Sodium bicarbonate is added to the blanching water when okra, green peas and some other green vegetables are blanched. The chemical raises the pH of the blanching water and prevents the fresh green colour of chlorophyll being changed into pheophytin which is unattractive brownish-green.

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The preservative solutions in the fruit and vegetable pre-treatment can only be used in enamelled, plastic or stainless-steel containers; never use ordinary metal because solutions will corrode this type of container.

As a general rule, preservatives are not used for treating onions, garlic, leeks, chilies and herbs.

Osmotic dehydration.

In osmotic dehydration the prepared fresh material is soaked in a heavy (thick liquid sugar solution) and/or a strong salt solution and then the material is sun or solar dried. During osmotic treatment the material loses some of its moisture. The syrup or salt solution has a protective effect on colour, flavour and texture.

This protective effect remains throughout the drying process and makes it possible to produce dried products of high quality. This process makes little use of sulphur dioxide.

Sun drying

The main problems for sun drying are dust, rain and cloudy weather. Therefore, drying areas should be dust-free and whenever there is a threat of a dust storm or rain, the drying trays should be stacked together and placed under cover.

In order to produce dust-free and hygienically clean products, fruit and vegetable material should be dried well above ground level so that they are not contaminated by dust, insects, livestock or people. All materials should be dried on trays designed for the purpose; the most common drying trays have wooden frames with a fitted base of nylon mosquito netting. Mesh made of woven grass can also be used. Metal netting must NOT be used because it discolours the product.

The trays should be placed on a framework at table height from the ground. This allows the air to circulate freely around the drying material and it also keeps the food product well away from dirt. Ideally the area should be exposed to wind and this speed up drying, but this can only be done if the wind is free of dust.

With 80 cm x 50 cm trays, the approximate load for a tray is 3 kg; the material should be spread in even layers. During the first part of the drying period, the material should be stirred and turned over at least once an hour.

This will help the material dry faster and more evenly, prevent it sticking together and improve the quality of the finished product. Products for sun drying should be prepared early in the day; this will ensure that the material enjoys the full effect of the sun during the early stages of drying.

At night the trays should be stacked in a ventilated room or covered with canvas. Plastic sheets should NEVER be used for covering individual trays during sun drying.

Dry or nearly dry products can be blown out of the tray by the wind. However, this can be protected by covering the loaded tray with an empty one; this also gives protection against insects and birds.

Shade drying

Shade drying is carried out for products which can lose their colour and/or turn brown if put in direct sunlight. Products which have naturally vivid colours like herbs, green and red sweet peppers, chilies, green beans and okra give a more attractive end-product when they are dried in the shade.

The principles for the shade drying are the same as for sun drying. The material to be dried requires full air circulation. Therefore, shade drying is carried out under a roof or thatch which has open sides; it CANNOT be done either inside conventional buildings with side walls or in compounds sheltered from wind. Under dry conditions when there is a good circulation of air, shade drying takes little more time than is normally required for drying in full sunlight.

5.2.4 Identification of suitable designs of solar dryers for different applications

In the selection of appropriate solar dryers for commercial scale operation, it is imperative that economics be kept in view at all time. A total Energy System concept should be employed and due consideration be given to parasitic energy consumption.

The following features have been identified:

• large scale dryers are more promising than small scale ones. However, small scale dryers should not be neglected.
• the dryer should be designed to maximise the utilisation factor of the capital investment, i.e. multi-products (fruit, vegetables and other raw material) and multi-use (e.g. drying and heating water for domestic use).
• in general, an auxiliary heat source should be provided to assure reliability, to handle peak loads and also to provide continuous drying during periods of no sunshine.
• forced convection indirect dryers are preferred because they offer better control, more uniform drying and because of their high heat collection efficiency result in smaller collector area. However, parasitic power should be kept to a minimum.

Two dryer systems have been identified:

• a cabinet type dryer with natural convection for internal air circulation for the processing of dried fruit such as mango, banana, pineapple, apricot, pear, apple, etc. and also for potato chips and other vegetables;
• a greenhouse type dryer with forced air circulation.

Some of the barriers to the commercial development of solar dryers have been attributed to:

• initial cost - poor farmers cannot afford them
• durability - constant breakdown due to using low cost building materials
• misuse - through lack of training and technical skills
• dependability and reliability - during the wet season when drying is critical there is not enough solar energy available
• the wider use of Solar Drying Systems has been limited by other factors which are not necessarily of a technical or technological nature. Among the most important are the lack of national policies directed to promoting the drying of produce at the production site, in order to reduce losses, improve quality and increase farmers' earnings.

5.2.5 Construction of solar dryers

In the case of simple natural convection dryers it may be more appropriate to build and operate a number of small units. Multiplicity allows diversity, since more than one crop can be dried at a time. A further advantage is that if one dryer is out of operation due to damage, drying can still continue at reduced capacity using the other dryers.

