Cover Image
Кітапты жабуTraining Programme for Women Entrepreneurs in the Food-processing Industry - Volume II (UNIDO, 1985, 286 p.)
Папканы жабуChapter 1 Dried Food
Құжатты қарау(introduction...)
Құжатты қарау1.1 Fruits and Vegetables
Құжатты қарау1.2 Meat and Fish
Құжатты қарау1.3 Herbs and Spices

1.3 Herbs and Spices




Harvest when mature: for black pepper when the oldest berries are just turning yellow or red and are hard to the touch; cardamom when the ripe capsule has black seeds, chillie when it starts turning red.


Remove dust, dirt and stones by hand using a winnowing basket, wash in clean water which is changed frequently (e.g. once every hour). Cardamom capsules are dipped in a solution of 4% sodium carbonate for 5-10 minutes and allowed to drain before drying.


Black pepper berries are dried in 3cm layers for 4-5 days to 8% moisture. Cardamom is dried to 8% moisture in the dark to retain the green colour of the capsules. A high humidity is maintained in the drier for 2 hours to inactivate enzymes that break down the chlorophyll, and then reduced. Chillie is dried to 10% moisture. Care is needed to prevent overdrying. Drying temperature is controlled below 50°C for pepper and cardamom and below 60°C for chillie to prevent loss of flavour components.


Chillie and cardamom are graded by colour and size; deep green colour and large capsules have higher grade for cardamom. For chillie, bright red colour has higher grade. Pepper is graded by size, colour and relative density. Machines are available to grade pepper by size or relative density but a trained person with a winnowing basket is preferred for small scale production. The optimal storage conditions are low temperature, low humidity and freedom from pests.



The rate at which food dries depends mostly on the properties of the drying air. These are:

1) relative humidity (RH),
2) temperature and
3) 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 lower temperatures, so to dry foods it is necessary to raise the air temperature and hence obtain a low 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. The air speed is increased by fans or by heating the air.

During initial stages of drying, water is lost from the surface of the food and the drying rate is controlled by all three of the above factors. In later stages moisture moves from inside the food to the surface. Here the rate is mostly controlled by the temperature of the food. The air speed and relative humidity have a much smaller effect.

NB: Not all foods need a high rate of drying. Some, for example, fruits and meat undergo 'case-hardening', if the rate of drying is too high. When this happens a layer of sugar and minerals forms on the surface of the food and seals it. This forms a barrier which prevents further water from leaving the food. So the food has a tough skin and is wet inside. Further drying is not possible. To overcome this the rate of drying is reduced either by reducing the temperature or the speed of the air (for example by drying in the shade).

In both initial and later drying stages the size of the pieces is important. In the first stage smaller pieces have a greater surface area for water to evaporate from. In the second stage the smaller the pieces the less distance the moisture must move to reach the surface of the food (Fig. 5).

Scale of Operation of Driers

The smaller volume of low volume/high value products means that smaller driers can be used (compared for example to cereal crop driers) and the higher value of the products may justify the additional expense of the drier and fuel. The scale of operation is determined by the demand for the product and the drying rate for a particular food in a particular drier. In general 5-10 kg food will need a drying area of 1 m2. This will allow calculation of the size of the drier that is needed for a given weight of food per day when the above information is known. In general higher drying rates and better control over the drying conditions can be found with artificial driers. However there are advantages to sun and solar drying which are described below.

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 can be compared in that region using graphs (for example Fig. 6). Sun drying is traditionally done only in regions 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 (because drying equipment and fuel are not used) and little expertise is required. The main problems with this method are as follows:

· theft or attack by insects, rodents and birds
· contamination with dust, dirt, dropping etc.
· no 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 be turned, moved if it rains, and
· animals must be kept away while drying takes place
· direct exposure to sunlight reduces the quality (colour and vitamin content) 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 pieces
· Cover to prevent insect damage
· Shade if necessary to protect colour or flavour of food
· Protect from animals
· Cover for rain or dew at night

Solar drying has advantages over sun drying as follows:

They operate by raising the temperature of the air to between 10 and 30°C above normal air temperature. This makes the air move through the drier (Fig. 7) and also reduces its humidity. The higher temperature deters insects and the faster drying rate reduces the risk of spoilage by microorganisms. These factors improve the quality of the product. The higher drying rate also gives a higher throughput of food and hence a smaller drying area.

