Why cassava needs processing
Constraints in the traditional processing of cassava
Traditional methods for processing cassava
Processing techniques and reduction of cyanide in cassava
Processed products
Processing equipment
Storage of processed products
Cassava leaves as vegetable
Utilization of processed products
Modes of consumption
Potentials of cassava as animal feed
The future of cassava
References
S.K. Hahn
Cassava is one of the most important staple food crops grown in tropical Africa. It plays a major role in efforts to alleviate the African food crisis because of its efficient production of food energy, year-round availability, tolerance to extreme stress conditions, and suitability to present farming and food systems in Africa (Hahn and Keyser 1985, Hahn et al. 1987).
Traditionally, cassava roots are processed by various methods into numerous products and utilized in various ways according to local customs and preferences. In some countries, the leaves are consumed as vegetables, and many traditional foods are processed from cassava roots and leaves.
Improvement of cassava processing and utilization techniques would greatly increase labor efficiency, incomes, and living standards of cassava farmers and the urban poor, as well as enhance the-shelf life of products, facilitate their transportation, increase marketing opportunities, and help improve human and livestock nutrition. This paper presents a general overview of traditional cassava processing and utilization methods now used by small-scale farmers and processors in Africa, and examines the opportunities for improving postharvest technologies.
Why cassava needs processing
Fresh cassava roots cannot be stored for long because they rot within 3-4 days of harvest. They are bulky with about 70% moisture content, and therefore transportation of the tubers to urban markets is difficult and expensive. The roots and leaves contain varying amounts of cyanide which is toxic to humans and animals, while the raw cassava roots and uncooked leaves are not palatable. Therefore, cassava must be processed into various forms in order to increase the shelf life of the products, facilitate transportation and marketing, reduce cyanide content and improve palatability. The nutritional status of cassava can also be improved through fortification with other protein-rich crops. Processing reduces food losses and stabilizes seasonal fluctuations in the supply of the crop.
Constraints in the traditional processing of cassava
Environmental factors
During the rainy season, sunshine and ambient temperatures are relatively low for processing cassava, particularly in lowland humid areas where cassava is mainly grown and utilized. In other localities, particularly in savanna zones, water which is essential for processing cassava, is not easily available. During the early rainy season, the dry matter content of roots is usually lower than in the dry season, which can result in a lower yield of products. In the dry season when the soil is hard, harvesting and peeling tubers for processing are difficult and result in more losses.
Varietal factors
Cassava shape varies among cultivars. Roots with irregular shapes are difficult to harvest and peel by hand, resulting in great losses of usable root materials. Root size also varies with cultivars although it depends more on environmental factors such as soil. Smaller roots require more labor for peeling. Varietal differences in dry matter content, and in starch content and quality influence the output and quality of the processed products. Cyanide content varies with varieties, but is also affected by the crop growth environment.
Agronomic factors
Time of planting and harvesting, and age of plant, from planting to harvesting, all affect starch content, yield and quality of products. Other agronomic practices such as intercropping, fertilizer application and spacing can also affect yield and crop quality.
Socioeconomic factors
Harvesting and transporting of roots from farm to homestead and subsequent processing are mainly done by women. Most of the steps in processing are carried out manually using simple and inexpensive tools and equipment that are available to small farmers. Cassava processing is labor intensive and productivity is usually very low. Transport of products to markets is made difficult by the poor condition of rural roads. The drudgery associated with traditional processing is enormous and the products from traditional processing methods are often contaminated with undesirable extraneous matter. Some of the products are therefore not hygienic and so are of poor market value. Better processing methods can improve the life-styles and health of rural people through higher processing efficiency, labor saving and reduced drudgery, all of which improve the quality of products.
Subsistence farmers harvest cassava when needed. Thus they leave the cassava in the ground for long periods, believing that the cassava is safer and would undergo less damage than when harvested. Although this system has certain merits, a delay in harvest can result in root losses due to root rots, damage by animals, and a decrease in the starch content in roots. Furthermore, keeping cassava in the ground prevents the use of that land for other purposes.
Traditional methods for processing cassava
Traditional cassava processing methods in use in Africa probably originated from tropical America, particularly northeastern Brazil and may have been adapted from indigenous techniques for processing yams (Jones 1959). The processing methods include peeling, boiling, steaming, slicing, grating, soaking or seeping, fermenting, pounding, roasting, pressing, drying, and milling. These traditional methods give low product yields which are also of low quality.
