Pre-Harvest and Postharvest Factors Affecting Quality and Shelf Life of Harvested Produce

Abstract

Food security and access to quality food are major challenges in the efforts against global hunger. Despite producing a large amount of food each year to boost the economy, a significant portion is lost due to pre-harvest and postharvest factors affecting produce’s quality and shelf life. Numerous interventions have been implemented to address this to improve postharvest management, but there is still an urgent need to identify and manage the various factors contributing to postharvest losses. Factors contributing to postharvest losses include agents of food deterioration inherent in the produce before harvesting, inappropriate cultural practices, genetic composition, harvesting methods, quality of water for irrigation, microbial invasion, insect pest inoculum remnants and more. Postharvest handling involves interactive activities from harvest to consumer’s final decision to eat or reject the food. Produce quality is determined by local conditions, policies, stakeholders’ cultural practices, market demand, road condition, handling methods, packaging materials, transportation methods and level of knowledge and awareness in that environment. This study is to elucidate, through literature, pre-harvest and postharvest factors affecting quality of harvested produce. This study showed that understanding and appropriate management of pre-harvest and postharvest factors would reduce quality losses and increase the shelf life of produce.

1. Introduction

One of the most urgent and essential needs of today is ensuring food security for the rapidly growing world population and, at the same time, ensuring long-term sustainable development in the reduction of food losses. Postharvest losses significantly increase food insecurity, reduce farmer’s income and enhance inefficiency in the global food system. The essential elements of postharvest losses challenge include problem of multiple points of intervention, multiple technologies, complex value chain, and poorly developed food systems [1]. In accordance with projections by FAO, food production will need to grow by 70% to feed the world population, which will reach over 10 billion by the year 2050. As efforts are being geared towards increased production, there must be corresponding efforts for an integrated and innovative approach to the global efforts to ensure sustainable food production, consumption and loss reduction [2]. One major way of strengthening food security is by reducing losses. Postharvest loss can be defined as degradation in both quantity and quality of food from harvest to consumption. Reduction in these losses would increase the amount of food available for human consumption and enhance global food security. Food losses occur due to poor infrastructure, logistics issues, lack of technology, lack of prompt access to markets, insufficient skills, and inadequate knowledge and management capacity of supply chain actors. Losses also occur at the production, postharvest and processing stages in the food supply chain [3, 4]. Food waste refers to food appropriate for human consumption being discarded along the food chain due to consumers’ behaviour [5, 6]. Damage restricts the use of a product, whereas loss makes its use impossible. These losses occur because harvested agricultural produce consists of living tissues that respire and undergo physiological changes caused by conditions such as high temperature, low atmospheric humidity, physical injury, biotic contamination and enzyme actions. Food losses reduce the food available for human consumption and incur costs of waste management; loss of scarce resources used in crop production generates about 6–10 per cent of human-greenhouse gas emissions in the land where food wastes decompose anaerobically [1]. Pre-harvest refers to every activity embarked on by the producer in the production of crops before harvest, and this includes site selection, land preparations, appropriate planting date, optimum seed rate, recommended spacing, appropriate tools and equipment used, proper tillage activities and seedbed preparation, pests, disease and weed management, irrigation, mulching, staking and use of hormones. An adequate supply of potassium nutrition in tomato production enhances titratable acidity and fruit colour quality and reduces the incidence of the yellow shoulder [7, 8], while the inadequate application of potassium in aqua-phonic tomato production results in ripening disorders [9]. An increase in nitrogen supply to tomatoes grown in a controlled environment may reduce fruit quality by decreasing the sugar content of the fruits [10]. A high nitrogen supply of about 250 kg/ha can impair some important quality traits of tomato fruits, such as total soluble solids [11], glucose, fructose, and pH [12]. The addition of ammonium in tomato production results in improved fruit flavours [13]. The quality of tomato fruit is also affected by the amount of boron used. Lower amounts of boron supply reduce fruit firmness [14]. The compositional quality of harvested produce is affected by maturity stage; Howard [15] observed that total vitamin C content of red pepper was about 30% higher compared to green pepper. Tomato fruits harvested green at table ripeness contain less vitamin C than those harvested at the full ripe stage. Tomato fruits at the ‘breaker’ stage contained only 69% of their vitamin C concentration. Quality refers to the state of excellence of a produce, which may be either good or bad. It refers to a property or group of properties that make a produce acceptable or desired by a consumer. It is subjective and changes according to culture, customs, environment, social status and mindset. These parameters change from one food commodity to the other. Several attributes have been used to describe quality: size, shape, colour, consistency, flavour or organoleptic properties like texture, smell and tastes. Other properties that are used to measure qualities include appearance as presentation, nutritive value, dependability and wholesomeness. Higher quality will translate to higher prices and more consumers’ satisfaction. Quality can be described based on produce usages, such as edible quality, dessert quality, shipping quality, table quality, nutritional quality, internal quality and appearance quality. The quality of harvested produce is a combination of characteristics, attributes and properties that give the produce value to humans for food. Quality standards are usually sets to essentially meet specification and demand of the consumer. Every change in food that causes it to lose its desired quality and eventually become unpalatable is called food spoilage or rot. Food spoilage is also a metabolic process that causes foods to be undesirable or unacceptable for human consumption due to changes in sensory characteristics [16]. Quality losses include those that affect the nutrient composition, acceptability, and edibility of a given product, while quantity losses refer to those that result in the loss of the amount of a product [3]. Reducing food losses requires an understanding of pre-harvest and postharvest factors resulting in loses, cultural background and economic level of the people involved. It is essential because all food losses occur at a particular socio-cultural environment. Reduction of postharvest food losses is critical in ensuring future global food security. The issue of food losses is of high importance in the efforts to combat hunger, raise income and improve food security in the world today. It is very important to know the pattern and scale of these losses across the world, especially in developing countries and identify their causes and possible solutions.