Food is enclosed in the drier and therefore protected from dust, insects, birds and animals. The driers are water-proof and the food does not therefore need to be moved when it rains, or at night to protect from dew. Finally, driers can be constructed from locally available materials at relatively low cost.

Situations where solar driers may be useful:

· where fuel/energy is expensive or erratic
· to supplement existing artificial driers and/or reduce fuel costs
· where land for sun drying is in short supply or expensive
· where sunshine is plentiful but humidity is high.

Situations where solar drying is not likely to be useful:

· where the quality of sun dried foods is acceptable

· where fuel is plentiful and cheap

· where the additional costs of a solar drier are not recovered from increased value of the food and there is no incentive for producers to risk higher amounts of money in a drier when there is not a greater return.

Other disadvantages of both solar (and artificial driers include greater space and labour requirements than traditional methods (for example loading, unloading of trays). They may require labour which is normally provided by children or mothers carrying babies in traditional methods. This is therefore additional work for women.

Available Designs of Drier

Designs vary from very simple direct solar driers (for example a box covered with plastic to trap the sun's heat) to more complex indirect designs which have separate heaters and drying chambers. In indirect designs solar radiation falls onto collectors which heat up air which is used to dry the food. The most common type of collector is a bare galvanised iron plate which is painted matt black. Glass covers are best but they break easily, are heavy and expensive. Plastic often has poor stability to sunlight and weather, but is about 10% of the weight of glass and does not break. The best types of plastic are polyester and polycarbonate when these are available. Polythene is cheaper and more widely available but is less strong and less resistant to damage by light and weather. It will need replacing about every six months.

Common designs include 1) tent driers (direct) (Fig. 8), 2) cabinet driers (direct or indirect) (Fig. 9) and 3) chimney driers (indirect) (Fig. 10). Each of these types uses natural air circulation (convection) although it is possible to fit an electric or wind powered fan to increase the speed of the air.


Raw material control

Details of the quality characteristics used in grading and sorting are in the process flow diagrams.

Foods should be harvested at the correct stage of maturity. If they are overripe they are easily damaged and may be difficult to dry; underripe and they have a poor flavour, colour and appearance. The main characteristics of fruit, vegetables, meat, fish and spices are shown in Table 1.

Table 1: Quality Characteristics of Raw Materials


Characteristic to be Examined







Freedom from infection



































All food should be prepared as soon as possible after harvest to minimise spoilage before drying begins. This is particularly so for wet low acid foods such as vegetables, meat, fish etc. 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 workrooms, washing hands, and removal of food wastes. Excepting vegetables that are blanched, any bacteria or moulds that contaminate the food before drying are likely to remain on the dried food because the temperature of drying is not high enough to kill them. Preparation procedures for foods from the three categories are summarised in Table 2 and production methods are shown in the flow diagrams.

Process control

The main control points are 1) uniform sized pieces of food, 2) correct preparation procedures, 3) correct drying conditions (air temperature, humidity and speed and amount of food loaded into the drier), 4) correct drying time. These will each vary according to the nature and size of the food, the type of drier, climate at different times of the year and the type(s) of preparation procedures used.

Table 2: Preparation procedures


cleaning skinning




blanching dioxide






































Product control

Table 3


Blanching time (min) using



Leafy vegetables, sliced beans



Squashes, cabbage















If products are dried correctly they should have an acceptable appearance, colour, texture, taste and microbiological quality. This is retained by adequate packaging and storage conditions for the expected shelf life. The main reason for loss of quality during storage is moisture pickup and this can be assessed by simple tests for dryness as follows:

FRUIT AND MEATS: A handful of fruit or meat when compressed should neither stick together nor leave any trace of moisture in the hand. It should feel leathery and flexible. Meat may be further dried to the point where it feels stiff.

VEGETABLES: Should feel flexible-leathery to brittle and generally drier than fruits. Pieces should fall apart after being squeezed together.

FISH: Should retain no imprint when squeezed between thumb and forefinger. It will feel either leathery or brittle.

