Post-harvest handling and processing of fresh vegetables 

N. C. Shahi  

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India is the second largest producer of both fruits and vegetables. Fruits and vegetables are the reservoir of vital nutrients. Being highly perishable, 20-40% of the total production of fruits and vegetables goes waste from the time of harvesting till they reach the consumers. It is, therefore, necessary to make them available for consumption throughout the year in processed or preserved form and to save the sizeable amount of losses. Fruits and vegetables have great potential for value addition and diversification to give boost to food industry, create employment opportunities and give better returns to the farmers.

Introduction   

World production of vegetables amounted to 486 million tonnes, while that of fruits reached 392 million tonnes. India is the second major producer of fruits and vegetables and ranks next to Brazil and China respectively, in the world. It contributes to 10 percent and 14 percent respectively of world's fruits and vegetables production. The country actually produces respectively about 50 million tonnes and 85 million tonnes of fruits and vegetables per year, but just about 2% of this goes for processing, while over 25% is spoiled due to improper handling and storage, and the rest is consumed in fresh form. India produces 41% of world's mangoes, 23% bananas, 36 % green peas and 10% onion. Mangoes, grapes, mushrooms are exported to United Kingdom, Middle East, Singapore and Hong Kong. Onion potatoes and green vegetables like okra, bitter gourd, green chillies also have good export potential. Reducing the post harvest losses is very important; ensuring that sufficient food, both in quantity and in quality is available to every inhabitant in our planet. It is projected that, the world population will grow to 8.3 billion inhabitants in 2025. Reduction of post-harvest losses also reduces the cost of production, trade and distribution, lowers the price for the consumer and increases the farmer's income.

Harvesting and pre-processing

When the vegetable are maturing in the field they change their form day to day. There is a time when the vegetable will be at peak quality from the stand-point of colour, texture and flavour. After the vegetable is harvested it may quickly pass beyond the peak quality condition. Production of heat is also the reason when large stockpiles of vegetables are transported or held prior to processing. Since the heat further deteriorates the vegetables and speeds micro-organism growth, the harvested vegetables must be cooled if not processed immediately. However, cooling only slows down the rate of deterioration, it does not prevent it. Each vegetable has its optimum cold storage temperature which may be between about 0-10⁰ C (32-50⁰ F). The continual loss of water by harvested vegetables due to transpiration, respiration and physical drying of cut surfaces result in wilting of leafy vegetables, loss of plumpness of fleshy vegetables and loss of their weight.

A 10°C increase in temperature will cause fruit and vegetables to deteriorate twice as fast, as well as encouraging disease organisms to grow twice as fast as well. This is why it is important to remove field heat from the produce as quickly as possible after harvest. Important as it regulates the quantity of refrigerant entering the evaporator coils. In many processing plants it is common practice to process vegetables immediately as they are received from the field. Cooling of vegetable in the field is common practice in some areas. Liquid nitrogen-cooled trucks may next provide transportation of fresh produce to the processing plant or directly to market.

Fresh Vegetable Storage

            In long term storage of fresh fruit and vegetable, it is necessary to reduce respiration and transpiration rate to a minimum possible and this can be achieved by:

• a) Maintaining temperature near to 0^O C,
• b) Increasing relative humidity of air to 85-95 % and
• c) Modifying CO₂ percentage in air.

The ideal temperature often depends on the geographic origin of the product.

Table 1:    Products and their recommended storage temperature

Recommended storage temperature a selection of fruits and vegetables
1 - 4 °C	5 - 9 °C	> 10 °C
Apple	Avocado (temperate origin)	Avocado (sub-tropical)
Peach and Plum	Egg plant	Banana
Mushroom	Potato	Pumpkin
Carnation	Protea	Ginger

 

Washing

                      Washing is used not only to remove field soil and surface micro-organism but also to remove fungicides, insecticide and other pesticides from vegetables. Washing water contains detergents or other sanitizers that can essentially completely remove these residues.

Sorting and Grading

This step covers two separate operations to

• Remove of non-standard vegetables and possible foreign bodies remaining after washing and
• Quality grading based on variety, dimensional organoleptical and maturity stage criterion.

 

Skin Removal/ Peeling

Some vegetables require skin removal. This can be done in various ways.

•a)      Mechanical

Various types of equipment which depend upon the characteristics of the fruit and vegetables are used.

