Yam (Discorea sp) drying with different cuts and temperatures: experimental and simulated results

Secagem de inhame (Discorea sp) com diferentes cortes e temperaturas: resultados experimentais e simulados

YAM (Discorea sp) DRYING WITH DIFFERENT CUTS AND TEMPERATURES: EXPERIMENTAL AND SIMULATED RESULTS¹

Romeu FIOREZE²,^*, Bruno MORINI³

SUMMARY

The yam (Discorea sp) is a tuber rich in carbohydrates, vitamins and mineral salts, besides several components that serve as raw material for medicines. It grows well in tropical and subtropical climates and develops well in zones with an annual pluvial precipitation of around 1300mm, and with cultural treatments, its productivity can exceed 30t/ha. When harvested, the tubers possess about 70% of moisture, and are merchandised "in natura", in the atmospheric temperature, which can cause its fast deterioration. The present work studied the drying of the yam in the form of slices of 1.0 and 2.5cm thickness, as well as in the form of fillets with 1.0 x 1.0 x 5.0cm, with the drying air varying from 40 to 70°C. The equating of the process was accomplished, allowing to simulate the drying as a function of the conditions of the drying air and of the initial and final moisture of the product. Also investigated was the expense of energy as function of the air temperature. The drying in the form of fillets, with the air in a temperature range between 45 and 50°C, was shown to be the most viable process when combining both the quality of the product and the expense of energy.

Keywords: yam; drying; time; energy.

RESUMO

SECAGEM DE INHAME (Discorea sp) COM DIFERENTES CORTES E TEMPERATURAS: RESULTADOS EXPERIMENTAIS E SIMULADOS. O inhame (Discorea sp) é um tubérculo rico em carboidratos, vitaminas e sais minerais, além de vários componentes que servem de matéria-prima para fármacos, cresce bem em climas tropicais e subtropicais, e desenvolve-se bem em zonas com precipitações pluviométricas em torno de 1300mm anuais, e com tratos culturais, sua produtividade pode passar de 30t/ha. Quando colhidos, os tubérculos possuem cerca de 70% de umidade, e são comercializados in natura, na temperatura ambiente, o que pode ocasionar sua rápida deterioração. No presente trabalho foi estudado a secagem do inhame na forma de rodelas de 1,0 e 2,5cm de espessura, assim como na forma de filetes com 1,0 x 1,0 x 5,0cm, com o ar de secagem variando de 35 a 70°C. Foi realizado o equacionamento do processo, permitindo simular a secagem em função das condições do ar de secagem e da umidade inicial e final do produto. Foi ainda investigado o gasto de energia em função da temperatura do ar. A secagem na forma de filetes, com o ar em uma faixa de temperatura entre 45 e 50°C, mostrou ser o processo mais viável, quando se combina qualidade do produto e gasto de energia.

Palavras-chave: inhame; secagem; tempo; energia.

1 — INTRODUCTION

The yam (Discorea sp.) is a plant still not very well known in Brazil. It is known as "inhame" in the Northeast and as "cará" in the South and Southeast [7]. Its edible part is the tubers, and it has a cycle of 210 to 270 days between planting and harvest [9].

The yam is an amylaceous root of the family of Dioscoreaceas that contains hundreds of species, and its rhizomes are rich in carbohydrates, vitamins and mineral salts. It is a plant that grows well in tropical and subtropical climates, and develops well in zones with an annual pluvial precipitation around 1300mm.

Its productivity exceeds 30t/ha in fertilised and irrigated cultivations [9], and in its traditional cultivation in the state of Paraíba, without special treatment, the productivity is about 12t/ha, compared with about 9t/ha for cassava under the same conditions [5]. An outline of the tuber, details of the leaves, and a photo of a cultivation [9], with the plants already developed, is shown in Figure 1.

Some yam species have medicinal use, employed to combat malaria, asthma, yellow fever, breakbone fever and diabetes; they are also used as cardiac tonics, sedatives and regulators of the intestinal function. The tubers of many species possess tannins, alkaloids, anti-allergic substances, mucilage and diosgenine. The diosgenine is the raw material used in the synthesis of certain steroids, responsible for its great pharmaceutical value [8].