2. Quality attributes

Appearance: This is the evaluation of quality by sense of sight, shape, wholesomeness and pattern. It includes quality traits such as:

Shape: The shape should not deviate from the typical accepted standard set for that produce. It must not have any damage or bruise, for example, cucumbers are supposed to be elongated in shape and also robust, not sickle or twisted in shape.

Size: The size of some harvested produce determines their market acceptability. Examples of such are plumb, robust banana finger, carrot, okra, plantain are more acceptable than a slim one; also in cauliflower, size and compactness of the head are quality parameters that determine market acceptability. Matured plants are harvested at about 15 cm in diameter, and protruding floral parts indicate over maturity.

Colour: Colour of harvested produce like fruits and vegetables is expressed through their various pigments and can be grouped into red: anthocyanin pigments, green: chlorophyll and yellow to orange: carotenoids. There are many factors that influence colour, such as genetic constitution, maturity, climate, environment, season, soil type, plant nutrition, plant density and postharvest treatment. Maturity is a very important factor in determining the colour of harvested produce. At early stages of development, colour is usually green and only attains its characteristic colour at full maturity. The chlorophyll colour is important in vegetables, and if this is lost, the vegetables are not acceptable. Examples of such green vegetables are celery, green bell peppers, chayote squash, cucumbers, collard greens, green beans, green onions, green peas, leeks, lettuce, mustard greens, endive, kale, kohlrabi, jalapenos, okra, snap peas, snow peas, Swiss chard, watercress and zucchini. It must show the true colour of the product in terms of lightness, transparency, turbidity, and glossy nature [17, 18].

Wholesomeness: This involves the sanitary factors of the product. It must be clean without impurities, extraneous matter, sediments or specks. Also, a product must be a whole product, not part of a whole.

Pattern: This also describes shape and size; it should follow the specific pattern peculiar to the product.

Firmness: For cauliflower, a firm and compact head of white to cream-white curds surrounded by a crown of well-trimmed, turgid green leaves are required quality indices with freedom from severe yellowing, defects due to handling and decay.

Texture: This is the hand and mouth feel, the assessment of quality by the sense of touch and taste. It also indicates coarseness or crispness, firmness, turgidity, density, viscosity, surface tension, juiciness or dryness, fibrousness or chewiness, softness, mildness or stickiness. The texture of tomato is majorly contributed by the insoluble solids derived from cell walls which determined the consistency, smoothness and juiciness of the fruit [24].

Consistency: This may also be considered by a sense of touch. This refers to visualisation, flow or spread proportion of the produce. Appearance and texture can be measured by a team of experts ranging in order of quality importance. It is, therefore, a very elusive factor to measure because it depends on individual judgement. This means of measurement is usually referred to as organoleptic test [19, 20, 21, 22].