Rapid urbanization in tropical Africa increased mobility in both rural and urban areas and the changing roles and status of women have resulted in an unprecedented demand for convenience foods. Added to these factors is the high cost of fuel for cooking in urban areas at a time when fuel wood is not only inconvenient to use but is becoming increasingly scarce. Therefore, cassava processing and utilization technologies for the future should improve traditional methods and develop low cost equipment with low energy demands. Improved processing and utilization technologies should address issues related to farmers' (producers') and consumers' needs (particularly urban needs in future), and also to economic factors and nutritional values. Knowledge of the current traditional processing and utilization methods and of present urban patterns of consumption and changing urban needs will guide future strategies for cassava processing and utilization.
Improvement of nutritional values of processed products also requires special attention from policymakers and researchers. Cassava is frequently denigrated because its roots are low in protein. However, protein may be supplemented from other sources, particularly legumes; for example, fortification of cassava flour or gari with protein-rich soyflour can be achieved. Such fortified products will be nutritionally advantageous, and thus economical and acceptable to consumers.
Although cassava is regarded as subsistence crop of low-income families or as a "famine-reserve crop", about 60 percent of the cassava output of households in the Oyo area of Nigeria is sold for processing (mostly into gari) while the remaining 40 percent is consumed at home (Ikpi et al. 1986). A high proportion (50 percent) of cassava was also sold to food processors in the western region of Cameroon (Okezie et al. 1988), suggesting a changing status for cassava.
Processing techniques and reduction of cyanide in cassava
Cassava contains the cyanogenic glucosides, linamarin and lotaustralin which are hydrolyzed after tissue damage, by the endogenous enzyme, linamarase to the corresponding cyanohydrins and further to hydrogen cyanide [HCN](Conn 1969). The hydrogen cyanide is responsible for chronic toxicity when inadequately processed cassava products are consumed by humans and animals for prolonged periods. Therefore, traditional processing procedures must aim at reducing cyanide and improving storability, convenience and palatability.
Cassava processing procedures vary, depending on products, from simple processing (peel, boil and eat) to complicated procedures for processing into gari, for example, which involve many more steps, namely peeling, grating, pressing, fermenting, sifting, and roasting. Some of these steps reduce cyanide more effectively than others. Processing techniques and procedures differ with countries and localities within a country according to food cultures, environmental factors such as availability of water and fuelwood, the cassava varieties used, and the types of processing equipment and technologies available. The most important traditional culinary preparations of cassava in Africa are "boiled or roasted roots", "fufu" (cassava flour stirred with boiled water over a low-heat fire to give a stiff dough), "eba" (gari soaked in hot water to produce a thick paste) and "chickwangue" (steamed fermented pulp wrapped in leaves).
Fermentation
Fermentation consists of two distinct methods: aerobic and anaerobic fermentation. For aerobic fermentation, the peeled and sliced cassava roots are first surface-dried for 1-2 hours and then heaped together, covered with straw or leaves and left to ferment in air for 3-4 days until the pieces become moldy. The fermented moldy pieces are sun-dried after the mold has been scraped off. The processed and dried pieces (called "Mokopa" in Uganda) are then milled into flour, which is prepared into a "fufu" called "kowan" in Uganda. The growth of mold on the root pieces, increases the protein content of the final products three to eight times (Amey 1987, Sauti et al. 1987). This fermentation method is also very popular in other parts of East Africa such as Tanzania, Rwanda, and Zaire.
In anaerobic fermentation, grated cassava for processing into "gari" is placed in sacks and pressed with stones or a jack between wooden platforms. Whole roots or pieces of peeled roots for processing into "fufu" are placed in water for 3-5 days. During the first stage of gari production, the bacterium Corynebacteria manihotattacks the starch of the roots, leading to the production of various organic acids (such as lactic and formic acids) and the lowering of substrate pH. In the second stage, the acidic condition stimulates the growth of a mold, Geotrichum candida, which proliferates rapidly, causing further acidification and production of a series of aldehydes and esters that are responsible for the taste and aroma of gari (Odunfa 1985). The optimum temperature for the fermentation for gari processing is 35°C, increasing up to 45°C.