HERBS AND SPICES: Crumble easily when rubbed together.

Other changes to selected foods during drying and storage are summarised in Table 4 together with summaries of the causes and correctiver actions.

Table 4 Quality Control Checks


Example of food




Fruits and vegetables

Enzyme activity

Blanch for the correct time and temperature, reduce the size of pieces

Loss of flavour

All foods

Temperature too high, time of drying too long, poor packaging

Lower the temperature, use gas barrier packaging

Loss of colour

Carrot, pepper

Enzyme activity air and light

Blanch, reduce the drying time, store away from sunlight, use a light-proof package or use sulphur dioxide treatment

Case hardening

Fish, fruit

Drying rate too high

Reduce the drying rate, make the pieces smaller

Slow rehydration

Any food

Pieces too large poor drying conditions, case hardening

Make the pieces smaller, optimise the drying conditions

Loss of texture

Fish, vegetables fruit

Enzyme activity, poor drying conditions

Blanch, optimise the drying conditions


All foods

Drying rate too high

Optimise drying conditions


All foods

Food too moist after drying

Dry to a lower moisture content


The factors that cause spoilage of dried foods are:

· moisture
· heat
· air
· odours
· light
· crushing
· micro-organisms
· insects and rodents
· shelf life

The need for packaging of dried foods depends on the nature of the food. Some (for example grains and tea, dried fish and spices) are called non-hydroscopic foods and these do not absorb much water from the air during storage. As a result the food remains dry and micro-organisms cannot grow on it. It does not therefore spoil and simple packages (baskets, jute sacks, wood or cardboard boxes) are sufficient These foods should be stored correctly, away from heat, temperature fluctuations, rodent and birds. Beans and grains can also be protected from insects by using hot pepper or bay leaves in the container.

Other types of food (for example salt, sugar, dried fruit) are hygroscopic and are able to pick up water from the air. The higher water content then allows moulds and some types of bacteria to grow on the food, and as a result it spoils. The need for packaging is therefore different for different types of food. Those that are hygroscopic need packaging in ceramic or glass jars, tins or plastic films to exclude moisture.

Some types of dried food, particularly those that contain fats or fat-soluble vitamins are also spoiled by oxygen in the air. The packaging for these types of food should also therefore exclude air. Polythene is often used but it is not the best material. Other types of packaging such as cellulose or polyprolene films are better for longer storage but these are more expensive and often unobtainable. In practice polythene can be used if it is the correct food grade quality and the food is not stored for more than a few weeks. 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 costs,

· greater convenience - high local demand for some products

· preparation of raw materials for further processing (e.g. dried fruit for bakeries, herbs for soups 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, fruit and meat)

· Small amounts of some products are used (herbs, spices) and therefore a relatively small market.

If drying is not traditionally done in an area, it is usually because the climate is not suitable and the food does not dry fast enough, or does not dry to a low enough moisture content to prevent it becoming mouldy during drying or storage. If a drying project is to be introduced to such an area, it is usually because new foods are grown in that area (e.g. new high yielding varieties or wheat replacing rice as a staple), production of a particular crop is cheaper in the new area than in the traditional areas in which the food was previously grown, or a second crop may be grown and harvested out of the normal season.

In such cases, it is necessary to heat the air using fuel, electricity or solar energy. The food will also need more protection to stop moulds and other types of spoilage during storage. This may mean additional packaging or a more sophisticated type of storage. All of these inputs increase the cost of the process and hence the cost of the product. Before a project is started it is therefore necessary to accurately find the demand for the dried food and also the price that customers are willing to pay for it. In other words a market survey and full financial evaluation are necessary to make sure that the work will not cause additional costs to the farmer and/or that it will actually generate income.

Income Generation

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

· Will the drier increase the processors' output for the same inputs (for example, by reducing losses of increasing drying rate)?

· If the costs increase to get a larger amount of dried foods, would the increased income cover the additional costs (i.e. will the processor find markets for the extra dried foods)?

· Would the use of a drier bring new opportunities for the processor (for example allowing the time saved to sell produce to other producers)?

