oper-ation that changes or converts raw material into a safe and edible form. Food processing also provides us with a means to extend the shelf life of otherwise perishable food; with-out food processing it would not be possible to sustain the needs of modern urban popu-lations and the choice of food available would be very limited and largely seasonal.
Changing lifestyles and family structure have resulted in a largely consumer-led de-mand for an ever-growing selection of food, particularly ready-prepared and partially prepared products. Virtually all food under-goes some form of processing before its con-sumption. At its most simple, processing can be peeling a piece of fruit or boiling po-tatoes. The oldest methods of food pro-cessing include sun drying, smoking, pick-ling and salting; these methods utilize the fact that water removal increases shelf life.
Fermentation and freezing are also among the traditional methods of food processing.
Table 8.1. Proximate composition of a sample of fresh chester leaves. From Alonge and Essien, 2014.
Table 8.3. Proximate composition of a sample of chester leaves oven dried at 105°C. From Alonge and Essien, 2014.
Table 8.2. Proximate composition of a sun-dried sample of chester leaves. From Alonge and Essien, 2014.
Canning, pasteurization and sterilization are techniques that have been used for many decades but are still important in the mod-ern food processing industry. However, many new processes are now joining them and a number of others are already waiting in the wings (Jones et al., 1979).
One of the problems with products is consumer interest in health and related issues, such as naturalness and added value. This interest has led to the development of prod-ucts that have specific vitamins or minerals added to them. Fortified foods and drinks are used around the world as a cost-effective way of ensuring the nutritional quality of food sup-ply. The addition of nutrients requires careful attention to food regulation and a suitable nu-tritional rationale so that the final product re-mains acceptable to consumers.
Food processing improves the nutri-tional value of certain foods. For example,
severe heat treatment destroys trypsin in-hibitors, which are anti-nutritional factors present in a range of foods. For instance, the harmful lectins present in red kidney beans are destroyed by extended boiling. Food processing can also increase the bioavail-ability of nutrients in foods and their or-ganoleptic qualities. However, any form of food processing, even slicing, washing and cooking in the home, can result in the loss of heat-sensitive, oxygen-sensitive and other-wise highly sensitive nutrients, especially certain vitamins. The leaching of minerals into the cooking water can also occur with vegetables. The main commercial processes that cause nutrient loss are blanching, other application of heat, processing and drying.
In some cases, processed food actually retains more nutrients than the unprocessed form. The best example is frozen vegetables, Table 8.7. Anti-nutrient (toxicant) composition of three varieties of chester as mg/100 g dry weight.
From Etuk et al., 1998.
Variety Hydrocyanide Total oxalate
Soluble oxalate Tannin
Black 10.75 13.25 9.76 11.45
Ekoi 10.00 29.17 25.65 6.18
White 8.14 17.05 14.32 5.01
Table 8.4. Proximate composition of three varieties of chester leaf. From Etuk et al., 1998.
Variety
Organic
compoundsa Moistureb Asha
Ether extract (fat)a
Crude
proteina Fibrea Carbohydratea
Calorific valuec
Black 95.00 42.4 5.00 4.20 11.80 14.76 79.00 401.00
Ekoi 95.50 33.00 4.50 3.40 9.45 13.92 82.65 399.00
White 96.00 45.20 4.00 1.40 14.70 12.50 79.90 391.00
a% dry matter; b% wet weight; ckcal/100 g.
Table 8.6. Vitamin A (retinal) and vitamin C (ascorbate) content of three varieties of chester leaf as mg/100 g dry weight. From Etuk et al., 1998.
Variety Vitamin A Vitamin C
Black 18.89 ± 1.93 133.33 ± 3.72
Ekoi NDa ND
White 11.00 ± 1.73 121.51 ± 1.86
aND = Not determined.
Table 8.5. Mineral content of three varieties of chester leaf (as ppm). From Etuk et al., 1998.