For "lafun" production in Nigeria, peeled or unpeeled cassava tubers are immersed in a stream, in stationary water (near a stream) or in an earthenware vessel, and fermented until the roots become soft. The peel and central fibres of the fermented roots are manually removed and the recovered pulp is hand mashed or pounded. The microorganisms involved in "lafun" production include four yeasts: Pichia onychis, Candida tropicalis, Geotrichum candida, and Rhodotorula sp.; two molds: Aspergillus niger and Penicillium sp.; and two bacteria: Leuconostoc sp. and Corynebacterium sp. (Nwachukwu and Edwards 1987). Moisture, pH and temperature conditions are critical for the growth of these microorganisms in roots and thus for fermentation.
Dewatering the fermented cassava
During or after fermentation of roots for gari production, the grated pulp is put in sacks (jute or polypropylene) on which stones are placed or jacked-wood platforms are set to drain or press off the excess liquid from the pulp. In Zaire, the cassava pulp is taken out and heaped up on the racks in the sun for further fermentation and draining of the excess moisture. In this way, much of the cyanide is effectively lost with the liquid.
Tissue disintegration
Tissue disintegration in the presence of excess moisture during grating or fermenting in water permits the rapid hydrolysis of glucosides, effectively reducing both free and residual cyanide in the products. Fermentation in water appears a more efficient method for reducing the cyanide of roots. For example, this process reduced cyanide by 70-95 percent of the original level after the roots were soaked in water for 3 days (Hahn et al. 1987). Gari obtained through the processing procedures involving grating and/or fermentation showed 80-90 percent reduction in total cyanide content relative to freshly peeled roots (Mahungu et al. 1987). Oke (1968) reported HCN content of 1.9 mg/100g for gari, 2.5 mg/100g for fufu (Nigeria) and 1.0 mg/100g for fufu (Zaire) or lafun (Nigeria). HCN concentration in 202 gari samples collected across the cassava growing areas of Nigeria had 0-3.2 mg/100g with a mean of 0.6 mg/100g. Akinrele et al. (1962) stated that 0.3 mg HCN/ 100g was an acceptable level in gari. Therefore, adequately processed gari in Nigeria would contain acceptable levels of HCN. When gari is prepared into "eba", HCN is further reduced to even safer levels. By processing roots into "chickwangue" cyanide reduction of at least 90 percent was achieved (Mahungu et al. 1987).
Drying
Drying is the simplest method of processing cassava. Drying reduces moisture, volume and cyanide content of roots, thereby prolonging product shelf life. This processing is practiced primarily in areas with less water supply.
Total cyanide content of cassava chips could be decreased by only 10-30 percent through fast air drying. Slow sun-drying, however, produces greater loss of cyanide. Sun-dying the peeled cut pieces of roots gave a HCN concentration lower than 10 mg/100g and loss was more effective than oven drying (Mahungu et al. 1987). Drying may be in the sun or over a fire. The former is more common because it is simple and does not require fuelwood.
Boiling
Boiling the peeled roots did not effectively remove HCN. Pounding the boiled roots into "pounded fufu" decreased the HCN concentration by only 10 percent. Therefore, only cultivars containing low cyanide are recommended for this method of preparation (Mahungu et al. 1987).
Processed leaves
Cyanide in pounded cassava leaves ("pondu" or "sakasaka") remained high at 8.6 mg/100g although 95.8 percent of total cyanide in leaves was removed through further processing into soup (Mahungu et se. 1987).
Milling
The dried root pieces and fermented/dried pulp are milled into flour by pounding in mortar or using hammer mills. Milling with hammer mills, done at village level, may also reduce cyanide. The dried cassava roots (both fermented and unfermented) are often mixed in a ratio of 2-3 parts cassava with one part of sorghum, millet and/or maize and milled into a composite flour. Mixing cassava with cereals increases food protein, and enhances palatability by improving consistency.