• A machine with abrasion device (potatoes, root vegetables);
• Equipment with knives (apples, pears, potatoes etc);
• Equipment with rotating sieve drums (root vegetables).

•b)      Chemical

Skin can also be softened from the underlying tissues by submerging vegetables in hot alkali solution. Lye may be used at a concentration of about 0.5-3 %, at about 93^OC for a short time period (0.5-3 min).

c)   Thermal

      Vegetables with thick skin as beets, potatoes, carrots and sweet potatoes may be peeled with steam under pressure (about 10 atm) as they pass through cylindrical vessels. When the pressure is suddenly released, steam under the skin expends and causes the skin to puff and crack. The skins are then washed away with jets of water at high pressure (up to 12 atm.).  

Manual peeling is used when the other methods are impossible or sometimes as a completion of the other three ways.

 

Blanching

            The special heat treatment to inactivate enzymes is known as blanching. Small vegetables may be adequately blanched in boiling water in a minute or two, large vegetables may require several minutes (Table 3).

            In order to reduce losses of hydro soluble substances (mineral salt, vitamins, sugars etc.) occurring during water blanching, several method have been developed :

• Temperature setting at 85-95^O C instead of 100⁰ C;
• Blanching time has to be just sufficient to inactivate enzymes catalase and peroxidase;

Table 2: Blanching parameters for some vegetables

Vegetables	Temperature, OC	Time, min.
Peas Green beans Cauliflower Carrots Pappers	85-90 90-95 Boiling 90 90	2-7 2-5 2 3-5 3

.

            Cooling of vegetables after water blanching or steam is performed in order to avoid excessive softening of the tissues and has to follow immediately after these operations.    

 Role of ethylene in the postharvest life

 The important role of ethylene as a plant growth regulator has only been established over the last 50 years. The most commonly know use of ethylene is to trigger ripening is some crops, such as bananas and avocados. The application of ethylene at a controlled rate means that these products can be presented to the customer as "ready to eat". The time and temperature of treatment also influences the rate of ripening with fruit being ripened at temperatures between 15 to 21°C and relative humidity of 85 - 90 %. Ethephon a chemical producing ethylene can promote several benefits such as fruit thinning (apples, cherries), fruit loosening prior to harvest (nuts), colour development (apples), degreening (citrus), flower induction (pineapples).

Vegetable drying / dehydration

In the process of drying or dehydration sufficient moisture is removed by the application of artificial heat under controlled conditions of temperature, humidity and air flow to protect the product from spoilage.

All dried vegetables are prepared by soaking and cooking. Good quality dried products, after cooking and if properly treated should be similar to cooked fresh produce. The following soaking/cooking methods are recommended:

i)  Quick method:       Cold water is added to the dried product ten times the weight of dry product.

ii)  Slow method :        Cold water is added to the dry vegetable and is left to soak for 1 to 2 hours before cooking. The product is then cooked in the same soaking water and actual cooking time is shorter than that for the quick method.

               

Packaging

The packaging of fruits and vegetables should protect them from injury and water loss, and be convenient for handling and marketing. The cost of the packaging is important, including whether the container can be recycled or reused.

Transport

Fresh horticultural products should be cooled after harvest and during transport. It is very important that the cold chain is continuous. Trucks for road transport may be refrigerated, or may sometimes just be insulated. It is difficult to control the temperature of air shipments, but produce shipped by air should be covered and precooled.

The use of controlled atmospheric conditions, as a way of reducing the use of chemical preservatives and pesticides, has great potential for the reduction of postharvest losses and the maintenance of nutritive value and organoleptic characteristics.

Vegetable powder Processing

This technology has been used mainly for potatoes (flour and flakes), carrots (powder) and red tomatoes (powder). In order to obtain these finished products there are two processes:

• a) Drying of vegetables down to a final water content below 4 % followed by grinding , sieving and packing of products;
• b) Vegetables are transformed by boiling and sieving in to purees which are then dried on heated surfaces (under vacuum preferably) or by spraying in hot air.

Packaging and storage of dried and powdered vegetables

During storage, the moisture content of dried vegetables is not constant because of relative humidity of air in storage rooms. The air relative humidity of the storage room should be below 78 %. Water vapour proof package should be used.

Table 3:     Moisture and density for some dehydrated vegetables

Bean (green)	5	1.6
Beet	5	1.6-1.9
Cabbage	4	0.7-0.9
Carrots	5	3-5
Onion	4	0.4-0.6
Pea (green)	5	3.4

 

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