The farm-to-table approach (other terms: system approach, whole chain approach, supply chain approach) is increasingly used to grow and market fresh produce. With this approach, factors before and after harvest are taken into account to obtain the desired product quantity and quality, reduce postharvest loss, and ensure delivery of good quality, safe produce to consumers. Any intervention that manipulates postharvest quality and shelf life could be considered under postharvest technology (PHT). This section introduces some production factors that influence product quality and shelf life, and discusses PHT innovations and recommendations, from harvesting to market handling.
Improved Crop Variety
Leafy vegetables have limited storage life even under optimum storage conditions. The potential shelf life is partly under genetic control and can be manipulated by breeding.
Breeding leafy vegetables for long shelf life and desired shipping quality is a particular need in developing countries with hot and humid climates where refrigerated facilities are lacking due to the high cost. Some specific shelf-life attributes that can be manipulated through breeding include green color retention and resistance to postharvest stress such as high temperature and microbial infection.
More effort is now being exerted to develop vegetable varieties with desired shelf life, shipping and processing attributes, and high levels of nutrients. Breeding leafy vegetables with high carotenoids content has been reported and results in lettuce were promising due to observed genetic variations in B-carotene and lutein contents (Fonseca, 2004). B-carotene and lutein were observed to be higher in leaves with higher chlorophyll content. Other nutrients in vegetables include those with therapeutic or pharmaceutical uses, hence the term nutraceuticals (e.g. glucosinolates). A growing concern is placed on the possibility of accidentally lowering beneficial non-target components while enhancing targeted ones with new technologies.
Production Factors
Environmental conditions and cultural practices during production have tremendous effects on produce quality, safety, and shelf life. For example, lettuce harvested during a period of rain does not ship well and product losses are increased.
Produce stressed by too much or too little water (by irrigation or rainfall), high rates of nitrogen fertilization, or mechanical injury (scrapes, bruises, abrasions) is susceptible to postharvest diseases. Brassicas are prone to bacterial soft rot if nitrogen is applied as foliar feed, thus nitrogen should be applied to the soil. This effect was not observed in pak choi (Jiang and Pearce, 2005); nitrogen above the optimal level did not result in reduced shelf life, while spraying nutrient solution appeared to be beneficial as it retarded yellowing. Potassium sulfate application also enhanced chlorophyll content and extended shelf life of pak choi.
Stress during growth has different effects on produce quality and shelf life. Sustained and intermittent water stress had mostly negative effects for pak choi (Jiang and Pearce, 2005).
Although shelf life of pak choi could be extended by these stresses, the plant fresh and dry weights were reduced. In Chinese cabbage, water stress did not affect the water content and weight loss during nine weeks of storage at 0°C. On the other hand, low light stress (shading) before harvest resulted in more rapid yellowing and wilting in pak choi, and lowered the leaf sugar, organic acids, and chlorophyll content. Increasing the period of shading before harvest further reduced sugar content and increased moisture loss during storage.
Ensuring safety of fresh leafy vegetables also begins in the field. Outbreaks of food-borne illnesses have been traced to contamination of produce in the field (Bachmann and Earles, 2000). Some preventive measures include (1) avoiding application of fresh animal manure or slurries to a field or to an area immediately adjacent to a field nearing harvest maturity, (2) cleaning equipment that has been used to apply manure on one field before moving it to another field, (3) avoiding using irrigation water from a farm pond used by livestock, and (4) avoiding contact of produce with soil during growth (by mulching) or harvest.
Harvesting
Harvest maturity
Quality cannot be improved after harvest, only maintained; therefore, it is important to harvest at the proper maturity stage and at peak quality. Immature or overmature produce may not last as long in storage as that picked at proper maturity. Common cabbage and Chinese cabbage heads are harvested when firm and mature (Bautista and Acedo, 1987; Boyette et al., 1992;
Stephens, 2003; Cantwell and Suslow, 2006). Compactness (firmness, hardness, solidity) of heads may be determined by hand pressure. A compact head can be only slightly
compressed with moderate hand pressure. Delaying harvest even a few days beyond maturity can result in split or cracked heads and increased incidence of rots. Immature heads are puffy or have hollow spaces because the inner leaves are not fully developed and hence, loosely arranged, which make them susceptible to damage (Bautista and Acedo, 1987). When harvested immature, yield decreases and shelf life is shorter than that of mature heads. In certain cases, some sample heads of common cabbage or Chinese cabbage are cut longitudinally to observe the internal stem; if the stem is too long, the head is already overmature (Chen, 2007). Harvest maturity of other vegetables such as leafy mustards, amaranth, and kangkong is based on plant size, number of days after planting (usually 25-30 days) and/or tenderness of leaves. They are harvested when they have developed to the fullest size, yet not so advanced in age that the leaves are tough and the flavor is bitter.