Variety Iron Magnesium Zinc Calcium Cadmium
Black 13.24 79.38 2.20 645.80 0.11
Ekoi 14.52 81.38 1.95 622.90 0.11
White 19.86 38.23 2.92 1056.25 0.21
which are harvested and frozen immedi-ately after collection, whereas fresh veget-ables may have been stored for several days before purchase or use. Even with unpro-cessed vegetables though, modern storage and transportation techniques can help to retain nutrients. An example of a potential health concern arising from food processing involves trans fatty acids (a group of unsat-urated fatty acids). Vegetable oils are often hydrogenated to improve their oxidative stability and functional properties, as in the manufacture of margarine, and during this process trans fatty acids can be produced.
These are metabolized in a similar manner to ordinary fatty acids after ingestion, and can cause an increase in ‘bad’ cholesterol, although the general consensus in the UK is that current intakes of trans fatty acid do not present a problem (Gooding, 1962).
Many new processing techniques have been developed in response to changing nu-tritional concepts and consumer demand for less processed food. The objective is to produce high-quality safe foods that are con-venient, fresher and considered more nat-ural. The nutritional implications of some of the techniques that are still in develop-ment have yet to be established and will al-most inevitably determine whether or not these techniques achieve commercial success (Umoh and Bassir, 1977).
Functional foods are the last refinement in a continuum of products developed to provide added value. One commonly used definition of a functional food is a dietary ingredient that affects its host in a targeted manner so as to exert positive effects that may include or justify certain health claims.
Within this context, there is increasing interest in probiotics and symbiotics. Probiot-ics are substances, e.g. oligosaccharides, that are not digested but which beneficially af-fect the host by selectively stimulating the growth of specific bacteria in the colon. It is now recognized that the composition of the bacterial population of the bowel is import-ant for human health and can potentially be manipulated by the type of food eaten. The incorporation of live microorganisms (as pro-biotics) into food such as yoghurts influences the bacterial population in the gut. A third
approach, the use of symbiotics, is a com-bination of the above two approaches (Umoh and Bassir, 1977).
The use of genetic modification in food production is a relatively new process and has many potential applications. For ex-ample, a plant can be modified to resist dis-eases, microbial attack or insect infestation, or to produce fruit with a better flavour and improved keeping qualities. Plants can also be developed to resist certain herbicides that are applied to kill weeds; other possi-bilities include development for drought resistance (very important in developing countries) and resistance to fruit damage.
The use of genetic modification offers sub-stantial potential benefits to the food indus-try and consumers, but it is recognized that some consumers may have reservations about this new and unfamiliar technology.
To help in the recognition of foods that contain genetically modified material, regu-lations have recently been published that require all foods containing ingredients pro-duced from genetically modified soya or maize to be labelled unless neither protein nor DNA resulting from the genetic modifi-cation is present in the food itself (van het Hof et al., 1999).
8.3.1 Processing
The dehydration of vegetables is one of the oldest forms of food preservation known to man. It involves the removal of moisture by sun drying or mechanical drying. Apart from extending shelf life, drying aids in eas-ier packaging and transportation due to the reduction in volume. The leaves of the ches-ter plant are sun dried and used in African countries because of their otherwise limited shelf life; they are used in the preparation of soups. The demand for dehydrated food is rising due the focus on instant and conveni-ence foods. (Gooding, 1962).
Dried vegetables can be produced by a variety of processes, which differ primarily in the type of drying method and depend on the type of food and quality required of the end products. Size reduction is an
important pretreatment prior to drying (Ene-Obong and Obizoba, 1996). The pre- drying treatment involves preparation of the raw product, and includes product selec-tion and sorting, and washing followed by size reduction.
Sun drying is a conventional method that has been practised for centuries. It is the cheapest method of drying for perish-able food. In this method, the food is dir-ectly exposed to sunlight by placing it on the floor, in yards and also on roof tops. Cas-sava leaves, sweet potato, okra and other green edible leaves are also dried in the sun by farm families. In countries like Nigeria, blanching and salting is carried out before sun drying. Nutritive quality, storage and palatability are the key consideration dur-ing the drydur-ing of any foodstuff. However, sun drying and shade drying result in loss of micronutrients (Umoh and Bassir, 1980).
Chester leaves are shade dried and mechan-ically dried after harvesting. They are then infused in water in different proportions.
Inyang (2015) has reported the effect of treatments on the mineral and vitamin con-tent of the leaves.
