Introduction

Starch is the main source of carbohydrates synthesized by higher plants. In fruits works as an energy reserve and its concentration varies with maturity (^{Taiz and Zeiger, 2010}), in the case of banana (Musa spp.) at physiological maturity starch makes up 60% of fruits dry matter (^{Jiménez-Vera, 2011}). In the food industry, starch is mainly used as a moisture retention agent, thickener, systems stabilizer and with low cost, moreover, it is the most used raw material for the preparation of biodegradable films, mainly because of its relative abundance and ease of use (^{Bello-Pérez, 2006}, ^{Lin y Zhao, 2007}, ^{Zamudio-Flores, 2011}).

Similarly, pectin is one of the complex carbohydrates found in the composition of the fruits. It is one of the main components of the primary and middle cell wall in plant tissues (^{Arellanes et al, 2011}) and has been employed as an edible coating because it is a non-toxic, biocompatible and biodegradable polysaccharide (^{Sriamornsak et al, 2008}).

Rheology is the science that studies the deformation and flow of materials. The definition of rheological parameters takes into account the laminar flow, in which a fluid is considered as a number of layers sliding over each other. In rheological studies the response of a material to an applied stress or strain is observed, thus the rheology is critical in food manufacturing to understand the way in which a substance moves and behaves to be transported and mixed during processing. Also, the rheology of a product dictates much of the consumer experience, for example, in connection with the texture and mouthfeel (^{Norton et al., 2011}).

To evaluate the rheological profile of polysaccharides such as starch and pectin, the most used model is the Ostwald de Waele or power law:

Where: a (Pa) is the shear stress at the interface of the fluid and the element that produces the shear; k is the consistency coefficient (Pa s ⁿ ); y (s^-1) is the shear rate or deformation in the interface, and n is the flow index (dimensionless). The k and n values describe the behavior of the fluid. The k consistency is an indicator of the viscous nature of the system, resulting that larger k is means more viscous material (^{Mechetti, 2011}). Flow index n is a measure of non-Newtonian behavior. If n < 1, the fluid is pseudo- plastic, if n > 1 the fluid is dilatant, and if n = 1 it is a Newtonian fluid.

In Mexico, Nayarit is the sixth largest producer of banana with a planted area of more than 3 671 hectares and a production of 42 387 tons in 2012 (^{SIAP, 2013}). The 'Pera' (Musa ABB) variety is the most cultivated with 54% of the domestic production, but has the lowest market prices compared to the 'Macho' and 'Manzano' cultivars (^{SNIIM, 2013}) therefore recently some alternatives have been displayed to improve the performance of this crop.

Obtaining polysaccharides from unconventional sources such as banana fruits (Musa spp.) could represent an alternative to give added value to this crop. Unconventional sources are sought to extract starch for two reasons: 1) meet the demand of the industries using starch as raw material or ingredient; and 2) find starches with different functional properties or better than conventional sources (^{Zhang et al., 2005}; ^{Bello-Pérez et al., 2006}); moreover, the potential use of starch and pectin in production of edible coatings could promote the postharvest conservation of fruits (^{Rojas-Graü et al., 2009}).

The literature has reported the rheological profile of polysaccharides in 'Macho' (Musa AAB) banana but no reports of rheological studies were found in 'Pera' banana fruits. Therefore, the objective of this research was to quantify and rheologically characterize the extraction of starch and pectin from 'Pera' (Musa ABB) banana fruits.

Preparation of samples and extraction of starch and pectin

'Pera' Banana fruits from the Mecatán locality of the San Blas municipality, Nayarit, harvested at physiological maturity on January 23^rd, 2012 were evaluated. Fruits were washed, their peel removed and were cut into pieces 0.5 cm wide along the fruit. Then they were stored in a freezer at -18 ± 2 °C, lyophilized (Free zone 2.5, Labconco®) and ground to obtain a flour-like powder, from which the polysaccharides were extracted using as reference the methodology of ^{Peña-Valdivia and Sánchez-Urdaneta (2006)} for polysaccharide extraction in nopalito (Opuntia ficus), adapted for the extraction of starch and pectin in 'Pera' (Musa ABB) banana fruits. The starch was extracted with distilled water and pectin with ammonium oxalate (0.05%) and pH 5.6 in a water bath with constant stirring. The aqueous mixture was centrifuged at 3 500 x 10 g revolutions per min (rpm) for five minutes, the supernatant was precipitated in cold alcohol (96°) at 1 : 4 ( v / v ) ratio and purified by dialysis against distilled water Membra using a Membra-Cel^® MD25 14 x 100 CLR membrane for 72 h. Subsequently, the sample was frozen, freeze dried and finally weighed to quantify the extraction of starch and pectin.