Flavour: This is the quality evaluation by the sense of taste or smell. It refers to terms such as odour, fragrance, acid, burnt or gutty; taste: sweet, sour, bitter, salty or bland, off-flavour, enzymatic reaction, physiological deterioration and chemical contamination, overcooked or stale. For example, apples should have a crunchy texture and a sweet flavour while pomegranates should have a juicy texture and a sweet-tart flavour while strawberries should have a juicy texture and a sweet flavour. Tomatoes are typically juicy and firm, but can range from soft and mushy to hard and crunchy feel. Cucumbers are typically crunchy and juicy; while carrots are typically crunchy and sweet. Beetroots are firm and sweet, and cabbage is crunchy and slightly bitter while lettuce is crisp and slightly bitter, and ginger and turmeric are fibrous and slightly spicy. Enzyme reactions also occur, resulting in desirable flavour. For example, hydroperoxide lyase catalyses the production of tomato flavours [23]. To enhance sales, these quality factors are essential for market acceptability and consumers’ choice to buy or reject the product. Sometimes, initial sales occur based on appearance, but repeated purchases are driven by expected quality factors determined by flavour compounds and texture [24, 25].

Nutritional value: This quality attribute is very important in ascertaining produce composition. It includes: total protein content, amino acid composition, mineral and vitamins, juice content, total soluble solids and vitamin C. Vitamin C content varies in fruits and vegetables from one to 150 mg/100 g [26]. In berry fruits, it ranges from 14 to 103 mg/100 g [27]. Rosehip, jujube, guava, kiwifruit, peppers, citrus fruit, spinach, broccoli and cabbage are rich in ascorbic acid. Tomato is about 93–95% water and 5–7% total solids. The lipids constituent in grapes is 0.1%, in bananas is 0.2% and in apple is 0.06%. Lipids content of 35 to 70% of dry mass is obtained in avocados, olives and nuts [28]. Fat-rich fruits and nuts include avocado, cauliflower, broccoli, carrots, hazelnuts, almond, walnut, Brazil nuts and chestnut [29]. Hazelnuts and almonds have flavonoid content of 18 and 15 mg/100 g, respectively [9]. Walnuts and Brazil nuts have phenolic acid content of 36 and 11 mg/100 g, respectively [10, 11, 12, 13, 14]. Good examples of fibre-rich foods are mango, orange, papaya, sweet lime, watermelon and apple [30]. Grains, fruits and vegetables are good sources of fibre. Fruits and vegetables provide 37% of the fibre in the diet and grains (36%) while legumes supplied 13% [31, 32]. Pro-vitamin A refers to precursors of vitamin A, obtained from fruits and vegetables such as carrot, pumpkin, peach and mango. Riboflavin is the central component of flavour proteins. It can be obtained from beans, beetroot, pepper and spinach. Niacin is derived from almonds, avocado and cape gooseberries. Vitamin B5 or pantothenic is obtained from meats, potatoes, oat cereals, tomato products and whole grains. Sources of vitamin B6 include beans, cabbage, cauliflower, spinach, sweet potato, grape, avocado and banana. It occurs in peas, beans, nuts, broccoli, mushrooms, potatoes, strawberries and sweet potatoes. Folic acid is essential for reproduction and normal growth. It is present in strawberry, tomato, avocado, spinach, cabbage and other green vegetables [33, 34]. Table 1 showed food sources supplying vitamins, carotenoids and phenolic.

Vitamin A	Vitamin B	Vitamin C	Vitamin D	Vitamin E	Carotenoid	Phenolic
Carrot	Beans	Strawberry	Cod liver oil	Almond	Pineapple	Plum
Pumpkin	Sunflower seed	Black currants.	Salmon	Corn	Carrot	Blackberry
Mango	Avocado	Kiwifruit	Sardine	Broccoli	Tomato	Apple
Peach	melons,	Orange	Tuna Fish	Spinach	Melon	Strawberry
Broccoli	Broccoli	Pepper	Beef Liver	Peanut	Mango	Raspberry
Red bell pepper	Cabbage	Guava	Egg yolk	Avocado	Peach	Blueberry
Lettuce	Spinach	Rosehip	Natural Sun	Grains	Plum	Gooseberry
Spinach	Peas	Persimmon	Mushroom	Nuts	Lemon	Soursop
Cantelope	Citrus	Straw berries	Fortified food	Leafy vegetables	Acerola	Curry
Sweet potato	Banana	Broccoli	Oil from Sheep wool	Seeds	Amta	Guava

Table 1.

Food sources supplying vitamins, carotenoids and phenolic.

Source: Adewoyin O.B.

2.1 Quality deterioration

This is associated with advance spoilage; any change in food quality, when optimum quality is not obtained, is referred to as spoilage. The major causes of food deterioration in harvested crops are microorganisms, natural food enzymes, insects, rodents and parasites, heat and cold, moisture and dryness, air, for exmaple, O₂, light and time.

1. Activities of microorganism: Several thousands of microorganisms are useful in food processing and fermentation in brewing while many others have undesirable changes in produce (referred to as spoilage), thereby reducing the shelf life of harvested produce. They are found on plants, in the soil at growth stages on the field, distorting roots and leaves, resulting in reduced yield and loss of quality. After harvest, microorganism activities continued everywhere: on peels of harvested produce in the hands of handlers and in water [35].

