Owing to high moisture content and tender nature vegetable pose characteristics post-harvest problem. High moisture content makes it difficult and expensive to conserve vegetables as dry products. They bruise easily and are metabolically active then the durables. The characteristics significantly limit the storage life of vegetables and postharvest life may therefore, only a few days.
A loss estimation is a tedious and in perishable the estimation are not as precise as they are in the durable commodities. The losses are highly locality specific and level of loss acceptable in rural market differs greatly from that acceptable in commercial post-harvest sector. Nevertheless, expert with long experience in the field have estimated 20 to 30 % losses in fruits and vegetables under Indian conditions. The loss estimations for onion (16-30%) , Tomato ( 20-25%) , Cabbage ( 37%), Cauliflower ( 49%) and Lettuce (62%) in developing countries have been reported.
A) Primary Causes of Losses:
i) Mechanical Losses:
Besides causing damage, bruising and cracking make the vegetable more prone to attack by organisms and significantly increase of water loss and gaseous exchange. Many a times, the mechanical received by the vegetable due to the pressure thrust during transportation, though not visible, leads to rupture of inner tissues and cells
And such produce is degraded faster during the natural senescence process. Processing operations such as spillage, abrasion, excessive polishing, peeling or trimming add to the loss of the commodity. Puncturing of the containers and defective seals amount to mechanical spoilage.
ii) Physio- biochemical Losses:
In fresh vegetables, transpiration and respiration and sprouting of tuber and bulb vegetables lead to direct food loss. The loss is accounted towards senescence process and represents major component of post harvest losses. The process is unavoidable. Post –harvest rooting in tuber vegetables, seed germination greening of potatoes leading to production of harmful compounds, and toughening and spongyness in green beans, sweet corn, carrot, radish that represents physiological loss through are not significant components of post harvest loss, down the quality of the vegetables. The undesirable chemical reactions between compounds presents in food such as Maillard reaction, fat autoxidation energy catalyzed reactions lead to significant losses. Contamination of vegetables with harmful substances such as pesticide or obnoxious substances makes the whole, lot inconsumable. Chemical reaction between canned vegetables and the container lead to spoilage of the pack.
iii) Microbial Losses:
Like any other food, vegetables are prone to microbial spoilage caused by fungi, bacteria, yeasts and moulds. A significant portion of losses of vegetables during post harvest is attributing to disease caused by fungi and bacteria. Succulent natures of the vegetables make them easily invadable by the organism. Besides attacking fresh vegetables, the organism also caused damaged to canned and processed products. Many serious post-harvest diseases of fresh vegetables occur rapidly and cause extensive breakdown of the commodity sometimes spoilage the entire package. It is estimated that 36% of the vegetable decay is caused by soft rot bacteria.
Obviously, the source of infection is soil in the field, water used for cleaning and surface contact with equipment and storage environment. The most common pathogens causing rots in vegetables are fungi such as Alternaria , Botrytis, Diplodia, Monilinia, Phomopsis, Pencillium , Rhizphus, and Fusarium and Bacteria, Erwinia, Ceratocystis , and Pseudomonas. While, most of the pathogens can invade only the damaged tissue, a few such as Colletotrichum are able to penetrate the skin of healthy tissue. Initially, only a few pathogens can invade and breakdown the tissue followed by attack of several weak pathogens. High temperature and relative humidity favour the development of post harvest decay organisms. More acidic tissue is generally attacked by bacteria , e.g. bacterial soft rot of potato by Erwinia spp and dry rot by Fusarium spp. Black rot of Sweet Potato by Ceratocystis Fimbriata, Water Soft rot of Carrot by Sclerotina Sclerotioorum , and leaf vegetables soft rot by Erwina carotovora and dry rot by Fusarium spp.
iv) Physical Looses:
Post-harvest technological requirement like refrigerated storage, temperature and relative humidity, composition and proportion of gases in controlled atmosphere storage, type of wrapper or packaging and others changes with the nature of the vegetables to be handled. Chilling injury by low but non-freezing temperature is observed common with tropical and sub tropical vegetables. The fruit vegetables such as beans, cucumber, okra, pepper and tomato are affected commonly. The symptoms of chilling injury may not be evident while the vegetable is held at chilling temperature but becomes noticeable only after the vegetables is transferred to room temperature (220C). Each vegetable has specific heat requirement during processing, improper cold undesirable gaseous composition of controlled atmosphere storage lead to physical damage to tissue breakdown i.e. Storage disorders.
v) Psychological Losses:
Human aversion of refusal to eat a vegetable for religious or personal reason may lead to the loss of the commodity at the consumer’s level.
B) Secondary Causes of Losses:
Inadequate harvesting transportation, storage and marketing and legislation lead to condition favourable for primary causes of losses. But inadequate harvesting facilitates and rough handling during harvesting result in bruising and increase possibilities of contact of the produce with the soil that leads to contamination with organisms. A prolonged time taken for harvesting and grading infield leaves the produce with field heat for longer time which causes faster senescence subsequently. Besides use of improper machinery and equipment in mechanical harvesting cause serious losses.
In Potato, for example many tubers are pushed around the side of the digger blade and are left in the field. These potatoes must be considered as lost. In addition to the produce left in the field, a substantial quantity is lost during harvesting and preparation of market. Johnson et al. ( 1995) demonstrated that machines equipped with standard blade resulted in 15% damage, whereas, a harvest equipped with vibrating blade had 11.7 % damage of potato tubers.
Mechanical harvesting of tomato has been reported to result in more cracking 33% than hand picking 10%.
Harvesting of vegetables during or immediately after rains creates condition favourable for decay organism. Harvesting during hotter part of the day leaves higher field heat in the produce that result in faster senescence , shrivelling and wilting of the vegetables as compared to those harvested in the early morning or late afternoon.