Physiological age of the vegetable or the leaves within a plant could affect the rate of postharvest quality loss. In pak choi, young leaves (20–25 days after emergence) are more prone to moisture loss and subsequent wilting than older ones (40 days) (O’Hare et al., 2001).
However, older leaves turned yellow more quickly than younger leaves. This response was related to initial sugar content, which was higher in younger leaves. In Chinese cabbage heads, intact mature leaves had a greater tendency to yellow than the same leaves, but detached from the head. Young leaves in intact heads began to swell and expand after one month of storage, causing some heads to crack, leading to rapid senescence of whole heads.
Time of harvesting
The time of the day when harvesting is done also affects produce quality and shelf life. In general, harvesting during the coolest time of the day (e.g. early morning) is desirable; the produce is not be exposed to the heat of the sun and the work efficiency of the harvesters is higher. If harvesting during the hotter part of the day cannot be avoided, the produce should be kept shaded in the field to minimize product heat, weight loss, and wilting.
Research showed that harvest time of day could affect quality of pak choi but not Chinese cabbage. Pak choi harvested at 0400h and 2000h contained the highest initial and final water content. Leaves harvested at these times maintained highest water potential, resulting in a slower rate of wilting than those with lower water potential (Jiang and Pearce, 2005). However, harvesting later in the day has an added advantage because sugar levels were found to be higher as a result of photosynthesis during the day (O’Hare et al., 2001). This could slow down leaf yellowing in pak choi, which has been associated with sugar depletion.
Harvesting method
Harvesting is done manually, hence the harvesters have a major influence on produce quality.
They should be made aware of the importance of good sanitation practices, proper maturity selection, and careful handling to avoid mechanical injuries.
A cabbage head is harvested by bending it to one side and cutting it with a knife, which should be sharpened frequently to reduce effort and lessen picker fatigue. In Yunnan, China,
cabbages and other leafy vegetables are harvested and trimmed using a special knife (Fig. 3) (Chen, 2007). The head should not be snapped or twisted, as this method damages the head and results in inconsistent stalk length and trim. Broken stalks are also more susceptible to decay. The stalk should be cut flat and as close to the head as possible, yet long enough to retain two to four wrapper leaves. Extra leaves act as a cushion during handling and may be desired in certain markets. Yellowed, damaged, or diseased wrapper leaves should be
removed. Heads with insect damage and other defects should be discarded. It is essential that heads not harvested be left undamaged because fields may be harvested as many as three times for maximum yield. Harvested cabbage can be placed in bags, boxes, wagons, or pallet bins. Harvesting aids can significantly reduce labor costs, improve harvest efficiency and cabbage quality, and speed the harvest and field handling operation.
Mustards and Chinese kale are harvested as single leaves or whole plants. Fields are usually harvested several times, so care is needed to prevent damage to the plants. The produce must be handled gently during harvesting and field handling to avoid physical damage.
Field Handling
The harvested produce is usually placed in collection containers, which may be plastic crates or bamboo baskets with cotton or paper cushioning or padding (Chen, 2007). Throwing harvested produce into the collection container or vehicle should be avoided to prevent physical injuries. Handling aids such as boxes, farm trailer, or a simple conveyer can be used.
Exposure of harvested produce to the heat of the sun is detrimental except in a few cases.
Leafy vegetables left in the sun after harvest may reach temperatures as high as 50oC (Kanlayanarat, 2007). High product temperature accelerates quality deterioration due to increased water loss and respiration. If packed and transported without cooling, wilting and other deteriorative processes rapidly set in.
Purposive water loss (2–3% water loss) may be imposed on harvested produce. In pak choi, this can be done by laying plants under the sun for 30 minutes immediately after harvest (Jiang and Pearce, 2005). This was found to significantly reduce mechanical damage (snapping of turgid outer leaves) when the produce was packed into bamboo baskets.
Subsequent washing to remove dirt was able to rehydrate the produce. Wilted pak choi could be re-hydrated (and cooled) by dipping in water and the general appearance, color, and original weight could be restored if moisture loss was less than 10%. Rehydration and controlled water loss led to a reduction in losses of 14.5%.