2. Natural Food Enzymes: This is also an important cause of food spoilage. Natural food enzymes like microorganism are controlled by heat, cold, dryness, certain chemicals and radiation. Treatments applied to inactive microorganism will partially or completely destroy the enzyme. Natural enzymes are present in harvested food crops, without which germination to fruit maturation would not have occurred. These enzymatic reactions persist after maturation. Some of these enzymes may be more active in fruit crops such as pepper, tomatoes, plantain and banana to cause fruit deterioration. It is not in all cases that low temperature is required to preserve most tropical produce. The colder, the worse for some produce. For example, if the avocado is stored in a fridge, it turns black without ripening because the respiratory pattern is upset and other biological processes take place, leading to deterioration. Specific conditions for preservation must be selected in accordance with the unique spoilage pattern of individual harvested produce [35, 36, 37].

3. Insects, rodents and parasites: The effect of insects, rodents and parasites depend on the environment. These factors can then be controlled easily by ensuring an appropriate environment free from pest invasion compared to microorganisms. Insects and rodents do physical damage to harvested produce such as scars, holes which provide entry point for secondary infection by microbes [38, 39, 40]

4. Heat and cold: Increase in temperature can double the rate of chemical and enzymatic reactions which lead to increase in microorganism invasion, and result in an increased loss of vitamins A, C and riboflavin. Under low temperature, banana, avocado and pear blacken in the freezer and during thawing the skin may crack and secondary invasion will set in bananas should be stored under conditions where the temperature range is 10–15°C [41].

5. Moisture and dryness: When a produce absorbs water, it losses quality and becomes moist. An example of such is caking and lumping in stored grain like soybean, rice, maize and beans. Quality changes occur in produce due to moisture pick up. Moisture picked up occurs during thawing at low temperatures but without control of relative humidity, which allows moisture to dense over the surfaces of the produce consequently resulting in the rapid multiplication of microorganism [37, 41, 42, 43]. Table 2 showed various crops and their safe moisture content.

6. Air: Oxygen in the air can be destructive due to its high oxidative power, and it supports multiplication and optimum living condition and reproductive cycles of insect pests, rodents and microorganisms. Off-colour can also occur due to oxygen, which is essential for microbial growth, especially mould, which grows on the surfaces of produce. Activities of pests and rodents increase rapidly when oxygen is adequately available in the storage system. Oxygen can be controlled from harvested produce by the waxing of fruits, packaging in O₂ impermeable skin, tight plastic, vacuum packaging with the removal of oxygen and supplementing with nitrogen.

7. Light: Some vitamins, such as vitamins A, C, and riboflavin, are destroyed by light and the only way to control this is to use dark-coloured containers for storage. Vitamins A, C and riboflavin, under light, change the colour of food, and react with lipids, fats and oils.

8. Time: All the above factors are affected by time. All the reactions proceed with time. Produce has to be preserved at the peak of quality, which is impossible due to time. It is always better to consume food immediately to avoid any loss, but this is not often possible due to time. Microorganisms have plenty of time to act and cause deterioration. Time is needed to produce toxic substances, exposure of food to cold or heat takes time, and length of time affects deterioration. In some food industry, fermentation may be beneficial in producing the required flavour and aroma; this also takes time to develop. All deterioration takes time, and all processes leading to deterioration must be arrested on time [43, 44].

3. Factors influencing quality

3.1 Pre-harvest factors

Site selection: The soil properties of a given site for crop production will determine the ultimate compositional and physical quality of the harvested produce. Appropriate site selection, free from heavy metals, toxic materials and adequate fertility level is essential for maximum quality. The soil should be analysed, and the soil condition should be determined before planting.

Genetic constituent of produce: The potential quality of harvested produce is a factor in the genetic constitution of the plant. Varieties with shorter shelf lives have higher postharvest losses while those with thick peel, high firmness quality, low respiration rate and low ethylene production rates have longer shelf life. Their different quality traits characterise each variety; these peculiar genetic quality character traits make some varieties more desirable to producers and consumers. The choice of adequate-yielding crop variety with desired qualities and longer shelf life is a vital decision for producers and an important pre-harvest factor that determines the shelf life of harvested produce [10]. The varieties that have the potential traits to withstand the rigours of marketing and distribution will have reduced losses after harvest. Varieties with resistance to low-temperature disorders, pathogens can be stored efficiently for longer duration with minimum storage losses. To prolong shelf life, enhance sustainable food availability and maintain good quality, producers must choose varieties that have inherently good quality and extended storage potential in addition to the high yield and pest resistance potentials. Failure to select an appropriate cultivar may lead to lower yield, low-quality fruits or less market acceptability. Fruits of different cultivars differ in size, colour, texture and flavour as well as storage potential. Getinet et al. [45] showed the influence of tomato cultivars on some postharvest qualities of tomatoes stored under different conditions, they established that tomato cultivar Roma VF has higher sugar content and lower weight loss compared to other cultivars. The genetic constituent of a produce is, therefore, critical to the postharvest storage life and utilisation qualities of such produce [37].