It is much consumed in countries of Asia as a substitute of rice, and when dehydrated in the form of flour yam can be used in the production of soups, cookies, breads, drinks, puddings, etc. In Hawaii and Indonesia, the yam is frequently consumed in the form of a fermented paste denominated "poi", or "chips", that is similar to fried potatoes, and also another product denominated "kulolo", that is a semi-solid product containing also coconut and sugar, and there are also those products made with the flour, like the yam bread [1].

In Brazil, the yam is usually ingested in the boiled form, in soups or in the form of cakes. However, the lack of industrialisation and conservation methods and the deficiency of popularisation of the nutritious qualities of yam reduce its importance among us, its consumption being limited to the production areas and during harvest time [10].

There are few studies and publications on the yam in Brazil because it is still considered a subsistence culture, not presenting significant commercial and industrial importance in spite of its cultivation being adapted to the several areas of the country, a very good productivity and its good characteristics as an energetic food.

In the Northeast, the largest productive area of yam in Brazil, it is cultivated and merchandised in an informal way, being taken directly from the producer to the outdoor markets, the supermarkets and greengroceries, which results in a lack of data about its production. It is consumed mainly in the boiled form, accompanying, or being the main plate of the meals, as well as in the form of cakes and puddings.

The tuber is merchandised in natura, and it is industrialised only in products like cakes and puddings, merchandised at public-houses and bakeries.

When harvested, the root has around 70% of moisture, and the product is exposed for sale at atmospheric temperature without any conservation, which can cause its fast deterioration. Being a raw material of high productivity per hectare, as well as for its qualities as functional food, it becomes of interest to know its behaviour in dehydration processes, for its more prolonged conservation, for export possibilities, or for the elaboration of new products, such as "chips" or flour for partial substitution of wheat flour in bakery products.

On the other hand, dehydration of products with high humidity can become costly due to the high cost and great quantity of thermal energy required. The objective of the present work was to study the behaviour of the yam tuber in the dehydration process, with different cuts and temperatures, and to derive out the equations to describe the process.

2 — METHODOLOGY

2.1 – Raw Material

The raw material used in the present work was the yam (Discorea alata), acquired at an outdoor market from the city of João Pessoa, Paraíba. At the purchasing time, it was verified that the tubers didn't present any signs of physical damage or deterioration.

In the laboratory, it was washed with water from the public system and was peeled with knives. Soon after, it was cut obliquely in slices of 1.0 and 2.5cm thickness. It was also cut into slices of 5.0cm thickness, which were put into a manual cutter of vegetables, whose matrix of the knives had dimensions of 1.0 x 1.0cm, resulting in fillets with dimensions of 1.0 x 1.0 x 5.0cm.

2.2 – Method

The drying system was constituted by a centrifugal fan, of 1.0 HP, that impelled the atmospheric air through a group of electric heaters (two of 1.0 kW and three of 0.5 kW), switched manually and individually, until approximately obtaining the desired temperature. A heater of small capacity (0.15 kW) was linked to a thermostat, whose bulb was located in the entrance of the drying chamber, for the automatic control of the air temperature.

The drying camber was made up of five trays, of 35 x 35cm, one above other. Four smaller trays were built in such a way that they occupied the whole area in one of those larger trays, which allowed drying four different samples simultaneously in a same test, with the same conditions of the drying air. For each test, one tray contained slices of 1.0cm of thickness, in another one slices of 2.5cm, in the third one fillets, with an initial height of layer of approximately 7cm, and in the fourth tray, the peels of the yam.

Tests were accomplished in a range of temperatures varying from 40 to 70°C with steps of 10°C, without control of the relative humidity of the air, with an airflow of 2.24m³/min passing through the samples.

The trays with the samples were removed periodically and individually from the drier, weighed in a semi-analytic balance, and returned to the drier to continue the drying. At the end of the process, they were placed in an oven at 105°C until they reached constant weight, to obtain the dry weight of the sample. During the drying process, periodically, were also measured the temperatures of the dry bulb and of the wet bulb of the atmospheric air with a psychrometer, as well as the temperature of the dry bulb of the air in the entrance of the drier with a common thermometer.

3 — RESULTS AND DISCUSSION

With the mean values of the temperatures of the dry bulb (T_a) and the wet bulb (T_wb) of the atmospheric air, and of the temperature of dry bulb of the air in the entrance of the dryer (T_d), and through a psychrometric diagram, the relative humidity (RH) of the air passing through the product was determined, as showed in Figure 2.