In Cambodia, the practice of leaving cabbage heads in the field for an hour or two with the cut butt end exposed to the sun may also work for the above purpose (Vanndy and Buntong, 2007). Additionally, this practice would dry out the cut butt end of the cabbage head, thereby depriving soft rot pathogens of water needed for their growth and development. However, the problem of heat accumulation within the produce has to be addressed. After the treatment, prompt transport to the packing shed should be done to dissipate field heat without the use of water for cooling. Washing is not advisable in common cabbage. Other leafy vegetables should be transported to the packing shed as soon as possible as they are particularly susceptible to wilting and other damage from high temperatures.
Packinghouse Operations
Trimming
Leaves that have yellowed, show signs of disease, or have other obvious defects, and some outer or wrapper leaves of common and Chinese cabbages are removed (Fig. 3). Removal of the four outer leaves of pak choi heads increased shelf life to over 14 days (Jiang and Pearce, 2005). In Chinese cabbage, farmers may remove all the outer leaves before transport to market. This is a traditional practice of some Chinese farmers, in which the outer nine leaves of Chinese cabbage are removed at harvest. A second trim of three leaves is made to remove mechanically damaged leaves following transport. A simple change to this practice by
removing only six leaves at harvest and leaving the other three outer leaves to protect the head from physical injury was found to reduce losses by 22% without any decline in product quality and increase the profits of the farmers. In common cabbage, outer leaves (wrapper leaves) are also trimmed off except for 3-4 wrapper leaves to protect the head from injury during handling and transport (Bautista and Acedo, 1987). However, wrapper leaves could not fully protect the head from too much force due to impact or compression, which usually results in head bursting. Care during handling is important.
Figure 3. Trimming, cleaning, and bundling leafy vegetables.
Table 1. Chinese cabbage grade standards in Yunnan, China (Chen, 2007) Grade Quality Specifications Head
Size/Weight (kg) Tolerance
First
Same variety; head tight or firm, of natural color and luster, fresh, clean,
and without rot, old stem, yellow leaf, peculiar smell, bolting,
Same variety; head tight or firm, of natural color and luster, fresh, clean, and without rot, old stem, yellow leaf, peculiar smell,
without rot, old stem, yellow leaf, peculiar smell, bolting, wormhole
In Yunnan, China, leafy vegetables are usually trimmed using a special knife (Fig. 3) (Chen 2007). Damaged and senescent leaves are removed and for some vegetables (e.g. cabbage, Chinese cabbage), the butt is trimmed. For leaf mustards, the roots are usually retained and cleaned by washing together with the leaves. The cleaned produce is wrapped or bundled before packaging (Fig. 3).
Sorting/Grading
Systematic sorting or grading coupled with appropriate packaging and storage, will extend shelf life, maintain wholesomeness, freshness, and quality, and substantially reduce losses and marketing costs. Sorting is done to separate poor produce from good produce, and further classify the good produce based on other quality parameters, such size (Bautista and Acedo, 1987). If this is done following quality standards set by product standards agencies or industry requirements, the process is referred to as grading. Leafy vegetables are usually sorted or graded based on maturity, size, shape, color, weight, and freedom from defects such as insect, disease and mechanical damage. Table 1 shows sample grade standards for Chinese
cabbage in Yunnan, China.
In many developing countries, implementation of grade standards as well as safety standards for leafy vegetables and other fresh horticultural produce faces formidable difficulties that contribute to the lingering problem of high postharvest losses. Grade standards, if enforced properly, are essential tools of quality assurance during marketing. They provide a common language for trade among farmers, handlers, processors, and marketers; maintain orderly marketing and equity in the marketplace; and protect consumers from inedible and poor quality produce (Kader, 2006).
Washing and sanitizing
Most leafy vegetables are washed in clean water to remove dirt and other debris and surface contaminants. This is especially important during rainy weather as the produce often is
contaminated with soil. In heading type of cabbages, washing is not advisable as it could favor bacterial soft rot if the heads are not properly dried. The wrapper leaves also keep the inner edible part clean.
Sanitation is essential to control the spread of diseases from one item to another and limit the pathogen load in wash water or in the packinghouse air. Waterborne microorganisms,
including postharvest plant pathogens and agents of human illness, can be rapidly acquired and taken up on plant surfaces (Kader, 2006). Natural plant surface contours, natural
openings, harvest and trimming wounds can be points of entry and provide safe harbor for microbes.