On the other hand, more sophisticated dryers, such as forced convection solar dryers, benefit from economies of scale due to the investment tied up in the fan and the source of heat.

Generally speaking, one large dryer will be more cost-effective than two smaller units. However, it should be taken into consideration that an oversized unit will be operating at less than full capacity, reducing any cost advantage. The drying area required will depend on local conditions, commodity and number of trays on each rack or trolley.

5.2.5.1 Construction methods and materials

Construction methods and available materials may vary considerably from location to location. It is not within the scope of this document to discuss individual, local circumstances. Some general guidelines regarding factors which must be considered can, however, be given:

- dimensions of standard materials. Where possible, design should take account of the sizes of material locally available. For example, it would be poor design to specify the width of a corrugated iron collector as 1.1 m if the standard width of a corrugated iron sheet is 1 m.

Before finalising a design the commercial availability of materials must be ascertained.

- use of rural materials. The cost of building of solar dryer can be minimised if the producer is able to use wood cut straight from the forest rather than prepared timber.

Careful design in the development stage of a dryer can often facilitate the use of cheaper materials. Difficulties caused by these materials are in joining pieces of the structure, in sealing the structure against air leaks, and in attaching the plastic sheet to the (wooden) frame. There is obvious scope for designs which use prepared timber for strategic points and unprepared at others.

Where the use of wood is necessary, remember to take environmental factors into consideration. For example, determine the effect of flash flooding or termites might be and take the appropriate preventive action.

- use of plastic sheets. For many solar dryers, the clear plastic sheet used is the major capital cost to the farmer; therefore, the type of plastic chosen is important.

A choice must be made between a relatively cheap plastic such as ordinary polyethylene which will last, at best, for one season due to photo-degradation and wear and tear; and a more expensive, better quality plastic less prone to photo-degradation; or even glass or a rigid plastic.

Attaching plastic sheet to the framework structure, so as to minimise the likelihood of the plastic being torn is, perhaps, the most difficult part of building a dryer. Listed below are some general points which should be followed to prolong the useful lifetime of plastic sheet on a solar dryer:

• when attaching plastic sheet to the framework, care should be taken to stretch the plastic at the points of attachment, but the plastic should not be so loose that it will flap about in the wind;
• rather than merely stapling or nailing the plastic directly to the framework, it is preferable to sandwich the plastic between the framework and a batten. This may not be practical when unprepared wood or other materials are being used.
• no sharp edges should come in contact with the plastic sheet since these will initiate tears;
• fold over the plastic at the point of attachment to the frame, so that there are two or more layers of plastic. This will help prevent tears;
• when fixing the sheet over the framework, sags and hollows in which water can collect should be avoided wherever possible;
• the dryer should be handled as carefully and as seldom as possible during operation and when not in use.

5.2.5.2 Technical criteria

The following design factors must be established:

• the throughput of the dryer over the productive season; the size of batch to be dried;
• the drying period(s) under stated conditions;
• the initial and desired final moisture content of the commodity (if known);
• the drying characteristics of the commodity, such as maximum drying temperature, effect of sunlight upon the product quality, etc.;
• climatic conditions during the drying season, i.e. sunlight intensity and duration; air temperature and humidity; wind speed (such data may be available from local meteorological stations);
• availability and reliability of electrical power;
• the availability, quality, durability and price of potential construction materials such as:

• glazing materials: glass, plastic sheet or film;
• wood (prepared or unprepared);
• nails, screw, bolts, etc.;
• metal sheet, flat or corrugated angle iron;
• bricks (burnt or mud), concrete blocks, stones, cement, sand, etc.
• roofing thatch;
• metal mesh, wire netting, etc.
• mosquito netting, muslin, etc.
• bamboo or fibre weave;
• black paint, other blackening materials;
• insulation material; sawdust, etc.;

• the type of labour available to build and operate the dryer;
• the availability of clean water at the site for preparation of the commodity prior to drying.

In any one situation there may well be other technical factors that need to be considered.

5.2.5.3 Socio-economic criteria

From the initial considerations, estimates of the capital costs of the dryer, the price of the commodity to be dried, and the likely selling price of the dried product will have been made. Other question that need to be considered are the following:

• who will own the dryer?
• is the dryer to be constructed by the end-user (with or without advice from extension agencies), local contractors, or other organisations?
• who will operate and maintain it?
• how can the drying operation be incorporated into current practices?
• are sources of finance from local authorities or extension agencies available, etc.?

Obviously there are many other socio-economic factors, particularly those of a local nature, which must be taken into account. It cannot be stressed too highly that if such factors are not taken into account and evaluated, then is every chance that an inappropriate dryer design may result. Equal emphasis must be placed on both technical and socio-economic factors.