With the dry weight for each sample, the moisture of the product was calculated for all the drying time, the results graphed as a function of time, and with a graphic extrapolation, the equilibrium moisture content, M_e, was determined for each drying condition.

With the value of the equilibrium moisture content for each test, the moisture ratio, MR, was calculated as function of time:

where:

MR = moisture ratio, decimal

M_(t) = moisture of the product as a function of time

M₀ = initial moisture of the product

M_e = equilibrium moisture content

With the values of the moisture ratio and the drying time, for each sample, there were calculated through linear regression, least squares method, the values of the constants "a" and "b" of Page's equation [6]:

where:

t = drying time, h

The comparison between the experimental and calculated results, for each sample, was done through the Relative Mean Error, RME:

where:

n = number of points

M_exp = experimental moisture

M_calc = calculated moisture, Eq. 2 and 1

For all the samples, the relative mean error was always below 5%, which demonstrated a good correlation between the calculated and the experimental data.

In all the experiments, a great difference was observed between the speed of drying of the slices of 1.0 and 2.5cm of thickness, and of the fillets, which was to a certain extent expected since this last form had a larger area to exchange energy and mass with the drying air, as well as the smallest distance for mass diffusion from inside the product. An illustrative example, for the temperature of 50°C, is shown in Figure 3.

The drying curve of the peel of the yam is not shown in the illustration since it is not the main object of this work; even so, in all the experiments it dried faster than the slices and the fillets since its particles had smaller dimensions.

As the behaviour in the three types of cuts, slices of 1.0 and 2.5cm and fillets, for all the tests were similar to that shown in the Figure 3, it was concluded that the form of fillets was the more suitable for the drying process.

The next step was to find a law of variation for the constants "a" and "b" in equation 2, for the form of fillets, in function of the Vapour Pressure Deficit, VPD [4]:

where:

P_vs = Pressure of water vapour at saturation

P_v = Pressure of water vapour

From the proper definition of relative humidity of the air:

therefore,

The vapour pressure at saturation of the air, is only a function of the temperature, represented by the Equation [2]:

where:

P_vs = KPa (Pa = N/m²)

T_a = absolute temperature, K

Through linear regression, least squares method, there was found the variation law for the constants "a" and "b" of Equation 2:

For the calculation of the relative humidity of the air, the equation of the proper definition of absolute humidity, X was used:

where:

P_b = barometric pressure, kPa

X = absolute humidity of the air, kg vapour/kg dry air

rearranging the equation above, there is obtained:

For the use of the Equation 2, with the constants a and b established by the equations 8a and 8b, there was still the need to have the law of variation of the equilibrium moisture content for the yam, as function of the conditions of the drying air. As this equation was not found in the literature, there were made two tests of yam and cassava drying simultaneously, in the form of fillets, for a long period of time, at 50 and 70°C. At the end of each test, when practically there did not occur any further variation in the moisture of the two products, the difference between the final moisture of the yam and the cassava was 0,4% or less, which allows the use of an equation of equilibrium moisture content obtained for cassava [3] to predict approximately the yam's equilibrium moisture content:

Written in the inverted form is as follows:

where:

M_e = equilibrium moisture content, wet basis

With the equating now complete, it is possible to calculate the drying process for different conditions of temperature and relative humidity of the drying air, as well as the initial and final moisture of the product. As an example of use of the equations, there will be presented a practical procedure. Consider the atmospheric air with the temperature of 30°C and relative humidity of 75%. From a psychometric diagram (schematised in Figure 2) the value of the absolute humidity (X = 0.0020 kg vapour/kg dry air) can be read.

Still consider the yam with an initial moisture of 70%, and that the same will be dried to a final moisture of 12%, with temperatures varying from 35 to 70°C. The model will be used to calculate the total drying time.

For each temperature, the following steps are executed to obtain the time of drying, calculating the variables:

a)Pressure of vapour at saturation, P_vs, Eq. 7

b)Relative humidity of the air, RH, Eq. 10

c)Vapour pressure deficit, VPD, Eq. 6

d)Equilibrium moisture content, M_e, Eq. 11a

e)Moisture ratio, MR, Eq. 1

f)Time of drying, t, Eq. 2

The data obtained in the simulation of the calculation of the time as function of the drying temperature were graphed, and the result shown in Figure 4.