Use these questions to produce a formula as follows:

Total costs = initial investment + maintainance + labour + cost of food + other expenses

Benefits = value of the food × weight dried in a given time + health benefits + other factors

Decide whether drying is economically viable on the basis of these costs and benefits and compare the economics of drying with other types of processing, for example, pickling, jam making (Is drying an economical option for the particular area?).

A number of different production systems can operate for income generation if there is a market for dried foods: for example small scale family drying for local sale, cooperative drying of grains or fish and service drying for local farmers or fishermen. It is unlikely that the quality of agricultural products such as grains and other staple crops will be suitable for an export market, but there may be opportunities for high value/lower volume products such as spices, enzymes, starches and other ingredients for food manufacturers.



Peeling machine (Fig. 2)
Blancher (Fig. 3)
Sulphuring Cabinet (Fig. 4)
Drier (Fig. 8 - 12)

Further Reading

BOWREY, R.G., BUCKLE. K.A., HAMEY AND PAYENAYOTIN, P., (1980) Use of solar energy for banana drying drying I food technology in Australia, 32, 6, 290-291

BROOME, R.H., (1952) In the orchard - sun drying of apricots Journal of Agriculture (Victoria), December

BUELOW, F.H., (1961) Drying crops with solar heated air Proceedings of UN conference on New Sources of Energy, Rome, Italy. Rome: FAO

BUELOW, F.H., (1962) Corrugated heat collectors for crop drying Sun at work, 1962, 8-9

CHAKRABORTY, P.K., (1976) Solar drying for drying fish and fishery products Research and Industry 21, 3.192-194

CHAKRABORTY, P.K. (1978) Technological development in artificial and solar dehydration of fish in India. Proceedings of symposiom of fish utilisation technology and marketing in the IPFC region (IPFC/78/SYMP/31), Manila, Phillipines, Rome, Italy, FAO

CHEEMA, L.S., AND RIBERTO, C.M.C. (1978) Solar driers of cashew, banana and pineapple. Proceedings of ISES conference "The sun: mankind's future source of energy" pp2075-2079. New Dehli, India. Australia: ISIS 2184pp

CLARK, C.S., (1981) Solar food drying: A rural industry Renewable energy review journal 3,1,23-26

CLARK, C.S., (1982) A solar food drier for Bangladesh Appropriate Technology 8,4,14-16

CLARK, C.S. AND SAHA, H., (1982) Solar drying of paddy Renewable energy review journal, 4. 1, 60-65

CLUCAS, I.J., (1981) Fish handling, preservation and processing in the tropics, Part I Report G144, TDRI, London, UK, 144pp

DOE, P.E., (1979) A polythene tent fish drier - a progress report Proceedings of conference "Agricultural Engineering in National Development". Paper 79-12, Selangor, Malaysia. Selangor: University Pertanian.

DOE P.E., AHMED, M., MUSLEMUDDIN, M., AND SACHITHINANTHAN, K., (1977) A polythene tent drier for improved sun drying of fish Food Technology in Australia, 29,437-441

EXCELL, RHB AND KORNSAKOO, S., A low cost solar rice drier Appropriate technology, 5,1,23-24

EZEKWE, C.I., (1981) Crop drying with solar air heater in tropical Nigeria Proceedings of ISES solar world forum, Brighton, UK. LONDON; ISES

GOMEZ, M.I, (1982) Effect of drying on the nutritive value of foods in Kenya in Food Drying., IDRC-195e (Ed.) Yaciuk G. Ottawa, Canada: International Development Research Centre. 104 pp.

GRAINGER. W., OTHIENO, H., AND TWIDDLE, J.W., (1981) Small scale solar crop driers for tropical village use - theory and practical experience Proceedings of ISES Solar world forum, Brighton, UK. LONDON; ISES

HARIGOPAL, U., AND TONAPI. K.V., (1980) Technology for villages - solar drier India Food Packer, 34,2, 48-49

HOPE, G.W., AND VITALE, D.G., (1972) Osmotic dehydration - a cheap and simple method of preserving mangoes, bananas and plantains. Report IDRC 004 el. International Development Research Centre, Ottawa, Canada. 13pp

