characterization of edible films of cassava starch by...

AUTORESAUTHORS

PALAVRAS-CHAVEKEY WORDS

Braz. J. Food Technol., 3:91-95, 2000 91 Recebido / Received: 10/04/2000. Aprovado / Approved: 27/07/2000.

✉ Marney Pascoli CEREDA

Celina Maria HENRIQUEMarcelo Álvares de OLIVEIRA

Marcelo Vieira FERRAZNívea Maria VICENTINI

Starch; Edible films; Post harvest; Foodstuffs / Amido;Filmes comestíveis; Pós-colheita; Alimentos.

On a world level there is a search for new ways to reduce post harvest losses, andprotect perishables in the most natural ways, without affecting the environment. This researchdescribes the replacement of some petroleum derivatives by edible films made from cassavastarch. Protein films have already been well studied. Starch films have been studied as a weakbarrier for reducing weight loss, however little is known about their sub-microscopic structure.Conflicting results require a better understanding of these films. This study aims to characterizethe starch films of naturally occurring and (highbred) modified cassava using scanning electronmicroscopy, with the objective of explaining their behaviours on the surfaces of fruits, vegetablesand flowers. The observations showed that the films can present both continuous and poroussurfaces. Some of these variations in surface texture could be explained by the type of starchused and the way the films are made.

Mundialmente buscam-se embalagens que reduzam as perdas pós-colheita, ao mesmotempo que protejam o alimento da forma mais natural possível, sem prejuízo do meio ambiente.Essa busca propõe a substituição de filmes obtidos a partir de derivados de petróleo por filmescomestíveis obtidos de proteína, amido, etc. Os filmes de amido já foram testados comoembalagem para frutas e hortaliças, porém pouco se conhece sobre a sua estruturasubmicroscópica. Os resultados de pesquisa têm-se mostrado conflitantes, exigindo uma melhorcompreensão destas películas. Este trabalho tem por objetivo caracterizar os filmes de fécula demandioca nativa e modificada, usando a microscopia eletrônica de varredura, na tentativa deelucidar o comportamento dos mesmos perante frutas, hortaliças e flores. Observou-se que osfilmes podem apresentar tanto uma superfície contínua, quanto uma superfície contendoporos, com variação também da textura conforme a natureza do amido utilizado, o que explicaas variações observadas.

Characterization of Edible Films of CassavaStarch by Electron Microscopy

Caracterização Microscópica de FilmesComestíveis de Fécula de Mandioca

SUMMARYProf.ª Dr.ª Pesquisadora do Centro de Raízes eAmidos Tropicais (Cerat) - FCA/UNESP

Caixa Postal 237 - 18603-970 Botucatu-SPFone/Fax: (014) 6821-9050

e-mail: [email protected]

Pós-graduandos da Faculdade de CiênciasAgronômicas da UNESP

Caixa Postal 237 - 18603-970 Botucatu-SPFone/Fax: (014) 6821-9050

e-mail: [email protected]

RESUMO

Braz. J. Food Technol., 3:91-95, 2000 92

M. P. CEREDAet al.


1. INTRODUCTION

The post harvest losses in Brazilian agriculturalproduction are as much as 30% of the total production. This isdue to the inadequate use of conservation techniques,fungicides, humidity and temperature control, waxes, etc.(OLIVEIRA, 1996).

The use of packaging, aimed at controlling excessiveweight loss in fruits, vegetables, and flowers, slows downevaporation and reduces both respiration and gas exchange.When these factors are controlled, the product senescence canbe reduced and its shelf life increased. Plastic films, waxes, andmore recently biofilms, are currently used to control thesefactors.

Literature about biofilms is very limited. Biofilms arebuilt from a biological macromolecular base that has the abilityto form a continuous matrix (KESTER, FENNEMA, 1986).Common biopolymers used to make these biofilms are starch,pectin, cellulose and its derivatives, collagen, gelatine, andmyofibrillar proteins. Depending on the raw material used,these films can be classified as either biodegradable or edible(CARVALHO et al., 1997).

To make edible films, it is necessary to disperse ordissolve the macromolecules in a solvent (water, alcohol ororganic acids) in order to obtain a film-forming solution whichcan be applied directly to the product. Evaporation of thesolvent leaves a membrane on the surface of the product(GONTARD, 1994).

Interest in edible films and coatings using biopolymershas increased in recent years, as these films are able to conserveand protect foodstuffs without causing a negativeenvironmental impact (SOUZA et al., 1997).