Summary

a) Situations where solar dryer may be useful:

• where the cost of conventional energy is prohibitive and/or the supply is erratic, to supplement existing artificial drying systems and reduce fuel costs;
• where land is in short supply or expensive;
• where the quality of existing sun dried products can be improved upon;
• where the labour is in short supply;
• where is plenty of sunshine, but high humidity.

b) Situations where solar dryers may not be useful:

• where conventional energy sources are abundant and cheap;
• where large amounts of combustible by-products or waste materials are freely available;
• where there is insufficient sunshine;
• where is plenty of sunshine and arid conditions (sun drying may suffice);
• where the quality of sun dried products already made cannot be improved upon;
• where local operators are insufficiently trained;
• where the ramifications of introducing a solar dryer have not been completely thought out.

5.2.6 Sun/solar drying tray

The drying tray described requires seasoned timber 22.5 mm thick x 50 mm wide.

The drawings that accompany these instructions are in Fig. 5.2.3.

Figure 5.2.3 Sun/solar drying tray

A sun drying tray requires 6 meters of seasoned timber 22.5 mm thick x 50 mm wide.

Cut the timber into lengths of 900 mm long for the sides of the tray and 600 mm long for the ends - 4 pieces of each length will be needed. The ends of each piece are cut as shown in the drawing - this is to make flush fitting joints. Join the corners using small brass screws 20 mm long. To make extra strong joints use good quality wood glue as well as the screws.

The nylon mosquito netting or grass woven mesh can be fitted between the frames as shown in the bottom drawing. Cut the mesh a little larger than the size of the frame. Using drawing pins, pin the mesh to the OUTSIDE edges of one of the frames - the mesh should be pulled tight as the pins are put in around the edges.

Lay the other frame on top and drill holes about 3 mm in diameter at the points marked X in the top drawing. Use nails that are a tight fit in the holes and tap gently into place leaving a portion standing above the frame.

Cut off the standing part to leave a piece about 12 mm long which is then bent over and tapped firmly down onto the frame. When the frame has been put together tightly, the drawing pins can be removed.

5.2.7 Dryers

Figures 5.2.4 to 5.2.19 illustrated various types of sun/solar dryers along with examples of drying and dehydration equipment.

5.2.8 Preservation by concentration

Foods are concentrated for many of the same reasons that they are dehydrated; concentration can be a form of preservation but this is true only for some foods. Concentration reduces weight and volume and results in immediate economic advantages.

Nearly all liquid foods which are dehydrated are concentrated before they are dried. This is because in the early stages of water removal, moisture can be more economically removed in highly efficient evaporators than in dehydration equipment. Further, increased viscosity from concentration often is needed to prevent liquids from running off drying surfaces or to facilitate foaming or puffing.

Foods are also concentrated because the concentrated forms have become desirable components of diet in their own right. Thus, fruit juices plus sugar with concentration becomes jelly. The more common concentrated fruit and vegetable products include items as fruit and vegetable juices and nectars, jams and jellies, tomato paste, many types of fruit purées used by bakers, candy makers and other food manufacturers.

5.2.8.1 Aspects of preservation by concentration

The level of water in virtually all concentrated foods is in itself more than enough to permit microbial growth. Yet while many concentrated foods such as non-acid fruit and vegetable purées may quickly undergo microbial spoilage unless additionally processed, such items as sugar syrups, jellies and jams are relatively "immune" to spoilage; the difference of course is in what is dissolved in the remaining water and what osmotic concentration is reached.

Figure 5.2.4 Sun drying tent

Figure 5.2.5 Solar tent dryer

Figure 5.2.6 Cabinet or tray dryer

Figure 5.2.7 Solar cabinet dryer

Figure 5.2.8 Solar dryer

Figure 5.2.9 Solar dryer in combined mode

Figure 5.2.10 Solar cabinet dryer with separate air heater

Figure 5.2.11 Basic form of flat-plate solar air heater

Figure 5.2.12 End-profile views of flat-plate solar air heater

Figure 5.2.13 Natural convection solar dryer

Figure 5.2.14 Simple sulphuring cell

1. Cell walls
2. Trays on a car
3. Metal plat with burning sulphur
4. Hole for sulphur dioxide fumes
5. Exhaust hole

Figure 5.2.15 Solar wind-ventilated dryer

Figure 5.2.16 Tunnel dryer for fruit and vegetables. Capacity: 6 to 12 cars with 25 or 18 trays each

Figure 5.2.17 Cabinet/cell dryer for fruit and vegetables. Capacity: 2 to 4 cars with 25 trays each; 1 tray = 1 m²