Planting period: The quality of crops planted during the dry season differ in size, firmness, fibre content and nutritional composition compared to those cultivated in rainy season when there is adequate water availability for chemical processes necessary for plant growth and development [46].

Irrigation: Some crops are not drought resistant hence, yield decreases in terms of size and nutritional quality after short periods of water stress. Proper irrigation planning is crucial for optimum crop development and adequate nutritional composition. Efficient water management scheme is vital to maintaining quality crop and maximum yield [47]. It is observed that deficit irrigation reduced fruit water accumulation and fresh fruit yield but increased fruit total soluble solids in tomatoes [48]. Mitchell et al. observed that deficit irrigation reduced fruit water accumulation and fresh fruit yield but increased fruit total soluble solids in tomato [48]. A higher level of moisture stress affects both yield and quality by decreasing cell enlargement. Crops which have higher moisture content generally have poorer storage characteristics. Some hybrid onions give a high yield of bulbs with low dry matter content and short storage life. Fully matured banana harvested soon after rainfall or irrigation may easily split during handling operations resulting in microorganism infection and rotting. If orange is too turgid at harvest, gland in the skin can be ruptured during harvesting, releasing phenolic compounds and causing oleocellosis or oil spotting (green spot on the yellow/orange coloured citrus fruit after degreening). In green leafy vegetables, too much rain or irrigation can make leaves harder and brittle, making them more susceptible to damage and decay during handling and transportation. Generally, crops with higher moisture or low dry matter content have poorer storage characteristics. Keeping quality of bulb crops like onion and garlic will be poor if irrigation is not stopped before 3 weeks of harvesting [11].

Thinning and Pruning: Thinning is a post-planting operation that reduces plant population, and competition between plants, increase maximum exposure to light, supply adequate water and nutrient to plants, consequently promoting good balance in the vegetative stage during fruit production and also improves the quality of harvested produce. It affects fruit texture and size due to inadequate exposure to sunlight. Thinning also improves the textural characteristics of harvested produce, which consequently greatly affect firmness. Studies also revealed that fruit firmness positively correlated with fruit size, implying that larger fruits were slightly firmer at harvest than smaller ones [18]. Appropriate pruning enhances fruit texture characteristics by optimise light distribution to all fruit on the tree. Inappropriate pruning may result in fruit shading with consequent smaller undesirable under, ripe fruit with a hard and grainy texture. Pruning is done to control the number of flowers and fruits by reducing the competition between fruits. Pruning, therefore, ensures nutrients are channelled to fewer fruits which can lead to increased fruit size [49] and increased sugar content of fruits in some cases [50]. On the other hand, the effect of pruning on other quality traits of the fruit produced depends on many factors, including the sink developmental stage, fruit-to-leaf ratio, truss position, and genetic composition of the plant [51].

Maturity Stage: Fruit maturity stage influences the total antioxidant capacity of the produce. These changes are determined by crop type and stage of maturity. For example, in tomato, pepper, mango, and prunus species, total antioxidant capacity increases as carotenoids and vitamin C accumulate during ripening [52]. Adewoyin and Babatola observed that pepper fruits harvested at 10% ripe (breakers stage) retained firmness, and weight loss was minimal compared to those harvested at 100% ripe stage (fully ripe) in all storage medium studied [53]. Wang and Lin observed that the shelf life of all tomatoes is longest when harvested at green mature stage though fruit nutritional values and appearance may be affected when harvested green [54]. Delays between harvest and consumption or processing can result in losses of flavour and nutritional quality. The magnitude of these losses increases with exposure to inappropriate temperatures, relative humidity and concentrations of O₂, CO₂, and C₂H₄ [26]. During berry ripening, anthocyanins accumulate while phenolic acids decrease [55] in products in which anthocyanins or chlorophylls dominate, carotenoids decrease during development; in cherry, ascorbic acid accumulates during ripening [56].

Climatic condition: Many plants are very sensitive to environmental conditions, and thus quality will not be optimised when crop is produced under adverse conditions. Poor weather at harvesting time affects the operations and functionality of harvesting machines or human labour and usually increases the moisture content of the harvested products, consequently resulting in loss of quality and reduced shelf life [57].