Chlorine in the form of sodium hypochlorite (NaOCl) solution (e.g. Chlorox or commercial bleach) or as a dry, powdered calcium hypochlorite, can be used in wash water as a disinfectant. For the majority of vegetables, chlorine in wash water should be maintained in the range of 75–150 ppm (Suslow, 1997; Bachmann and Earles, 2000). The antimicrobial form, hypochlorous acid, is most available in water with a neutral pH (6.5 to 7.5).
Concentrations above 200 ppm may injure some vegetables (e.g. leafy greens and celery) or leave undesirable off-flavors. A 100 ppm chlorine solution can be prepared by mixing 4 tablespoons of commercial bleach (5.25% NaOCl) per gallon of water (Bautista and Acedo, 1987). Chlorine is routinely used as a sanitizer in wash, spray, and flume waters used in the fresh fruit and vegetable industry (Beuchat and Ryu, 1997). Antimicrobial activity depends on the amount of free available chlorine (as hypochlorous acid) in water that comes in contact with microbial cells.
Chlorinated water can also be used during hydrocooling, and to disinfect packinghouse, packaging and transport facilities. Furthermore, use of sanitized wash water can help to prevent postharvest diseases and food-borne illnesses. E. coli 0157:H7, Salmonella, Cryptosporidium, Hepatitis, and Cyclospera are among the disease-causing organisms that have been transferred via fresh fruits and vegetables.
A standard procedure for washing lettuce leaves in tap water was reported to remove 92.4%
of the microflora (Adams et al., 1989). Including 100 ppm available free chlorine in wash water reduced the count by 97.8%. Adjusting the pH from 9 to 4.5-5.0 with inorganic and organic acids resulted in a 1.5- to 4.0-fold increase in microbiocidal effect. Increasing the washing time in hypochlorite solution from 5 to 30 minutes did not decrease microbial levels further,
whereas extended washing in tap water produced a reduction comparable to hypochlorite.
The addition of 100 ppm of a surfactant (Tween 80) to a hypochlorite washing solution enhanced lethality but adversely affected sensory qualities of lettuce.
Hydrogen peroxide (food grade) also can be used as a disinfectant. Concentrations of 0.5% or less are effective for inhibiting development of postharvest decay caused by a number of fungi (Bachmann and Earles, 2000). Hydrogen peroxide has a low toxicity rating and is generally recognized as having little potential for environmental damage. Ozone as a sanitizer may not be practical in developing countries because of its high cost.
Other commodity treatments
Rehydration by dipping in clean water or water added with chlorine can be done to restore the fresh and turgid appearance of some leafy vegetables such as Chinese kale, kangkong, and mustards.
Common cabbage and Chinese cabbage can be applied with antibacterial treatments to control bacterial soft rot. The use of saturated alum solution and lime paste has been found very effective in controlling soft rot in common cabbage (Bautista and Acedo, 1987). Alum has a two-fold function to control bacterial soft rot: as an antimicrobial agent by direct kill, and as a moisture-withdrawing substance that deprives the bacterial pathogens of water. However, alum is phytotoxic and causes black spotting on affected leaf tissues, thus care must be taken to apply alum only on the butt end of cabbage. On the other hand, lime is only moisture-withdrawing and usually cannot control soft rot if the pathogen already has entered into the tissues through wounds. The use of alum is now a commercial practice of common cabbage growers in the Philippines. Lime paste is used by commercial cabbage growers in Indonesia.
In Thailand, lime (CaCO3) paste is used in commercial packinghouses for Chinese cabbage (Kanlayanarat, 2007). The paste is applied at the butt end and allowed to dry before packing.
Other potential low-cost techniques to control bacterial soft rot in cabbages include the use of botanicals or leaf extracts from plants that are known to be edible or consumed as medicinal plants. One effective treatment is the use of guava leaf extract, the effect of which in common cabbage and Chinese cabbage was comparable to alum treatment (Acedo et al., 1999, 2003;
Acedo and Capuno, 2004). Cabbages treated with guava leaf extract were free of bacterial soft rot, while untreated heads all developed soft rot symptoms after pathogen inoculation, which resulted in trimming losses of more than 30%. Other plants known to be edible or
Acedo and Capuno, 2004). Cabbages treated with guava leaf extract were free of bacterial soft rot, while untreated heads all developed soft rot symptoms after pathogen inoculation, which resulted in trimming losses of more than 30%. Other plants known to be edible or