Processed products
Gari
Fresh roots are peeled and grated. The grated pulp is put in sacks (Jute or polypropylene) and the sacks are placed under heavy stones or pressed with a hydraulic lack between wooden platforms for 3-4 days to express excess liquid from the pulp while it is fermenting. Fermentation imparts an acidic taste to the final product. The dewatered and fermented lumps of pulp are crumbled by hand and most of the fibrous matter is removed. The remaining mass is sieved with traditional sieves (made of woven splinters of cane) or iron or polyethylene mesh. After being sieved, the fine pulp is then roasted in an iron pan or earthen pot over a fire. If the sieved pulp is too wet, it takes longer to roast resulting in a finished lumpy product with dull colour. Palm oil may be added to prevent the pulp from burning during roasting and to give a light yellow colour to the gari. When palm oil is not added, a white gari is produced. Palm oil contains substantial quantities of vitamin A, therefore, yellow gari is 10-30 percent more nutritious and expensive than white gari. The garification or conversion rate of fresh roots into gari is 15-20 %. This value varies with cassava varieties, time of harvesting, age of plant and other environmental factors. Gari is very popular in Nigeria and less so in Cameroon, Benin, Togo, Ghana, Liberia, and Sierra Leone. In Brazil, this method is used for the production of "farinha de madioca".
Peeling is done mainly by women and children. The peeled roots are grated by women, using a simple traditional grater, but it is done by men if a power driven grater is used. Pressing is done by women in the traditional way but done by men when a hydraulic presser is used. The sieved fermented pulp is roasted almost exclusively by women in a pan or pot on the fire with fuelwood as the energy source.
Fermented and dried cassava pulp
"Lafun" in Nigeria, "cossettes" in Zaire and Rwanda, "kanyanga. and "mapanga. in Malawi, and "makopa" in Tanzania are various names for fermented and dried cassava products. The processing method to ferment and dry cassava pulp is very simple and does not require much labor. It is thus widely used for processing high cyanide cassava varieties in many parts of Africa where water for soaking is available. Whole or peeled roots are immersed in water for 3-4 days for fermentation and softening the tissues. The fermenting roots are then removed and broken into small crumbs, sun-dried on mats, racks, fiat rocks, cement floors or roofs of houses. Drying the fermented roots takes 1-3 days, depending on the prevailing weather. The dried crumbs are then milled into flour.
Wet pulp
The processing procedures for "wet pulp" and of fermented and dried pulp production are similar except for the drying. The wet pulp may be molded into balls, 3-5 cm diameter, put in boiling water and stirred thoroughly to obtain a stiff Wet pulp of about 0.5-1.0 kg is packed in a plastic or polypropylene bag and marketed in cities in Nigeria, Ghana and Cameroon. Urban dwellers therefore do not need to buy fresh roots for processing into wet pulp to prepare wet fufu.
Smoked cassava balls ("kumkum")
Cassava is processed into smoked cassava balls in the same way as fermented and dried pulp is produced except that the fermented wet pulp is pounded and molded into round balls of about 4-7 cm diameter. These balls are then smoked and dried on a platform above the fire place in a special structure hung above the hearth. The dark coating caused by smoke is cleaned off and the cleaned balls are milled into flour before reconstitution into fufu (Numfor end Ay 1987).
Chickwangue
"Chickwangue" is the most popular processed food from cassava in Zaire. "Myondo" and "Bobolo" in Cameroon belong to this "Chickwangue" group. Similar products are produced in Congo, Central African Republic, Sudan, Gabon, and Angola.
Cassava roots are peeled, steeped in water for 3-5 days to ferment and become soft. The fermented pulp is taken out and the flares are removed from it. The pulp is then heaped on racks for further fermentation or the heap is covered with leaves and pressed with heavy objects to drain off excess liquid. The pulp is then ground on a stone or pounded in a mortar to obtain a finer pulp. The fine pulp is wrapped in leaves of plantain or any plant of the Zingiberaceae family and tied firmly with fibres from banana. These are steamed in pots. Chickwangue is about 10 cm wide and 20 cm long. Myondo has a diameter of 1.5-2.0 cm and a length of 15 cm to 20 cm. Bobolo has a diameter of 2-4 cm and a length of 30-40 cm. The Gabon "Chickwangue" is smaller in size than that of Zaire.
Starch
Cassava roots are peeled, washed and grated. The grated pulp is steeped for 2-3 days in a large quantity of water, stirred and filtered through a piece of cloth. The filtrate stands overnight and the supernatant is then decanted. The starch sediments are air-dried under shade.
Dried cassava
The roots are peeled, sliced into small pieces and sun-dried on racks or roofs for 4-5 days or sometimes up to 3 weeks, depending on the weather and the size of pieces. Later, sun-dried pieces are milled into flour. This processing system is very simple but the processed products contain considerable amounts of cyanide. This method is widely used in many areas in Africa, particularly where water supply for fermentation is seriously limited.