Courtesy of U.T.A. Industrie

Figure 5.2.18 Tunnel type dehydration unit - tunnel dryer

1. Control and switch-board
2. Burner
3. Platform for burner
4. Frontal plate
5. Air flow regulating plate
6. Burner's cylinder
7. Air circulating fan
8. Air direction conveying plates
9. Cars rail
10. Tunnel "feeding" door: inlet of cars with fresh product
11. Tunnel "evacuating" door: outlet of cars with dry product
12. Cars pushing device
13. Cars with trays
14. Drying trays

Figure 5.2.19 Typical counterflow tunnel dryer construction

Sugar and salt in concentrated solutions have high osmotic pressure. When these are sufficient to draw water from microbial cells or prevent normal diffusion of water into these cells, a preservative condition exists.

The critical concentration of sugar in water to prevent microbial growth will vary depending upon the type of micro-organisms and the presence of other food constituents, but usually 70% sucrose in solution will stop growth of all micro-organisms in foods. Less than this concentration may be effective but for short periods of time unless the foods contain acid or they are refrigerated.

Salt becomes a preservative when its concentration is increased and levels of about 18% to 25% in solution generally will prevent all growth of micro-organisms in foods.

Except in the case of certain briny condiments, however, this level is rarely tolerated in foods.

Removal of water by concentration also increases the level of food acids in solution (particularly significant in concentrated fruit juices).

5.2.8.2 Reduced weight and volume by concentration

While the preservation effects of food concentration are important, the main reason of most food concentration is to reduce food weight and bulk. Tomato pulp which is ground tomato minus the skins and seeds, has a solid content of only 6 % and so a 3.785 litre can would contain only 231 g of tomato solids (See Table 5.2.6.1).

TABLE 5.2.8.1 Specific gravity and solids. Tomato pulp and commercial tomato concentrates

Source: Adapted from National Canners Assoc. (1956)

Concentrated to 32% solids, the same can would contain 1.38 kg of tomato solids or six times the value of product. For a manufacturer needing tomato solids such a producer of soups, canned spaghetti or frozen pizza the saving from concentration are enormous in cans, transportation costs, warehousing costs and handling costs throughout his operation.

5.2.8.3 Methods of concentration

1. Solar concentration. As in food dehydration, one of the simplest methods of evaporating water is with solar energy. A typical example of this method is production at farm level in developing countries of fruit pastes/leathers (such as apricot or plum pastes).
2. Open Kettles. Some foods can be satisfactorily concentrated in open kettles that are heated by steam. This is the case for jellies and jams, tomato juices and purées and for certain types of soups. High temperatures and long concentration times should be avoided in order to reduce or eliminate damage. It is also necessary to avoid thickening and burn-on of product to the kettle wall as these gradually lower the efficiency of heat transfer and slow the concentration process.
   However, when the process is under control, this type of evaporation is still highly recommended for small scale operations in developing countries. It is a quite widely used system, mainly for jellies, jams and marmalades (Fig. 5.2.20).
   Figure 5.2.20 Open kettle
3. Flash evaporators.
4. Thin-film Evaporators.
5. Vacuum evaporators. - It is common to construct several vacuumised vessels in series so that the product moves from one vacuum chamber to the next and thereby becomes progressively more concentrated in stages.
   With such an arrangement the successive stages are maintained at progressively higher degrees of vacuum, and the hot water vapour produced by the first stage is used to heat the second stage, the vapour from the second stage heats the third stage and so on. In this way maximum use of heat energy is made. Such system is called a multiple effect vacuum evaporator and is illustrated in Fig. 5.2.21; it is a widely used system for concentrated tomato paste.
   Figure 5.2.21 Multiple effect vacuum evaporator
6. Freeze Concentration. This process has been known for many years and has been applied commercially to orange juice. However, high processing costs due largely to losses of juice occlude [unclear] to the ice crystals, have limited the number of installations to date.
7. Ultrafiltration and reverse Osmosis.

5.2.8.4 Changes from concentration

Obviously concentration that exposes food to 100° C or higher temperatures for prolonged periods can cause major changes in organoleptic and nutritional properties. Cooked foods and darkening of colour are two of the more common heat induced results which must be kept under control during a well designed process with an efficient evaporator which is still "safe" .

Microbial destruction is another type of change that may occur during concentration and will be largely dependent upon temperature. Concentration at a temperature of 100 °C or slightly above will kill many micro-organisms but cannot be depended on to destroy bacterial spores. When the food contains acid, such as fruit juices, the kill will be greater but again sterility is unlikely.

On the other hand, when concentration is done under vacuum many bacterial types not only survive the low temperatures but multiply in the concentrating equipment. It is therefore necessary to stop frequently and sanitise low temperature evaporators and where sterile concentrated foods are required, to resort to an additional preservation treatment.

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