Biofilms made from starch biopolymers began to bemore intensively studied when cassava starch was selected asthe most suitable raw material (CEREDA et al. 1992). The processof obtaining biofilms from cassava starch is based on the starchgelation at high temperatures with an excess water. Aftergelation, the amylose chains tend to be sufficiently close thatthey link together by means of hydrogen bonding. On drying,there is a contraction and consequent expulsion of the waterlinked to the molecules, allowing the gel to form stable films(BOBBIO, BOBBIO, 1992). This is also an alternative source formaking edible films to be used in the conservation of fruits,vegetables, flowers and other agricultural, natural or processedproducts.

The granules of cassava starch observed by SARMENTO(1997) using optical microscopy had various shapes, the mostcommon being round.

The main factor controlling the swelling behaviour ofthe starches is the strength and character of the mycelial netwithin the granule, which that is dependant on the level andtype of the association (LEACH, 1965).

When the granules start to swell by the application ofheat, there is a large increase in the solution viscosity. Thetemperature at which this change starts, is called the setting

point, while the temperature at which the initial viscosityincreases by 10 Brabender units is called the thickeningtemperature. SARMENTO (1997) found that values from 60.5°Cto 65.8°C initiate gelatinization, whilst 61.2°C to 66.5°C wasthe thickening temperature range. With cooling to 50°C, asmall rise in viscosity was noted, indicating a small retrogradetendency, or even crystallisation (SARMENTO, 1997).

The initial work of CEREDA et al. (1992, 1995) showedthat the use of cassava starch films was promising, giving agood appearance, without stickiness) exhibiting shininessand transparency; improving the visual appearance ofvegetables, without being toxic, being ingested together withthe protected product or easily removed by washing withwater, and reducing the commercial unit costs by somewherebetween US$ 0.30 and 0.40. This earlier research pioneeredthe use of cassava starch coatings and has given a number ofdifferent results when used to treat fresh vegetable products.

Results from post harvest research have already shownthat cassava starch films demonstrate not only variedbehaviour as a water loss barrier but also influence thecoloration of the products.

Research has shown that the concentration of thestarch suspension influences film formation. On the guava,this concentration is somewhere between 1 and 2%(OLIVEIRA, 1996). Fruits covered with cassava starch filmshave shown less weight loss than controls and with lowerlevels of respiration. Coloration changes in these treated fruitswere very slow.

HENRIQUE (1999) studied the effect of the use ofsuspensions of cassava starch at concentrations of 0, 1, 2and 3% on the discoloration of Sicilian lemons using Ethrel.The best conser vation level was obtained at aconcentration of 3%, with the best shine and lowest waterloss. The chemical composition was not changed, and therewas only partial discoloration. Nevertheless, after 10 daysstorage, the film began to attack the fruit. The 2%concentration films were proportionally less shiny and thediscoloration was complete.

VICENTINI et al. (1999) working with 1 and 3%concentration cassava starch films in the coating of redpeppers c.v. Magali, stored at room temperatures (25ºC),observed that the fruits coated with both concentration filmslost the same weight as the controls. The weight losses wasreduced but the red peppers had good colour at the end ofthe storage and the natural discoloration of the fruits wasnot occured.

The use of natural and modified starch in theconservation of cut rose flowers has also been well studied.In this case, in addition to the use of natural cassava starch, aderivative CMA (carboxymethyl starch) was used because ofits dispersion properties in cold water (FERRAZ, 1998).

The objective of this study was to characterize coatingsof both natural and modified cassava starch using a scanningelectron microscopy (SEM), as a method of learning moreabout their structures on the surface of fruits, vegetables andflowers.


M. P. CEREDAet al.


c

Commercial, natural and modified starches providedby the Brazilian industry were used in this study, includingAmylum 320, an esterified derivative and CMA, an oxidisedcassava starch. These two commercial products are obtainedby similar reactions of esterification and oxidation, respectively.All products were used at the same 3% concentration.

Starch suspensions (3% weight/volume) were obtainedby heating to 70°C, with constant mixing. Starch gelatinizationwas complete after 15 to 20 minutes. The mixtures were thencooled to room temperature, approximately 25°C (OLIVEIRA,1996). With CMA, heat was not necessary, so the suspensionwas obtained by mixing at room temperature. To obtain theAmylum 320 suspension, the product was first dissolved inalcohol and then mixed with water. The film-forming solutionswere then dispersed on 10 cm diameter polystyrene platesand subjected to blow drying at 50°C for 24 hours under sterileconditions. The thickness of the film was controlled by volume/area ratio of the film-forming solution used.