Processing equipment
Traditional cassava processing does not require sophisticated equipment. Processing cassava into gari requires equipment such as grater, presser and fryer. The traditional cassava grater is made of a flattened kerosine tin or iron sheet perforated with nails and fastened onto a wooden board with handles. Grating is done by rubbing the peeled roots against the rough perforated surface of the iron sheet which tears off the peeled cassava root flesh into mash. In recent years, various attempts have been made to improve graters. Graters which are belt-driven from a static 5 HP Lister type engine have been developed and are being extensively used in Nigeria. Its capacity to grate cassava is about one ton of fresh peeled roots per hour.
For draining excess liquid from the grated pulp the sacks containing the grated pulpy mass are slowly pressed down using a 30-ton hydraulic jack press with wooden platforms, before sieving and roasting into gari. Stones are used in traditional processing to press out the excess moisture from the grated pulp. Tied wooden frames are used for this purpose in places where stones are not available. Pans made from iron or earthen pots are used for roasting the fermented pulp. Fuelwood is the mad or source of energy for boiling, roasting, steaming and frying. Fuelwood may not be easily and cheaply obtained in the future because of rapid deforestation.
Slight changes in the equipment used in processing can help to save fuel and lessen the discomfort, health hazard, and drudgery for the operating women. The economic success of any future commercial development of cassava processing would depend upon the adaptability of each processing stage to mechanization. However, the first step to take for improvement of cassava technologies should be to improve or modify the simple processing equipment or systems presently used, rather than to change entirely to new, sophisticated, and expensive equipment.
Storage of processed products
Processing, particularly drying and roasting, increases shelf life of cassava products. Good storage depends on the moisture content of the products and temperature and relative humidity of the storage environment. The moisture content of gari for safe storage is belong 12.7%. When temperature and relative humidity are above 27°C and 70% respectively, gari goes bad (Igbeka 1987). The type of bag used for packing also affects shelf life depending on the ability of the material to maintain safe product moisture levels.
Jute and hessian bags are recommended in dry cool environments because they allow good ventilation (Igbeka 1987). When gari, dried pulp and flour are well dried and properly packed, they can be stored without loss of quality for over one year. Dried cassava balls ("kumkum") can be stored for up to 2 years (Numfor end Ay 1987). "Chickwangue", "Myondo" and "Bobolo" can be preserved for up to 1 week but they can be kept for several more days when recooked.
Cassava leaves as vegetable
Cassava shoots of 30 cm length (measured from the apex) are harvested from the plants. The hard petioles are removed and the blades and young petioles are pounded with a pestle in a mortar. A variation of this process involves blanching the leaves before pounding. The resulting pulp is then boded for about 30-60 minutes. In some countries, the first boiled water is decanted and replaced. Pepper, palm-oil and other aromatic ingredients are added. The mixture is then boiled for 30 minutes (Numfor and Ay 1987). Unlike the roots that are essentially carbohydrate, cassava leaves are a good source of protein and vitamins which can provide a valuable supplement to predominantly starchy diets. Cassava leaves are rich in protein, calcium, iron and vitamins, comparing favorably with other green vegetables generally regarded as good protein sources. The amino acid composition of cassava leaves shows that, except for methionine, the essential amino acid values in cassava exceed those of the FAO reference protein (Lancaster and Brooks 1983).
The total essential amino acid content for cassava leaf protein is similar to that found in hen's egg and is greater than that in oat and rice grain, soybean seed, and spinach leaf (Yeoh and Chew 1976). While the vitamin content of the leaves is high, the processing techniques for preparing the leaves for consumption can lead to huge losses. For example, the prolonged boiling involved in making African soups or stews, results in considerable loss of vitamin C.
Cassava leaves form a significant part of the diets in many countries in Africa. They are used as one of the preferred vegetables in most cassava growing countries, particularly in Zaire, Congo, Gabon, Central African Republic, Angola, Sierra Leone, and Liberia. The cassava leaves prepared as vegetable are called "sakasaka" or "pondu" in Zaire, Congo, Central African Republic and Sudan, "Kizaka" in Angola, "Mathapa" in Mozambique, "Chigwada" in Malawi, "Chombo" or "Ngwada" in Zambia, "Gweri" in Cameroon, "Kisanby" in Tanzania, "Cassada leaves" in Sierra Leone, "Banankou boulou nan" in Mali, "Mafe haako bantare" in Guinea, and "Isombe" in Rwanda. They are mostly served as a sauce which is eaten with chickwangue, fufu, and boiled cassava.
