Design and Maintenance of Green House 3(2+1)

Lesson 29 Post Production Handling of Greenhouse Production

29.2 POSTHARVEST HANDLING

29.2.1 Postharvest Handling of Fruits and Vegetables

The ideal management of vegetables starts with proper handling at harvest, an operation that should be done preferably in the morning, when the ambient temperature is lower. The harvested product must be protected from the sun, and whenever possible, to proceed   immediately   to    their pre-cooling (fast cooling before processing) if such facilities are available. The   most   popular pre-cooling procedure is by forced air, which circulates air at low temperature (Tompson, 2003). Many  vegetables are  sensitive to  cooling  (chilling), that   is  they   get  damaged  if exposed to low  temperature (but  above  the freezing  point)   for   a   certain  minimum period of time. Depending on their origin, tropical and   subtropical fruits have   their threshold for chilling at 10–15°C, whereas the threshold is lower for fruits that originate in temperate areas.

At   7°C   chilling damage occurs in cucumber, aubergine, pepper, melon or ripened tomatoes, whereas for green tomatoes damage occurs at higher temperatures (Wang, 2003).  Chilling damage can be very relevant if the low temperatures last a long time. If the duration of low temperatures is short, normal metabolic capacity in these plants is limited or cancelled, affecting their shelf life, although the damage is only evident when the product goes back to normal temperatures.

In  general, tomatoes can  be  conserved well  with a RH of 90%,  but the optimal thermal  regime varies depending on  the  ripening   stage,   the   recommendation  being   for less   than 15°C  for  green   and   early   pink- colour stage  tomatoes, and  less  than  10°C for late pink-colour stage and  ripened fruits (Chaux and   Foury, 1994a, b).  The storage temperature allows for regulating the ripening speed; for instance, for pink-colour stage tomatoes, a   temperature of   10°C   allows them to ripen in 10–20   days,   whereas at 20°C ripening is shortened to 8–10   days (Chaux and  Foury, 1994b).

The optimal storage temperature of greenhouse cucumbers is from 12 to 13°C, because lower temperatures cause the fruits to wilt and higher temperatures accelerate their respiration and dehydration (Chaux and Foury, 1994b). Covering them in a plastic film extends   their shelf life.

The optimal storage temperature of leafy vegetables is lower than for fruit vegetables. In general, while temperatures of 0–2°C, with RH of 90–98%, are  optimal for  some  (artichokes, asparagus, broccoli, cabbage, cauliflower,  Chinese  cabbage, endives,  lettuce, carrots and  cantaloupe melons), the  fruit vegetables are  better conserved at  7–10°C, with RH  of  85–95% (Tompson and  Kader,2003).  The usual greenhouse vegetables are all very sensitive to freezing (Wang, 2003). In fresh  products, postharvest treatments with  high   temperatures  can   be  of interest to  control  pests  and   insects and fungal diseases, before their storage or long- distance  shipping.  Washing the   peppers with water at temperatures between 50 and 65°C, while simultaneously brushing them, has  proved to be efficient for the  control of postharvest diseases; the same is true  for the treatment  of  tomatoes with  hot   water  at 50°C for 2 min  (Lurie  and  Klein,  2003).

A proper environmental humidity has a notable influence on maintaining the post- harvest quality of fruits and vegetables, especially during cold storage. An inappropriate humidity can increase the incidence of fungal diseases, alter   the   organoleptic characteristics and induce the cracking or cork-like texture of the fruits and vegetables (Scharz, 1994).

Storage in a controlled atmosphere involves the modification of the normal composition of the air (78% N₂, 21% O₂ and0.03% CO2), in order to have less than 8% of O₂  and  more  than 1% of CO₂, while keeping low   temperature  and  adequate humidity according  to the  product  being    stored, which decreases the  respiration rate  of the product and  the  production of ethylene (Kader, 2003).

An optimum atmospheric composition delays: (i) the loss of chlorophyll (green  colour);  (ii) the  biosynthesis of carotenoids (yellow and  orange colours) and  anthocyanins (red  and  blue  colours); and  (iii) the biosynthesis and  oxidation of phenolic compounds  (brown colour) (Kader, 2003). Low levels of O₂  and/or high  concentrations of CO₂  in the  air affect  the  flavour, decreasing  the  loss  of  acidity, the  conversion  of starch into   sugar,   the   inter-conversions  of sugars and  the biosynthesis of volatiles that affect  the  flavour and  aromas, resulting in an  improvement of the  nutritional flavour, as  the   ascorbic  acid   and   other  vitamins remain in the fruits (Kader, 2003).

