars.

There are several traditional storage structures used for yam storage including: (a) leaving the tubers in the ground until required, (b) the yam barn, and (c) Underground structures (Opara, 1999). Leaving the tubers in the ground until required is the simplest storage technique practised by rural small-scale farmers. When carried out on-farm, this type of storage prevents the use of the farmland for further cropping. Harvested yams can also be put in ashes and covered with soil, with or without grass mulch until required.

The yam barn is the principal traditional yam storage structures in the major producing areas. Barns are usually located in a shaded areas and constructed so as to facilitate adequate ventilation while protecting tubers from flooding and insect attack. Barns consist of a vertical wooden framework to which the tubers are individually attached (Fig. 2). Two tubers are tied to a rope at each end hung on horizontal poles 1-2 m high. Barns up to 4 m high are uncommon. Depending on the quantity of tuber to be stored, frames can be 2 m or more in length. The ropes are usually fibrous, but in Southeastern Nigeria, they are made from the raffia obtained from top part of Palm wine tree. Many farmers have permanent barns, which need annual maintenance during the year’s harvest. In these situations, growing trees are used as vertical posts, which are trimmed periodically to remove excessive leaves and branches. Palm fronds and other materials are used to provide shade. The vegetative growth on the vertical trees also shades the tubers from excessive solar heat and rain. The use of open-sided shelves made from live poles, bamboo poles or sawn wood has been recommended to enable careful handling and easy inspection in comparison with tying tubers to poles which can cause physical damage and rotting (Bencini, 1991). In barn storage, yams have a maximum storage life of 6 months and are therefore most suited for long-term varieties. Storage losses can be high and up to 10-15% in 3 months, and 30-50% after 6 months if tubers are not treated for rotting using fungicides such as Benlate, Captan or Thiabendazole.

Fig. 2. Tipical Yam Barn in West Africa.

Yams are also stored in underground structures such as pits, ditches and clamps. These are suitable for limited storage periods, especially the early varieties that are often harvested before the end of the rainy season.  During construction of pits, the earth dug out is used to build a low wall around the edge. The temperature in the storage space can also be moderated by placing cut vegetation over the ditch, clamp or pit. In these structures, ventilation and rodent attack of tubers is a major problem, and it is difficult to inspect the tubers.

Well-ventilated, weatherproof, and stronger shelters can be built as to improve the performance of the traditional shelters described above. New features may also be provided to exclude pests and rodents. A typical improved yam barn has sidewall 1.2 m high and wire mesh to ward off rodents and birds (Akoroda and Hahn, 1995). The roof was double thatch and extended to the eaves with smooth floor of cement or mud, and only one entry door was provided to guard against entry of rodents. Tubers were stored on platforms or shelves. Tubers stored in such improved structures had only 10% spoilage after 5-6 months.

2.7      Processing

Industrial uses of yam includes starch, poultry and livestock feed, and production of yam flour. Readers interested in detailed information on specific yam processing methods, equipment, and packaging techniques can find these information in an FAO technical compendium (Bencini, 1991). Residues from sifting and peels are used as animal feed in many rural areas. One of the major disadvantages of industrial processing of yam for food is that nutrient losses in these products can be high, particularly minerals and vitamins. In products obtained from secondary processing such as biscuits and fufu, the amount of loss depends principally on the amount of edible surface exposed during processing operations. Primary unit operations such as milling affect the thiamine and riboflavin contents of D. rotundata, with average losses of 22% and 37%, respectively. Sun drying results in high losses of B vitamins with little change in mineral content. Pounding yam flour in a traditional wooden mortar or grinding in an electric mixer had similar effects.

2.8      Dormancy in Yams

Dormancy is the temporary suspension of visible growth of any plant structure containing a meristem, and in stored yam tubers, it is the period during which sprouting is inhibited. Knowledge of the potential length of dormancy for stored tuber is important because once dormancy breaks, the tubers also senesce rapidly with loss of the stored food (carbohydrate) (Passam and Noon, 1977). Yam tuber does not sprout during the early part of storage, even under suitable growth conditions. The environmental conditions affecting yam tuber dormancy are photoperiod, white and coloured lights, temperature, relative humidity, and partial oxygen pressure. The length of tuber dormancy is endogenously controlled and conditions such as availability of soil moisture or cool temperature are ineffective triggers of sprouting. Physiological age of tubers affects their readiness to sprout, but by approximately 6 months after harvesting, dormancy disappears completely and budless setts planted after that period will require nearly the same time to sprout (Onwueme, 1975). The length of dormant period is affected by the yam species (Table 7). These data are useful for developing suitable storage and marketing strategies, and also for scheduling the next planting.

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