Utilization of processed products
Utilization in this paper includes cooking or preparation, and consumption. Cooking cassava consists of boiling, steaming, roasting and pounding. The peeled fresh cassava roots are eaten raw or eaten boiled and roasted. The fresh roots are boiled and pounded to obtain "pounded fufu". This is most popular in Ghana, and to some extent, in Nigeria and Cameroon. The processed cassava, either in the form of flour, wet pulp or gari is cooked and eaten in three main food forms: "fufu", "eba" and "chickwangue". The "fufu" group includes "amala" in Nigeria, "fufu" in Zaire, Congo, Cameroon and Gabon, "ugali" and "kowon" in Uganda and Tanzania, "nchima" in Mozambique, "nsima" in Malawi, "ubugali" in Rwanda, and "funge" in Angola.
Gari can be eaten dry or it may be soaked in cold water to which sugar is added. "Eba" is a very popular food in Nigeria and is gaining popularity in Cameroon, Benin, Ghana, Liberia and Sierra Leone because of its fast and easy reconstitution into a convenient food.
"Chickwangue" is a very stiff paste or porridge and is much stiffer than "fufu" and "eba". The size, shape and texture of the "chickwangue" food group vary among countries. "Myondo" and "bobolo" in Cameroon are essentially the same as "chickwangue" in preparation although shapes and sizes are different. "Chickwangue" and its analogues are produced from more hygienic procedures and contain less cyanide but they require much more labor for processing and preparation.
People in Zaire call maniac "all sufficient" because "we get bread from the root and meat from the leaves".
Modes of consumption
Cassava root based foods are all consumed with soups or stews. The soup is essential in the food system in Africa without which most foods cannot be eaten. Soup made of cassava leaves is often eaten with cassava root based foods. Therefore, the cassava root based foods which are essentially carbohydrate are supplemented for protein when consumed with protein rich soups or stews.
Potentials of cassava as animal feed
The major future market for increased cassava production is as livestock feed. Cassava has long been recognized by researchers in Africa as an appropriate animal feed and it has been used as an important and cheap feed in many European countries. Both roots and leaves are usable as food to livestock. Cassava is one of the most drought tolerant crops and can be successfully grown on marginal soils, giving reasonable yields where many other crops cannot do well. It is estimated that approximately 4 million tonnes of cassava peeling-useful as livestock feed-are annually produced as a by-product in Nigeria alone during processing of cassava roots. Therefore, cassava offers tremendous potentials as a cheap source of food energy for animals, provided it is well balanced with other nutrients. There is a great deal of current interest in supplementing feeding of animals with cassava in Africa.
The future of cassava
The future of cassava depends very much upon development of improved processing technologies and of improved products that can meet the changing needs of urban people, and, on its suitability for alternative uses such as animal feeds. Also important is the overall ratings of different products to meet the expectations of producers, transporters and consumers (table 1). Whereas the future is bright, more quantitative information on postharvest aspects of cassava culture in tropical Africa will help scientists orient their efforts to satisfy the many needs of both rural and urban dwellers.
Table 1: Subjective ratings of traditionally processed Cassava products In Africa based on selected features
Feature (parameter)
|
Boiled cassava
|
Fufu
|
Gari
|
Chickwangue
| ||
fermented
|
unfermented
| |||||
| Shelf life | 1 | 5 | 5 | 4 | 3 | |
| Transportability | 1 | 5 | 5 | 5 | 4 | |
| HCN content | 2 | 5 | 3 | 4 | 5 | |
| Ease of: | ||||||
| Processing | 5 | 5 | 5 | 2 | 1 | |
| Preparation | 4 | 4 | 4 | 5 | 2 | |
| Utilization | 4 | 4 | 4 | 5 | 5 | |
| Processing hygiene 5 | 1 | 3 | 5 | 5 | ||
| Total (ratings) | 22 | 29 | 29 | 30 | 25 | |
Note: Subjective ratings for desirability based on scale 1-5, 1 = lowest and 5 = highest.
References
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