Rheological characterization of starch and pectin

For rheological characterization of starch and pectin from 'Pera' banana fruits, a solution was prepared in distilled water at 1% (w/v) at 50 °C under constant stirring for 24 h (^{Del-Valle et al., 2005}). From this solution 2 mL were taken and placed on the flat plate heater of a controlled stress rheometer (AR2000, TA Instruments®, USA) using an acrylic cone geometry (60 mm diameter, 2° angle, "Gap"= 54 mm and temperature=25 °C), with shear rate range from 0.1 to 300 s^-1.

Data were plotted with the SigmaPlot software (Version 12.0) and the curves were fitted to the Ostwald-De Waele model (power law).

Extraction of starch and pectin polysaccharide

The extraction yield of the starch and the pectin extracted from 'Pera' banana fruits at physiological maturity is presented in Table 1, where the high content of starch contained by the fruit is observed with a extraction yield of 56.53% based on dry matter. These results are higher to those reported by ^{Bello-Pérez et al. (2000}) who reported a 43.8% yield of starch in 'Macho' (Musa AAB) banana. In general, these high starch content is attributed to the fact that this compound is the main reserve carbohydrate synthesized by plants (^{Taiz and Zeiger, 2010}). Also, it has been reported that banana fruits at physiological maturity contain between 50 and 60% starch, thus it may represent an unconventional extraction alternative and could be used in the food industry as an ingredient or to produce edible coatings (^{Bello-Pérez et al, 2000}).

Table 1 Extraction yield of starch and pectin per every 100 g of 'Pera' (Musa ABB) banana dried flour. 

Concerning pectin (Table 1), the extraction yield was 9.73%. These values are higher than those reported by ^{Arellanes et al. (2011)}, who found a 6.64% yield of pectin in 'Manzano' banana and also than those obtained by ^{Vásquez et al. (2008)} who obtained 7.65% of pectin in banana peel (Musa AAB Hartón clone). These results show differences in the extraction efficiency of pectins according to the banana type and part of the fruit used for analysis.

Rheological characterization of starch and pectin from 'Pera' banana

The rheological profile of starch and pectin from 'Pera' banana is presented in Table 2. The rheograms of these polysaccharides were fitted to the Ostwald-De Waele model (power law). The rheology of starch and pectin in suspension in none of the 1 % solutions showed time dependency during deformation. The starch and pectin had a flow index (n) lesser than 1, defined as non-Newtonian fluids, the viscosity of these polysaccharides decreased with increasing shear rate (Figure 1). This is called a shear-thinning behavior and occurs when the applied stress disrupts the arrangement of the matrix molecules.

Table 2 Parameters of the Ostwald-de Waele model (power law) for starch and pectin from 'Pera' (Musa ABB) banana. 

Figure 1 Viscosity of starch and pectin as a function of shear rate of 1% aqueous solutions (w/v) of 'Pera' (Musa ABB) banana 

^{Guerra-Della Valle et al. (2009)} assessed the flow index of three types of starch: native (0.465), acetylated (0.479) and oxidized (0.566) from 'Macho' (Musa AAB) banana, obtaining a non-Newtonian behavior of the shear-thinning type, similar to those reported in the present investigation. The shear thinning behavior of pectin from 'Pera' banana was similar to those reported in Campomanesia xanthocarpa fruits, extracted with water and 0.5% citric acid, where flow index values of 0.710 and 0.690 were obtained, respectively (^{Silva et al, 2010}).

Finally, Figure 2 shows the appearance of 'Pera' banana fruits, starch and pectin evaluated in this study.

Figure 2 (A and B) 'Pera' (Musa ABB) banana fruits; (C) starch and (D) pectin. 

Conclusions

Per every 100 g of dried flour from 'Pera' (Musa ABB) banana fruits an extraction yield of 56.53 % of starch 9.73 % of pectin and was obtained.

The rheological profile of the polysaccharides had a consistency index (Pa s ⁿ ) of 0.0325 (starch) and 0.0140 (pectin) and flow indexes (n, dimensionless) of 0.7225 (starch) and 0.7800 (pectin). Both polysaccharides showed a non-Newtonian flow and pseudoplastic behavior.

The 'Pera' (Musa ABB) banana fruits may represent an alternative source for the extraction of starch and pectin, and could be potentially used in the food industry as an ingredient or to produce edible coatings.