From that figure, it can be observed that the time to dry the product from 70 to 12% at 70°C was 5.4 h, while to do the same at 50°C was 9.3 h. The next stage was to verify if the difference of time corresponded in the same proportion to the difference of air-heating energy.

To do this, for a fixed airflow of 2.24m³/min, one electric resistance was turned on; the temperature of the airflow at the entrance of the dryer was left to stabilise, and then a reading was made. A second electric heater was also turned on, the temperature stabilised, and its value was read. The first and the second heaters together were read, and so on. The results obtained are shown in Figure 5.

It can be observed from Figure 5, that at the temperature of 50°C, 1.4 kW is needed, while for 70°C, 3.0 kW is needed.

From these results it can be observed that the drying time decreased 42% when the temperature was raised from 50°C to 70°C, but the consumption of energy increased 53%.

Still in Figure 4 it can be observed that to dry at 45°C, the total time is 11.5 hs, which is a reasonable time for a process of drying the yam before its deterioration. These results appear to suggest that in a range of temperatures between 45 and 50°C, the product can be dehydrated without problems of safety and without excessive expenditure of energy.

4 — CONCLUSIONS

- The drying of the yam in the form of fillets, cut in the dimensions of 1.0 x 1.0 x 5.0cm, proved to be more viable than the format of slices with 1.0 and 2.5cm of thickness.

- At the temperature of drying at 70°C, the fillets dehydrated in a time 42% smaller when compared with the process at 50°C, but the expense of energy was 53% larger.

- The mathematical simulation showed that the drying of yam fillets in a temperature range from 45 to 50°C is adequate in terms of the quality of the product and in relation to the energy consumed, and evidently, there could still be one tray put above the other to improve energy savings.

5 — REFERENCES

² Professor, PhD - Universidade Federal da Paraíba CT/DTQA – Curso de Pós-graduação em Ciência e Tecnologia de Alimentos 58051-970, João Pessoa, PB - [e-mail]

³ Ex-discente, MSc.

* A quem a correspondência deve ser enviada.

• [1] BEGLEY, W.B. Taro Processing in Hawai: An Economical and Historical Perspective. Westview Tropical Agricultural Series, n. 1, p. 357-372, 1979.
• [2] FIOREZE, R. Adaptaçăo de modelo de Thompson para simulaçăo de secagem de raspas de mandioca (Manihot esculenta sp.) Joăo Pessoa, 1982, 95p. Dissertaçăo (Mestrado em Engenharia Mecânica), Universidade Federal da Paraíba.
• [3] FIOREZE, R. Equaçăo para o teor de umidade de equilíbrio estático para produtos biológicos. Tecnologia e Cięncia, Joăo Pessoa, UFPb, v. 3, p. 9-11, 1994.
• [4] FIOREZE, R. The intermittent drying of agricultural crops with particular reference to energy requirements. PhD Thesis. Cranfield Institute Of Technology, UK, 1986, 153p.
• [5] IBGE. Anuário Estatístico do Brasil v. 55, 1955.
• [6] PAGE, J. Factors Influencing the Maximum Rates of Air Drying Shelled Corn in Thin Layers. Purdue University, MSc Thesis, 1949.
• [7] PAIVA, W.O.; MENEZES, M.T. Comportamento agronômico de clones de inhame no estado de Rondônia. ACTA Amazônica, Manaus. v. 22, n. 1, p. 191-195, 1992.
• [8] PEDRALLI, M. Inhame, esse desconhecido. Cięncia Hoje Rio de Janeiro, v. 8, n. 46, p. 58-62, 1988.
• [9] SANTOS, E.S. Inhame (Discorea sp.) Aspectos Básicos da Cultura EMEPA, Joăo Pessoa, PB, 1996. 154p.
• [10] SILVA, A. A Cultura do Cará da Costa Fortaleza. Banco do Nordeste do Brasil, 1971, 65p.

1

Recebido para publicação em 29/03/00. Aceito para publicação em 03/10/00.

Publication Dates

• Publication in this collection
  20 Apr 2001
• Date of issue
  Aug 2000

History

• Received
  29 Mar 2000
• Accepted
  10 Mar 2000