In  addition to  a  delay in  senescence, storage under a controlled atmosphere decreases the  sensitivity to ethylene (if the O₂  level  is below 8%,  and  if the CO₂  level  is above   1%),   and   can   be  useful to  control pests and  diseases.

Disadvantages of storage in a controlled atmosphere include: (i) irregular ripening   in   some cases; (ii) the modification of the   organoleptic characteristics (as a result of anaerobic respiration); and (iii) the increase, sometimes, of physiological disorders and chilling damage (Mir and Beaudry, 2003).

When the aim is to accelerate the ripening of a product, such as tomato, ripening chambers may be used, adding ethylene and keeping a proper temperature. These chambers are often used in ripening citrus (degreening).

MAP (modified atmosphere packaging) of fresh vegetables allows for isolating fresh products, with active respiration, in plastic film packages to modify the O₂ and CO₂ levels in the   atmosphere inside the package. At the time of packaging, levels of 2–3% of O₂ and 5% of CO₂ are usual (Kader et al., 1989).

In addition, MAP decreases the dehydration of the product and insulates it from the external environment, limiting its exposure to pathogens and contaminants, which contributes to maintaining its quality. It is often used in fresh cut products.

The  modification of the  atmosphere in MAP   requires  active  respiration  of   the plant tissues, on  one  side,   and  the  existence  of a barrier that  prevents the gas exchange, on  the  other side. The  creation of such barriers is achieved by using plastic  films   of  a  characteristic  permeability (that  controls the  entrance and  exit  of O₂ and  CO₂ into/from  the   package)  and   by means  of  micro-perforated  plastic films (Mir and  Beaudry, 2003).

In MAP, the control of temperature is essential. The decrease of water losses prevents the product   becoming desiccated but in some cases may increase how much it wilts. Not all products are suitable for MAP.

A  variant of  MAP  is  partial vacuum packaging, which  keeps the  normal composition of the  atmosphere, but  at a lower than normal pressure in  impermeable packages, at low  temperature (Gorris  et al., 1994). This system stabilizes the quality, decreasing the   metabolic activity of the products avoiding the increase of undesired microorganisms.

29.2.2 Post Harvest Handling of Cut Flowers

Once harvested, there are a series of steps or tasks done to prepare the flowers for market. These are collectively called handling. These handling steps include;

Not all of these steps are done to all flowers, and whether they are used or not depends on the market the flowers are going to be sold to. Where and how the steps are done depends on the market and the facilities of the operation. Flowers can have all the handling steps performed in the field, only some done in the field with the rest in the packing shed, or have all handling steps done in the packing shed.

Field handling usually is limited to leaf removal, grading, bunching, hydrating, and packing with immediate transport to market or cold storage for brief holding. Flowers for local retail markets often are packed this way since they are marketed immediately after harvest. Flowers also can have these steps performed in the field and then be transported to a packing shed where recutting, special treatments, precooling and dry packing can be performed.

All the handling steps can be done in a packing shed, too. It often makes for a better flow of activities if they are all done in the same place. Some of the steps can only be feasibly done in the packing shed, such as special treatments, precooling, cold storage and recutting. These extra steps usually are done for flowers going to wholesale markets.

The packing shed may be an ultra-modern air conditioned building or an open air covered porch. The handling space should

• be shaded or covered to keep temperatures lower and prevent direct sunlight on the flowers.

• be well lit so you can see well when grading the flowers.

• have a clean water source for preparing harvest, treatment and holding solutions, and for use in cleaning the area.

• have ample space so all handling activities can be performed smoothly, such that workers are not crossing over each other.

• have a cold storage or at least a cooler, shaded place to store the flowers until they are ready for market.

• have a place to prepare for harvest activities.

Although not previously listed, the first step after cutting the stem, whether you are going to handle them in the field or in the packing shed, should be to place them in water or a harvest solution. This solution may be acidified (pH 3.5), tepid water, citric acid works well, or a floral preservative. The harvest containers should be clean and disinfected after each use. Flowers should never be laid on the bare ground. After the harvest container is full of flowers, place them in a cool place until they can be handled or taken to market. The cool place can be a shady area in the field or a refrigerated cold storage. Do not over fill the containers. This will bruise your flowers and cause some to tangle with each other.

Leaves should be stripped from the stem. If the flowers are being field handled this can be done before they are placed in the harvest containers or before they are bunched into marketable bouquets. Usually, leaves are stripped from the bottom one- third of the stem, or at least the ones that would be in any holding solution.

