Microscopy and texture of raw and cooked cassava (Manihot esculenta Crantz) roots.

Helayne Aparecida Maievesa, Daiana Cardoso De Oliveiraa, Claudia Bernardob, Carmen Maria De Oliveira Müllera, Edna Regina Amanteb*.

aChemistry and Food Engineering Department, Federal University of Santa Catarina, Florianópolis, Santa

Catarina, Brazil.

bFood Science and Technology Department, Federal University of Santa Catarina,

Rod. Admar Gonzaga, 1346, Florianópolis, Santa Catarina, Brazil. CEP 88034001.

E-mail: eamante@cca.ufsc.br.

ABSTRACT

The main goal of this work was to show the relationship between the microscopic characteristic and the hardness of raw and of cooked cassava roots in order to suggest their use in different industrial sectors that produce cassava derivatives. Roots that require shorter cooking time can be suggested for industrial processes that involve heat treatment, such as in cassava ethanol production. Scanning Electron Microscopy (SEM) was used to define the causes of the differences in cooking time and in hardness of the roots studied. Cassava cultivars that have a greater relationship with parenchymatous tissues, pectin, and cellulosic material tend to be harder either raw or cooked. One of the cultivars studied in this work stood out for its high starch yield, which can suggest its use for starch production. Meanwhile, the other cultivars, which can be cooked more easily, are suggested for processes that involve heat treatment of the roots. SEM in conjunction with texture evaluation can discriminate the roots that are ideal for industrial application. Also, information on their agricultural yield and physical and physicochemical properties can help determine the best cultivars for other specific applications.

Keywords: cassava; starch; flour; texture; composition.

INTRODUCTION

Cassava (Manihot esculenta Crantz) is a bushy perennial heliophilous plant of the family Euphorbiaceae. It is resistant to droughts and can easily adapt to very different climate and soil conditions. The part of the plant that is mostly used is its tuberous root, which is rich in starch and is used as food, animal feed, or raw material in several industries [1]. There is a growing interest by industries due to its advantages of easy extraction of starch and also easy access for chemical and enzymatic modifications [2]. The technological responses of cassava roots go beyond their agricultural performance, which is no less interesting, but such responses depend on the properties of this raw material when used in manufacturing processes. When considering the consumption of starchy roots, either raw or cooked, the texture of food is one of the primary attributes to consumers’ acceptance and it is also important when defining their different industrial applications. Besides their technological properties, each food or product has well defined characteristics that are generally perceived as textural properties [3; 4]. In addition to their agricultural yield, the choice among cultivars for requires consideration regarding the properties of plant tissues, in order to meet the expectations for industrial processes with either uncooked or cooked raw materials. Additionally, discussions about cooking time and microscopic characteristics of the roots can help elucidate specific properties of several cultivars of cassava roots. Therefore, the aim of this work was to compare texture properties of raw and of cooked cassava roots of ten different cultivars, in order to establish some relationship among cooking time, texture, and microscopic characteristics.

MATERIALS & METHODS

Cassava cultivars were provided by the EPAGRI (Agency of Agricultural Research and Rural Extension) Experimental Station in Urussanga, Santa Catarina State, Brazil. Harvesting of each of the ten cultivars was performed in triplicate, in different locations and amounting to thirty samples.

Samples from the 2009 harvest were transported from their cultivation sites to the Laboratory of Fruits and Vegetables Technology, at the Federal University of Santa Catarina, Brazil, respecting a maximum time period of 24 hours between harvesting, which was performed in the cooler hours of the day, and processing. After removing excess soil with running water, the roots were weighed and their gross weight was recorded. Then they were peeled and used for the evaluation of hardness of raw roots. The cooking times of cassava roots were determined according to the methodology adapted by Pereira et al. (1985) [5]. For each repetition, five slices (50mm) of roots were put in an aluminium container and cooked in boiling water (1 litre). The same material used for the determination of cooking time was also used for the evaluation for texture of the roots after cooking.

The instrumental texture profile analysis (TPA) of raw and cooked cassava roots occurred as the measure of strength and compression in a texture analyzer TA-ST2i, using a probe P/2 (cylindrical) and an HDP/90 platform, operating under the following conditions: pre-test speed: 2.00 mm/s, test speed: 3.30 mm/s post-test speed: 5.00 mm/s; deformation: 10 mm/s.