ISLAM, M.N., AND FLINK, JM., (1982) Dehydration of ptato. I - air and solar drying at low air velocities Journal of Food Technology, 17, 373-385

ISMAIL, M.S., LAH T.A.T. AND BUYONG, A.A., (1982) Solar drying of marine products Regional journal of energy, heat and mass transfer, 4, 1,59-70

JOHNSTOON, J.C, (1979) Solar roofs dry african crops Sunworld, 3,6,161 -163

KEENER, H.M., SABBAH, M.A., MEYER, G.E., AND ROLLER, W.L., (1977) Plastic film solar collectors for grain drying Journal article no 14-77, Ohio Agricultural Research and Development Centre, Wooster, USA, 21pp

KOK, R., AND KWENDAKWENA, N., (1983) The construction and testing of a solar food drier in Zambia Paper no 83-4538, ASAE, St. Joseph, USA, 14pp

LAWAND. T.A., (1966) A solar cabinet drier Solar Energy 10,4,158-164

McBEAN, M.G., JOSLYN, M.A., AND NURY, FS., (1970) Dehydrated Fruit Report, Commonwealth Scientific Industrial and Research Organization, Sydney, Australia

McDOWELL, J., (1973) Solar drying of crops and foods in humid tropical climates Report CFNI-T-7-73, Caribbean Food and Nutrition Institute, Kingston, Jamaica. 42pp

MENDOZA, E.R., (1979) Performance of a low cost solar crop drier Proceedings of inter-regional symposium on solar energy for development, paper B-7, Tokyo, Japan, TOKYO: JSETA

MAHWALI, M., (1966) The drying of yams with solar energy Technical report T27, Bravce Research Institute, Saint Anne de Bellevue, Canada. 17pp

OTHIENO, H. GRAINGER, W., AND TWIDDELL, I.W., (1981) Application of small scale solar crop driers to maize drying in Kenya. Proceedings of 2nd conference oon energy for rural and island communities. Inverness, UK. LONDON Pergamon Press

OZISIC, M.N., HUANG, B.K.. AND TOKSOY, M., (1980) Solar grain drying Solar energy, 24,397-401

PABLO, I.S., (1978) The practicality of solar drying of tropical fruits and arine products as income generation for rural development. Proceedings of UNESCO solar drying workshop, Manila, Phillipines. MANILA; BED

PATTERSON, G., AND PEREZ, P., (1981) Solar drying in the tropics Santa Monica, USA: Meals for millions/Freedom From Hunger Foundation. 13pp

PONTING, J.D., WAITERS, G.G., FORREY, R.R., AND STANLEY, W.L., (1966) Omotic dehydration of fruits. Journal of Food Technology, 1, 10, 125-128

PUIGGALI, J.R., AND VARICHON, B., (1982) First prototypes for small fruit and vegetable country solar driers (in) Drying '82, (ed) Majumdar, A.S., LONDON, UK, Hemisphere Publishing Corp.

RAHMAN, R.K., (1981) Solar drying technoloogy for fruits and vegetable preservation Bangladesh Quarterly, 2,1,7-10

SHAW, R., (1981) Solar drying potatoes Appropriate Technology, 7, 4, 26-27

SINGH, H AND ALAM, A., (1981) Solar cabinet drier for chilli drying seeds and farms, VII, 7,25-26, 31

TRIM, D.S. K.O. H.Y., (1982) Development of a forced convection solar drier for red peppers Tropical Agriculture (Trinidad) 49,4,319-323

VOIROL, F., (1972) The blanching of vegetables and fruit Food Process Industries, August

WAGNER, C.J., COLEMAN, R.L., AND BERRY, R.E., (1980) Pretreatment for solar and hot-air drying. Proceedings of Florida Horticultural Society, 93,336-338

WATERMAN, JJ. (1976) The production of dried fish Fisheries Technical Paper 160. FAO, Rome, Italy, 52pp

WHILLIER, A., (1964) Black Painted solar air heater of conventional design Solar Energyu, 8,1, 31-37

WILHEIM, W.G., (1980) Low-cost solar collectors using thin-film plastic absorbers and glazing Proceedings of 1980 Annual Meeting of American Section ISES, Phoenix, USA, New York: ISES