The removal of moisture from a typical food product will follow a series of drying rates, as illustrated in Figs 8.2 and 8.3. The initial removal of moisture (AB in Figs 8.2 and 8.3) occurs as the product and the water within the product experience a slight tem-perature increase. Following the initial stages
of drying, significant reductions in moisture content will occur at a constant rate (BC) and at a constant product temperature. The constant-rate drying period occurs with the product at the wet bulb temperature of the air. In most situations, the constant-rate dry-ing period will continue until the critical moisture content (WC in Fig. 8.2) is reached.
Below the critical moisture content, the rate of moisture removal decreases with time. The falling-rate drying period (CE, see Fig. 8.1) will follow.
The choice of drying method is based on economic considerations and on the quality and characteristics of the raw material. Some of the drying methods adopted are sun or solar drying, stationary or batch drying pro-cesses (kiln, tower and cabinet driers) and continuous drying (tunnel drying, continu-ous belt trough drying, fluidized bed drying, explosion puffing, foam mat drying, spray drying, drum and microwave-heated drying and subatmospheric dehydration, viz. vac-uum shelf, vacvac-uum belt, vacvac-uum drum and freeze drying.
Sun drying is susceptible to the vagaries of weather, though it is much used (see Fig. 8.4).
Certain fruits, such as prunes, grapes, dates, figs, apricots and pears are sun dried. These crops are processed in substantial quantities without much in the way of technical aid.
Sun-dried products do, however, have limited shelf life because the moisture is not com-pletely removed from the product.
A B
C D
Moisture content
Drying time
Fig. 8.3. Drying rate curve 2. A–B, initial removal of moisture; B–C, moisture content reduction at a constant rate; C–D, lower rate of drying towards end of drying process. From Singh and Heldman, 2001, p. 151.
WC
Free moisture (kg H2O/kg dry solid) Drying rate (kg water/m2/h)
E D
C B
A
Fig. 8.2. Drying rate curve 1. A–B, initial removal of moisture; B–C, moisture content reduction at a constant rate; C–E, lower rate of drying towards end of drying process. WC, critical moisture content. From Singh and Heldman, 2001, p. 151.
Mechanical drying involves quicker and complete drying, though the capital in-vestment is higher.
8.3.2 Value addition
Chester leaves are harvested and processed for preparation of soup. During the produc-tion season, they can be harvested, dried and stored (Fig. 8.5). Alonge and Essien (2014) noted that the extent of moisture removal de-pended on the temperature and humidity of the drying air. Both the sun drying method and the oven drying method were investi-gated and the products were compared with fresh samples to ascertain the changes that took place during drying of the leaves. It was concluded that the oven drying method was more effective, as the amount of nutrients in-creases with drying time, although the fat and carbohydrate content both decrease. Hence, the nutritive value and keeping quality of chester leaves can be increased by drying.
8.4 Uses
Generally, chester is consumed as cooked complement to major foodstuffs such as cas-sava, cocoyam, guinea corn, maize, millet, rice and plantain. In fact, most of the meals
prepared using these staples are seen as in-complete if a good amount of cooked green leaves does not accompany them. As a result of the method of processing chester for preser-vation, the nutritive value of the vegetable can be affected. For example, the period of time that leaves spread in the sun are left to dry are likely to affect their nutritive value. However, there is little or no information on the effect of drying of the vegetable for preservation before it is being used for preparing soups. Ajibesin et al. (2008) reported that the Efiks in south-ern Nigeria use the leaves in vegetable soup and also to treat hypertension and abscesses.
8.4.1 General uses
Atama (chester in the Efik local dialect of Nigeria) has for several hundreds of years been exploited by traditional herbalists for the treatment of various ailments, including typhoid fever, diarrhoea and candidiasis (Andy et al., 2008). Chester plants can be used in various forms to treat various dis-eases and this qualifies it to be called a me-dicinal plant (Andy et al., 2008).
8.4.2 Pharmacological uses Chester has a characteristic aroma which could possibly be attributed to the presence of Fig. 8.4. Solar drying of chester leaves in a trough.
terpenes and essential oils that convey its distinctive taste and aroma, and also con-tribute to its very high medicinal value as summarized below:
1. Oral enquiries of the indigenous people of