Samples were prepared through drying in a forced air oven (Fabbe) at 60o C and afterwards they were ground to 60 mesh. Similar samples were prepared for Scanning Electron Microscopy Analysis. A pool of three repetitions of each of the ten powder samples was fixed with double-side tape onto aluminium cylinders and coated with a layer of gold 350 Å thick in a Polaron E5000 sputter coater. A Philips XL30 scanning electron microscope was used at 20 kV.

All the assays were performed in triplicate, totalling thirty samples, thus corresponding to the ten cultivars. The results were shown as average and standard deviation. Averages were compared by variance analysis, and when any significant difference was detected, they were compared by Tukey test at 5% of significance.

RESULTS & DISCUSSION

The climatic conditions and the age of the cassava plant can affect the quality of its roots. The influence of these factors on the composition of cassava roots originates from the physiological process of growth as well as from the accumulation and mobilization of substances in the roots themselves [6]. Although the cultivars in this study are classified as being appropriate for industrial use, they can be classified as cultivars that require shorter cooking time ranging between 10,40 and 18,56 minutes and showing no statistically significant difference among them for this feature.

Scanning Electron Microscopy images can be useful as reference of the presence of cellulosic and pectic materials linked to starch granules. These materials can hinder their swelling in the cooking process. Figure 1 shows the SEM images of the samples STS 2/03-10 (white root) and STS 2/03 - 7, where it is possible to note that the starch granules are wrapped in a thin parenchymatous structure.

Structures of the cultivars STS 1311/96 – 1, STS 1302/96 – 4, Preta, and STS 1309/96 - 7 visibly show to be more linked with cellulosic and pectic material than the cultivars STS 2/3-10 (white root) and STS 2/03-7. The third group of samples with highest cooking time, represented by the cultivars SCS 252 - Jaguaruna, Mandim Branca STS 1302/96 3 - Vermelhinha, and SCS 253 Sangão showed microscopic structure slightly different from all the other cultivars, as shown in Figure 3.

The samples of STS 1311/96-1, STS 1302/96-4, Preta, and STS 1309/96-7 represent the group that cooked within the second shortest time; however, not in statistical terms. Figure 2 shows the SEM micrographs of these cultivars. The cultivars STS 1302/96-3 - Vermelhinha and SCS 253 Sangão respond differently to the kind of soil. The SCS 253 Sangão does not show statistical difference in terms of texture while the cultivar STS 1302/96 3 – Vermelhinha may have suffered such effect.

The cultivar STS 1302/96 - 4, classified as “difficult to cook”, was superior to the others in terms of hardness, corroborating the results obtained also for microscopic features.

Figure 1 Micrography (SEM) of the cultivars STS 2/03-10 (white root) (A) and STS 2/03 – 7 (B) (1000X).

Samples of raw cassava roots of the cultivars SCS 252 – Jaguaruna, Mandim Branca (“Argissolo” and Neossolo Quartzênico), STS 1302/96 3 – Vermelhinha (“Argissolo”), SCS 253 Sangão (“Argissolo” and “Neossolo Quartzênico”) achieved the lowest values for hardness and they do not show any significant difference regarding this aspect. The cultivars STS 2/03-10 (white roots), STS 1302/96 3 – Vermelhinha (“Neossolo Quartzênico”), STS 1311/96 – 1, Preta, STS 1309/96 – 7, and STS 2/03 – 7 showed intermediary values for hardness. This profile of results differs slightly from the results for cooking time, which reveals that, besides cellulosic and pectic material, the presence, shape, and distribution of starch and of moisture in the plant tissue contribute to the rheological behaviour of the cooked roots. This happens because while the starch occurs in granular shape in the raw tissue, the cooked roots show gelatinized starch, which is the natural characteristic of plant tissue influenced by changes in texture during cooking. Therefore, in addition to the cooking time and hardness of raw cassava roots, the texture of cooked samples must also be known.

Figure 2 Micrography (SEM) of the cultivars STS 1311/96 – 1 (A), STS 1302/96 – 4 (B), Preta (C), and STS 1309/96 – 7 (D) (1000X).