Heat management: Physiological and biochemical processes involved in plant growth, yield and maturation is influenced by temperature. Higher temperature during field conditions decreases shelf life and quality of the produce. At high temperatures, plants respire at a faster rate, and stored carbohydrates in harvested produce are depleted rapidly during respiration. High temperature during the fruiting season of tomato leads to quick ripening of fruits. Orange grown in the tropics have higher sugar content and total soluble solids than those grown in the subtropics. Tropically grown oranges tend to be green in colour and peel less easily. This is due to the higher temperature that occurs in the tropics, which results in rapid maturation of fruit which halts the process of the typical temperate orange colour development [58].

Light: Light regulates several physiological processes like chlorophyll synthesis, phototropism, respiration and stomatal opening. The duration, intensity and quality of light affect the quality of fruits and vegetables at harvest. Most of the produce needs high light intensity. Absorption of red light through pigments, phytochrome, is essential for carbohydrate synthesis, which determines the shelf life of the produce. Citrus and mango fruits produced in full sun generally had thinner skin, a lower weight, low juice content and lower acidity but a higher total soluble solid. Citrus fruits grown in the shade may be less susceptible to chilling injury when stored in cold storage. In tomatoes, leaf shading of fruits produced a deeper red colour during the ripening than in the case of those exposed to light. The side of the fruit that was exposed to the sun was firmer than those that were not exposed to sunlight, the lower the light intensity, the lower the ascorbic acid content of plant tissues. In leafy vegetables, leaves are larger for those exposed to adequate light and thinner under conditions of low light intensity.

Humidity: High humidity during the growing season results in thin rind and increased size in some horticultural produce, and this produce is more prone to a high incidence of disease during postharvest period. Humid atmosphere may cause the development of fungal and bacterial diseases, which damages produce during storage and transport. Damaged produce removes water very quickly and emits a larger ethylene concentration than healthy ones. Low humidity may cause browning of leaf edge on plants with thin leaves or leaflets. High humidity can maintain the water-borne pollutants in a condition so that they can be more easily absorbed through the cuticles or stomata. Reduced transpiration leads to calcium and other elemental deficiency [59].

Rainfall: Rainfall affects the water supply to the plant and influences the composition of the harvested plant part. This affects its susceptibility to mechanical damage and decay during subsequent harvesting and handling operations. Excess water supply to plants results in the cracking of fruits such as orange, cherries, plums and tomatoes. If root and bulb crops are harvested during heavy rainfall, the storage losses will be higher [60].

Seasons: Seasonal fluctuation and time of the day at harvest will greatly affect the postharvest quality of produce. Synthesis of higher amount of carbohydrates during the day and its utilisation through translocation and respiration at night is responsible for the variation in the longevity of some harvested produce. Roses and tuberose have been found to show longer keeping quality in the winter under ambient conditions than in the summer. Produce harvested early in the morning or in the evening hours exhibits longer postharvest life than produce harvested during hot time of the day. If long-day onion (temperate) is grown during short-day (tropics) conditions, it will result in very poor storage quality [61].

Fertiliser application: Poor fertiliser management will increase physiological disorders due to deficiencies of some minerals or increase of others, leading to toxicity. In both cases, quality will be negatively affected. The use of trace elements or the practice of soilless tomato production can be made possible during irrigation, where fertilisers are added to the irrigation water in a form of solution and administered. These trace elements are selected depending on the specific postharvest quality traits needed in the fruits. Nutrient balance is crucial for maintaining optimal fruit texture and size: fruits from nitrogen deficient trees are usually smaller with firmer texture, while excess nitrogen leads to rapid loss of firmness and decreased storability. Potassium deficiency also leads to textural changes resulting in small, poorly coloured fruit that may not ripen, leaving fruit hard and inedible. A lack of boron can result in fruits with a mealy texture [62, 63, 64].

Pest and Disease Management: Pathogens and insects have very negative effects on quality of harvested produce. The effect of insect is more pronounced on grains but can also cause a lot of damages in fruits and vegetables. Nematodes cause various injuries to fruits and vegetables and continue the deterioration during storage. Parasites are therefore seen to be important in damage to farm produce as well as food preservation. In the case of insects, produce attacked by them in agriculture may consume over 50% of the harvest. Insects at times lay eggs in the produce, making it almost impossible to eliminate all insects pest in the produce. Parasites like nematodes and amoeba may infect the produce, and the same is true when produce comes in contact with water; this is very common in Africa. Rodents contaminate food with their urine and droppings. They also produce large litter, for example, two rats can give up to 30,000 litres per year. Through their contamination, they spread diseases like salmonellosis, plague and typhoid fever. Various efforts to control the detrimental effects of these organisms have resulted in great hazard to human health due to misuse of the chemicals [65].