The cassava starch granules and microstructures of thefilm surfaces were analysed using an scanning electronmicroscopy (SEM). The samples were mounted on aluminiumstubs and coated with a gold film (40 � 50nm).

2. MATERIALS AND METHODS

3. RESULTS AND DISCUSSION

Granules of the three types of cassava starch; natural(a), esterified -CMA (b) and oxidised - Amylum 320 (c), can beseen in Figure 1.

This confirms that the appearance of the granules aresimilar, presenting shapes as cited by SARMENTO (1997), exceptfor (b) which shows deformations in the centre with somesurface roughness, contrasting with both (a) and (c), whichappear smooth.

Photomicrographs of the films formed showcontinuous surfaces for natural starch (Figure 2), CMA (Figure3) and Amylum 320 (Figure 4). However, small pores can alsobe seen in the natural starch film (Figure 2b), probably causedby micro bubbles of air dissolved during heating and coating.These pores could explain some of the contradictory resultsfrom earlier studies; such as CEREDA et al. (1992), CEREDA etal. (1995), OLIVEIRA (1996), HENRIQUE (1999), and VICENTINIet al. (1999). The photomicrographs show that the starch filmscould provide a continuous surface with controlled porosity,similar to other films, such as polyethylene.

Figure 4 shows that films of Amylum 320 maintainedan intact granular structure and did not disperse in water. Thisbehaviour is different from natural starch and CMA. Thisobservation could explain the appearance of the film (Figure3), that shows plates of granular starch with little structuralchange attached to thin films. Probably, during gelation, largequantities of amylose and amylopectin were able to migrate tothe surface of the granule. These bonds formed a fragile networkwhich was even destroyed by the impact of the electron beamduring microscopy.

b

a

FIGURE 1. Micrographs of the cassava starch granules:(a) natural, (b) esterified - CMA, and (c) oxidised - Amylum 320,taken by SEM at magnifications of 137x, 242x and 264x,respectively.

FIGURE 2. Micrographs of the surface of natural cassava starchfilms at 3% concentration (wt/vol), using SEM at (a) 372x and(b) 552x magnifications.

a

b


M. P. CEREDAet al.


a

b

FIGURE 3. Micrographs of the surface of cassava starch filmsusing CMA at 3% concentration (wt/vol), by SEM at (a) 8x and(b) 463x magnification.

FIGURE 4. Micrographs of the surface of the cassava starchfilm Amylum 320 at 3% concentration (wt/vol), by SEM at 405xmagnification.

The structure of the starch film, could be explained bythe theory of EL-DASH (1996), which states that the three-dimensional structure of polysaccharides displays visco-elasticproperties, since they expand under tension and return to theiroriginal size when the tension is relaxed.

Obviously if the applied force is too strong, thehydrogen bonds can be broken causing a collapse of thestructure. The structure can probably be strengthened by thenumber of hydrogen bonds, but this would probably reducethe capacity of the structure to absorb water and gases. Theequilibrium between strength and the capacity to absorb waterand gases must be determined for each different product. Alsoit is possible to strengthen the connection between the layersof the polysaccharide lattice by modifying the starch moleculestructure, hence permitting the formation of covalent bonds atcertain points along the length of the molecule.

The hypothesis proposed by EL-DASH (1996) and theabove photomicrographs allow for the clarification of thevariations found in the use of cassava starch films, and for thebuilding of a model for the water diffusion of starch coatingsfor fruit, vegetables and flowers. Since the gelified starch ishygroscopic, the starch film will absorb vegetable moistureand this moisture will migrate to its surface and disperse intothe atmosphere. In the case of coating, having pores, thetransfer of water to the atmosphere is more accentuated. Onthe other hand, controlling the agitation during the heating/cooling cycle and de-aerating the suspension before use, couldproduce films with different water and gas dispersionproperties.

In relation to other types of film used as barriers,additive free starch should behave as a weak barrier to waterloss, unless other special films have been made. If this theoryis confirmed, it may be possible to modify this starch film,adapting them as necessary for each raw material and workcondition.

4. CONCLUSION

Suspensions of natural cassava starch or those of derivedstarch such as oxidised (Amylum 320) or esterified (CMA) at3% concentration, have the ability to form continuous orporous films, depending on the method of preparation.Suspensions of CMA (esterified cassava starch) formdiscontinuous and fragile structures, without preventing theformation of individual granules.

ACKNOWLEDGEMENTS

The authors would like to thank FAPESP for the financialsupport.

REFERENCES

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