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The cultivars showed the same pattern of texture, which is typical of samples of raw cassava (data not shown). However, they differed in the force needed to break the samples. The maximum forces or rupture points values ranging between 0.19 and 0.30 N.

Figure 3 Micrography (SEM) of the cultivars SCS 252 – Jaguaruna, Mandim Branca, STS 1302/96 3 – Vermelhinha, and SCS 253 Sangão (1000X).

The cooked samples of cassava roots had a behaviour pattern with typical oscillations in hardness obtained according to different cultivars. The range of cooking time observed with the cultivars studied in this work classified them as being of “normal cooking time” according to Cereda and Vilpoux (2004) [1], who described well-cooked roots as being those that have opaque and floury appearance and typical aroma and flavour. These authors classified cassava root cooking, within an industrial profile, as a process that follows three types of behaviour. Type “A” is of normal cooking - when roots becomes soft within 30 minutes at water ebullition temperature at atmospheric pressure. Type “B” is characterized by roots that are cooked at atmospheric pressure and whose consistency is vitreous or waxy. The roots open in the lamellae and show translucence and tastelessness, whereas when cooked in a pressure cooker the cooking process occurs normally or near normally. Type “C” would be those roots with which the cooking process does not occur at all, even when cooked in a pressure cooker. According to the results of this work, there are microscopic differences among the different cultivars of cassava roots studied. According to this evaluation, the texture of cooked samples does not differ statistically. Subsequently, the results for the texture of the cassava roots, both raw and cooked, will be discussed in this paper seeking a greater distinction between the different cultivars.

The texture of raw plant tissues depends on the composition and integrity of the plant itself. The tests for texture were performed with different cultivars whereas their age, standard conditions of storage, means of transportation, and types of cuts were the same. Therefore, the different characteristics detected in the tests were due to the differences among the cultivars of the roots studied. The behaviour of the raw cassava samples, according to the plotting of hardness (N) by distance (mm) (540 graphics were plotted), follows the model where the superficial layer is harder than the underlying layers. Unlike the raw samples, cooked cassava roots showed a typical pattern of texture, with the superficial layers softer than the

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underlying layers. Curves of hardness (N) by distance (mm) (540 graphics were plotted) revealed the profile of the cooked samples showing typical differences according to the cultivars.

CONCLUSION

Cultivars that show shorter cooking times may be suitable for industrial processes that require pre gelatinization of raw materials. However, the use of a texturometer to determine the hardness of the cooked roots can significantly facilitate the decision on the softer varieties for industrial processes involving heat treatment of raw materials. The Scanning Electron Microscope (SEM) helped to define the causes of differences in cooking time and hardness of the roots in this study. Cultivars that are more deeply related with parenchymatous tissues, pectin, and cellulose, tend to be harder, both in raw and in cooked cassava roots.

ACKNOWLEDGMENTS

The authors are grateful to the financial support of CNPq and FAPESC and also to EPAGRI for their partnership and their donation of the samples for this work.

REFERENCES

[1] Cereda, M. P. & Vilpoux, O. 2003. Processamento de Amiláceas Latino Americanas. Cultura de Tuberosas amiláceas Latino Americanas. Fundação Cargill. 4.

[2] Marcon, M.J.A., Avancini, S.R.P. & Amante, E.R. 2007. Propriedades químicas e tecnológicas do amido de mandioca e do polvilho azedo. Editora da Universidade Federal de Santa Catarina, UFSC, Florianópolis, SC, BR. 101p.

[3] Smewing, J. 2001. Hidrocoloides. Textura de los Alimentos. Zaragoza: Ed. Acribia, 273-290.

[4] Revilla, I. & Vivar-Quintana, A.M. 2007. Effect of canning on texture of Faba beans (Vicia Faba). Food Chemistry 106, 310 - 314.

[5] Pereira, A.S., Lorenzi, J.O. & Valle, T.L. 1985. Avaliação do tempo para cozimento e padrão de massa cozida em mandioca de mesa. Revista Brasileira da Mandioca.Cruz das Almas, Bahia, Brazil 47, 27-32.

[6] Lorenzi, J. O. 1994. Variação na qualidade culinária das raízes de mandioca. Bragantia. 53, 237-245.