Weed management: Failure to control weeds will result in a lot of damage to crop quantity and quality. Weeds harbour diseases and pests that easily infest crops both on the field and in the store. Weeds also contaminate harvested produce by mixing with the seeds [66].

Presence of heavy metals: The site for crop production must not be just any site or dump site which is loaded with heavy metals. Appropriate soil tests should be done to ascertain the soil condition because some crops absorb heavy metals, which are easily assimilated by human system [66, 67, 68, 69].

Harvesting methods and time: The time of the day at which harvesting is done must be considered to avoid excessive field heat, which can cause rapid deterioration of the harvested produce. Loss is also caused by improper harvesting methods such as rough handling, untimely harvesting, lack of appropriate and poorly-designed harvesting tools, equipment, and harvesting containers [70].

Method of Processing: Processing can decrease phenolic antioxidants [71]. Anthocyanin losses in processed berries are reduced by blanching, indicating enzymatic degradation [71]. A study conducted comparing manually tearing lettuce into strips to shredding with a sharp knife showed that the retention of ascorbate in lettuce sliced by a machine was 25–63% lower than in lettuce shredded by manual tearing [72]. Effects of slicing and shredding radishes on quality during storage at 1, 5, and 10°C were determined [73]; on the 10th day, intact radishes stored at 1°C had the lowest respiration rate, while sliced radishes stored at 10°C had the highest. Shredded radishes showed the most undesirably low levels soluble solids, higher weight loss, ascorbic acid, and lightness as compared to intact or sliced radishes [74].

4. Postharvest factors

Harvested produce are living tissues and subject to continual changes after harvest. Such changes cannot be stopped but can be controlled within certain limits by using various postharvest procedures. Postharvest factors that affect quality of harvested produce include the following: temperature is the most important tool to extend shelf life and maintain quality of harvested produce [75]. Delays between harvesting and cooling or processing can result in direct loss due to water loss and decay; indirect losses such as off-flavour and deterioration in nutritional quality can also occur. For instance, the temperature range and the extent of vitamin C loss depended on the type of citrus fruit. In general, the extent of loss in ascorbic acid (AA) content in response to elevated temperature was greater in vegetables than in acidic fruits such as citrus. Ascorbic acid is more stable under acidic conditions [76]. Reported that retention of vitamin C ranged from 56 to 98% for six broccoli cultivars stored at 2°C for 3 weeks.

Ethylene management: Ethylene (C₂H₄) is an odourless and colourless two-carbon natural plant hormone which is triggered at maturity in climacteric fruits. It is also known as the ‘natural ageing, death or ripening hormone’, it is active at small traces and its accumulation can led to fruit decay and waste during postharvest stage of harvested produce. It is regulated in various physiological processes of plant growth, germination, development, ripening, maturity and senescence. It also plays a major role in the abscission of plant organs. Several strategies of crop management, coordination of postharvest and pre-harvest factors and various techniques of plant breeding have been investigated to understand ethylene regulation pathways, biochemical and physiological processes in extending produce shelf life and improve the postharvest quality of harvested produce [77]. Some fruits are either ethylene producers or absorbers. Fruits such as apples, bananas, melons, pears and peaches are ethylene producers, while broccoli, cabbage, and cauliflower are ethylene sensitive. The respiration rate of non-climacteric crops is not influenced by the presence ethylene. Examples of non-climacteric produce are leafy vegetables, watermelon, strawberries and grapes. Non-climacteric crops will not respond to ripening with ethylene gas. Exposure of climacteric fruits to ethylene will advance the onset of an irreversible rise in respiration rate and rapid ripening. Appropriate packaging can delay the onset of climacteric and prolong shelf life of fruits by reducing ethylene production and sensitivity [28]. Ethylene production rates is influenced by type of produce, and it increases with maturity at harvest, physical injuries, disease incidence, increased temperature up to 30°C and water stress [30]. The response of various types of crops to ethylene is shown in Table 3.

Product	Safe moisture content (%)
Maize flour	11.5
Millet	16.0
Rice (milled)	13.0
Rice	15.0
Sorghum	13.5
Wheat	13.5
Wheat flour	12.0
Pulses	15.0
Lentil, pea	14.0
Carrot	12.7
Cabbage	21.6
Banana	9.95
Plantain	9.97
Mango	9.147
Lettuce	12
Spinach	4.7
Guava	6.94
Yellow Yam	59.3
White Yam	64.97
Bitter Yam	64.9

Table 2.