Grading starts with deciding which flowers to harvest. Only marketable flowers should be harvested. Marketable flowers are free of blemishes, including both leaves and petals. The flowers can be grouped or graded by stem length if there are differences and also by developmental stage. More mature ones should be sold as soon as possible, while others can be held in cold storage for later sales.

How the flowers are bunched and packaged depends on the market you are using. If you are selling in a local retail market you have a lot of flexibility, but your customers will let you know what sells the best. Mixed bunches and single type bunches are both popular. Larger flowers such as lilies, gladiolus and sunflowers often are sold as single stems. Sleeving or wrapping the bunches helps prevent the different bunches and flowers from becoming tangled. Columbine, larkspur, delphinium, baby primrose, forget-me-nots and buddleia are flowers that should be wrapped or sleeved prior to marketing to prevent tangling.

Wholesale markets have a set of guidelines for the methods of bunching and packaging flowers. Most are bunched by 10‘s or 5‘s . Some, like roses and carnations, are bunched by 25‘s. Lilies-of-the- Valley are bunched in 25‘s and Sweet Violets are bunched in 100‘s with a collar of leaves underneath the flowers. Large, expensive to grow flowers can be sold by single stems. As stated before, some should be wrapped to prevent tangling. Most are boxed and shipped dry.

Proper pre-shipping handling is important in order to get flowers to the market in good shape. The flowers should be well hydrated but not wet when packed. Most spike flowers like snapdragons and gladiolus need to be packed upright to prevent the tips from curving. Special boxes or hampers are made for these types of flowers.

Once bunched, flowers should be hydrated, placed in water for a while before they are packed dry. The hydrating step should include a step where, after the flowers are bunched, the stems are recut under water to eliminate any air bubbles in the xylem that can block the uptake of water. These air bubbles can occur when the flowers were harvested. Once recut, the flower can be placed in a general holding solution used to hydrate the flowers or receive a special treatment such as silver thiosulfate. Flowers usually are not packed dry into boxes in the field but are in the packing shed for distant wholesale markets. When flowers are packed into boxes, the bunches are sleeved or wrapped and then packed tightly so the bunches do not move or vibrate in transit (causes bruising). The standard flower box is 12×12×48 inches. There are smaller sizes, too, called half or quarter boxes that are 6×12×48 inches and 6×6×48 inches, respectively.

Precooling is a step that rapidly brings the temperature of the flowers down from the field temperature to a proper storage temperature. A low temperature slows the respiration rate of the flowers which in turn helps them last longer.

Forced-air cooling is the best method for flowers cool air is actively forced with fans through the bunched flower. This can be done when the flowers are in a bucket or when they are packed dry into boxes. The precooling of flowers is a very important step for individuals selling to a large wholesale market, distant markets and if their crop is to be stored for a long time such as peonies. Individuals who sell at a local retail market usually do not need to worry about this step since their flowers will be in the customer’s home the day they are picked.

Cold storage is recommended for all flowers that will not be in the market immediately and any flowers sold wholesale. As stated before, low temperatures slow the respiration rate of the flowers and prolong the vase life of the flowers. In general, temperatures should be 32 to 40°F and have a relative humidity of 85 to 90 percent, for most flowers. Flowers should never be stored with fruits and vegetables. Some fruits and vegetables produce ethylene that can dramatically shorten the life of the flowers. Once flowers are bunched into marketable units they should be placed in cold storage. As a new grower using local retail markets, a refrigerated cold storage may not be available or affordable. Since most of their flowers will be sold within hours a cool place such as an air conditioned room, cellar or basement could be used.

Flower storage life and vase life are considered to be two different things. The customer wants to know the vase life i.e. how long would the flowers last in his home while the grower needs to know both: to determine how long flowers can be kept in cold storage and to be able to tell customers how long the flowers will last. If flowers have to be stored before marketing, a cool place (preferably a refrigerated cold storage, especially for flowers) should be used.

There are many flowers that are not commonly found in the wholesale market because they do not store well, ship well or last long. These should only be used for local markets. These include foxglove, garden phlox, lupine, clarkia, stevia, common stocks, candytuft, cornflower, feverfew, blue lace flower, English daisy, calendula, pot marigold, sweet violets and gaillardia.

REFERENCES

1. Nicolas Castilla, 2013. “Greenhouse Technology and Management.” Ediciones Mundi-Prensa, Madrid (Spain) and Mexico.PP. 243-245

2. Karen L.B., 1997. “Post -harvest handling of fresh cut flowers and plant material” Cooperative extension service, Manhattan, Kansas.