Family Alliance For Development And Cooperation, Tanzania

PRODUCT/PROCESS DESCRIPTION

Drying is a process that removes most of the water from a food to give it a long shelf life and/or to increase the convenience and value of the food. Different categories of dried foods can be identified e.g. high volume-low value crops such as cereals, and low volume high value foods such as fruits.

FRUITS AND VEGETABLES:

Process Notes

Harvest

This should be done when the fruit is mature. Harvesting should be done as carefully as possible to prevent spoilage, bruising and loss of quality during transport.

Wash

Use clean water to remove dirt e.g. soil, dust, etc. It is recommended that fruit is stored in clean or tidy environments.

Sort / Grade

This is necessary to produce a uniform dried product. Foods are sorted for colour, size, shape and maturity. Uniform size and maturity influence the drying time. Sorting is done by hand.

Peel

Peeling increases the rate of drying (peel prevents moisture leaving the food). This is done by hand using sharp stainless steel knives.

Cut / Slice

Cut to reduce the size of pieces and allow faster drying. This is done by hand.

The Drying Process:

• The rate at which food dries depends mostly on the properties of the drying air.
  
  Relative Humidity (RH)
  Temperature and
  Air speed
  

Temperature and relative humidity:

Air contains water vapour and the amount of water carried by the air varies with its temperature. At higher temperatures, the air can carry more water vapour than at low temperatures. So to dry foods, it is necessary to raise the air temperature and hence obtain a lower RH.

Air Speed:

Faster moving air carries away moisture more quickly from the food and the food therefore dries faster than in slow moving or stationary air. In the solar dryers, this is done by natural convection currents.

Scale of Operation of Solar Dryers:

The smaller volume of low/high value products means that smaller dryers can be used (compared for example to cereal crop dryers); and the higher value of the products may justify therefore, the additional expense of the dryer.

The scale of operation is determined by the demand for the product and the drying rate of that particular product. In general, 2-3 Kg of food will need a drying area of 1 m. This allows for calculation of the size of the dryer that is needed for a particular quantity of product.

SUN AND SOLAR DRYING

To find out whether (and when) sun/ solar drying is possible in the participants communities, the harvest times, food prices and weather patterns should be considered. Sun drying is favourable in areas where in an average year, the weather allows the food to be fully dried after harvest.

The main advantages of sun drying are the low capital and operating costs and little expertise is required. However, the increasing demand for quality dried product is calling in for the use of improved sun drying technologies of which solar drying is one.

Main problems associated with Traditional sun drying:

• · Theft or attack by insects, rodents and birds
• Contamination with dust, dirt, droppings, etc.
• No or limited protection from rain
• Slow or intermittent drying which increases the likelihood of spoilage
• Relatively low quality products and variable quality due to over / under drying
• Relatively high final moisture content and spoilage during storage
• Large areas of land needed for the shallow layers of food
• Laborious because the crop must turned, moved if it rains, etc.
• Animals must be kept away while drying takes place otherwise they eat the product
• Direct exposure to sunlight reduces the quality ( colour and vitamin contents) of some foods.
  

The quality of sun dried foods can be improved by the following methods:-

• Sort, grade and clean the food
• Reduce the size of the pieces
• Cover to prevent insect / vermin damage
• Shade if necessary to protect colour or flavour of the food
• Protect from animals or birds
• Cover for rain or dew at night.

Solar drying has advantages over sun drying as follows :

Solar dryers operate by raising the temperature of the air between 10 and 30° C above normal air temperatures. This makes the air to move through the dryer and also reduces humidity. The higher temperatures created in the dryer deters insects and the faster drying rate reduces the risk of spoilage by micro-organisms. The higher drying rate also gives a higher through output of food and hence a smaller drying area.

Food is enclosed in the dryer and therefore protected from dust, insects, birds and animals. The dryers are water proof and the food does not therefore need to be moved when it rains or at night to protect from dew.

Dryers can be constructed from locally available materials at relatively low cost except for the solar plastic that has to be imported.

QUALITY CONTROL CHECKS

Foods should be harvested at the correct stage of maturity. If they are overripe, they are easily damaged and may be difficult to dry. If they are under ripe, they may present a poor flavour, colour and appearance.

All foods should be prepared as soon as possible after harvest to minimize spoilage before drying begins. This is particularly so for wet low acid foods such as vegetables which can support the growth of food poisoning bacteria.

Strict hygiene is necessary at all stages to reduce the risk of food spoilage and food poisoning. This must include proper cleaning of tools, equipment and working places, washing hands and removal of food wastes ( and peelings). Note that, any bacteria or moulds that contaminate the food before drying are likely to remain on the dried foods because the temperature in the dryer may not high enough to kill them

PACKAGING AND STORAGE

The factors that cause spoilage of dried foods are:-

• Moisture
• Heat
• Air
• Odours
• Light
• Crushing or bruising
• Micro-organisms
• Insects and rodents
• Shelf life

The need for packaging of dried foods depend on the nature of the food. Most foods are hygroscopic and these absorb much water from the atmosphere during storage. The higher water content allows moulds and bacteria to grow on the food and as a result it spoils it.

The need for packaging is therefore very important especially for these hygroscopic foods. Those foods that are hygroscopic need specialised packaging in ceramic or glass jars, tins or plastic films to exclude moisture.

In practice, polythene is often used but it is not the best material. But if the correct grade and gauge size is used, the risk of spoilage is reduced. Unsuitable polythene will give off flavours to the food and allows air to penetrate. All types of packaging should be properly sealed to exclude air, moisture and insects. For example, plastic is heat sealed and not folded or stapled.

Advantages and limitations of drying:

• Long shelf life
• Reduced weight and bulk for transport and distribution
• Greater convenience - high local demand for some products
• Preparation of raw materials for further processing ( e.g. for bakeries, etc)

Limitations of dried foods

• Loss of quality (vitamins, colour, flavour and texture are all lower than in fresh foods)
• Some products are preferred fresh (e.g. vegetables and fruits)
• Small amounts of some products are used (herbs and spices) and therefore a relatively small market.

Income generation

A judgement on the economics of the drying process will include some of the following questions:-

• Will the dryer increase the farmer's output for the same inputs
• Will the farmer find markets for extra dried foods
• Would the use of the dryer bring new opportunities for the farmer
  

Conclusion:

There is a lot more that one needs to know about food processing. This is only a very small introduction. One thing to keep in mind is that, a strict code of conduct needs to be adhered to when processing food, to avoid contamination and to make it suitable for human consumption.

Contact us for more information.