Crops and their safe moisture content.

Source: Adewoyin O.B.

Class	(μl C2H4/kg-h at 20°C (68°F)	Commodities
Very low	< 0.1	Artichoke, asparagus, cauliflower, leafy vegetables, root vegetables, potato, cherry, citrus, grape, jujube, strawberry, pomegranate
Low	0.1–1.0	Cucumber, eggplant, okra, watermelon, chilli, bell pepper, pumpkin, watermelon, olive, pineapple, blueberry, raspberry
Moderate	1.0–10.0	Tomato, banana, fig, guava, honeydew melon, mango, plantain
High	10.0–100.0	Apple, passim, apricot, avocado, cantaloupe, kiwifruit, nectarine, papaya, peach, pear, plum
Very high	> 100.0	Sapota, mammee apple, passion fruit, cherimoya

Table 3.

Classification of crop according to ethylene production.

Source: Adewoyin O.B.

Chemical Treatments: Calcium dips may be used to reduce physiological disorders and maintain firmness in apples and cherries. Dehydrated pineapples and guava pre-treated with cysteine hydrochloride had increased vitamin C retention and reduced colour change during storage [76, 78].

Irradiation: Ionising radiation may be used for sprout inhibition, insect control or delay of ripening of certain fruits and vegetables. Irradiation of horticultural crops at relatively low doses of 75–100 krad irreversibly inhibited the sprouting of potatoes regardless of storage temperature. Losses in vitamin C were lower in potato irradiated for sprout control and subsequently stored at 15°C than in non-irradiated tubers stored at 2–4°C 78].

Respiration: The respiration rate in harvested produce will influence its metabolic activities. High temperatures can hasten the rate of respiration and CO₂ production in harvested produce. CO₂ production in stored climacteric products like tomatoes can trigger ethylene production, depending on other factors like O₂ or CO₂ levels, exposure time, and ripening stage [79].

Relative Humidity: Moisture loss from harvested produce is predominantly caused by the amount of moisture present in the air expressed as relative humidity [80]. Harvested fruits and vegetables maintain their nutritional quality, appearance, weight and flavour at very high relative humidity. On the contrary, this is adverse for grain crops such as maize, millet, wheat, rice, beans and soybean. Harvested fruits shrivel with little percentage moisture loss while grain crops require low moisture content for optimum storage conditions after harvest. The optimal values of relative humidity to maintain quality of mature green tomatoes are within the range of 85–95% (v/v) and 90–95% (v/v) for firmer ripe fruits [81].

Physical Handling: The handling of produce from the moment of detachment from the parent plant, packing from the farm gate to the market, and then the final consumption of produce are significantly associated with mechanical injuries such as bruising, scarring, scuffing, cutting or puncturing. Some of these mechanical injuries may result from careless handling, the use of inappropriate harvesting containers, inappropriate vehicles, careless loading and unloading and packaging materials. According to Miller [82], the consequences of these mechanical injuries are cumulative, leading to a total breakdown of the cell structure accompanied by unwanted metabolic activities such as increased ethylene production, accelerated respiration rates, and ripening, resulting in either reduced shelf life or poor quality [83]. It is, therefore, important to handle harvested produce with care during harvest and postharvest activities to minimise mechanical injuries to avoid losses.

Gases: The combination of different gases in a storage environment is very important in extending the shelf life of harvested produce. The optimal ambient condition required to inhibit senescence in matured harvested produce is very low supply of oxygen [84, 85, 86]. Carbon monoxide (CO) has been investigated as a gas to speed up ripening, and it is therefore necessary to balance the carbon monoxide with low oxygen to delay senescence in harvested produce [86].

5. Conclusion

Huge amount of crops are being produced annually, but most is lost at various stages of handling due to pre-harvest and postharvest factors affecting quality and shelf life of harvested produce. Every unit of produce preserved converts to added unit available for productive utilisation and food security. The use of any postharvest method or handling practices can only maintain quality. Understanding and controlling the various roles of pre-harvest factors like fertiliser application, pruning, maturity stage, cultivar selection, and irrigation can play major role in improving quality of harvested produce and prolonging their shelf life after harvest. Using best postharvest handling practices or factors such as optimum temperature, right relative humidity, right gases in storage and the best physical handling procedures to maintain the quality after harvest is also critical. Postharvest factors alone cannot maintain quality but the pre-harvest factors during production are also important. Until both factors are managed properly, quality loss will still be a major challenge in maintaining quality and prolonging shelf life of harvested produce.