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Starch stability under the processing conditions can be improved by modifying the granule structure using chemical and/or physical processes. The effect of heat-moisture treatment (HMT) on the physicochemical, morphological, pasting and thermal properties of green banana (

The main sources of commercial starch are corn, wheat, potato and cassava. However, there is a trend towards different sources of natural starch with characteristics and properties that meet the consumer demand and different industrial segments. Thus, the processing of unexplored raw materials, like the starch extracted from green banana (

Several factors can impair the industrial applications of native starch, including low shear stress resistance, thermal decomposition, high retrogradation and syneresis, and structure instability under different temperatures, pH, and pressure conditions (

The HMT involves the treatment of starch under insufficient moisture and temperature values higher than those of gelatinization for a given period of time, leading to structural modifications in the granules without destruction of the structure, with significant impact on the general properties, including starch digestibility (

Several authors have studied heat treatment to modify starch granules, including rice starch (

Green banana (

The heat-moisture treatment (HMT) of banana (

X1
X2
X3
Temperature/°C
CP
100
13
25
Delta
12
6.5
6
Coded level
Actual level
Trial
Repetition
X1
X2
X3
Temperature/
1
1
–1
–1
–1
88
6.5
19
2
1
1
–1
–1
112
6.5
19
3
1
–1
1
–1
88
19.5
19
4
1
1
1
–1
112
19.5
19
5
1
–1
–1
1
88
6.5
31
6
1
1
–1
1
112
6.5
31
7
1
–1
1
1
88
19.5
31
8
1
1
1
1
112
19.5
31
9
1
–1.68
0
0
79.84
13
25
10
1
1.68
0
0
120.16
13
25
11
1
0
–1.68
0
100
2.08
25
12
1
0
1.68
0
100
23.92
25
13
1
0
0
–1.68
100
13
14.92
14
1
0
0
1.68
100
13
35.08
15
1
0
0
0
100
13
25
15
2
0
0
0
100
13
25
15
3
0
0
0
100
13
25
15
4
0
0
0
100
13
25
15
5
0
0
0
100
13
25

CP=central point

where_{w}(g) is the mass of water to be added,

_{i}(%) is the initial moisture of the sample,

_{f}(%) is the desired moisture, and

A mass of 60 g native starch was placed in a stainless steel reactor, and distilled water, according to the experimental planning shown in

The moisture content (AOAC method 945.15) (

The total lipids were determined as described by Bligh and Dyer (

Resistant starch and non-resistant starch were determined using the Megazyme International Reagent Kit (Bray, Ireland), according to the method described by the manufacturer. Samples were incubated in a shaking water bath (Q215m2; Quimis) at 200 rpm with pancreatic α-amylase and amyloglucosidase at 37 °C for 16 h, during which time non-resistant starch was solubilised and hydrolysed to

X-ray diffraction (XRD) analysis of starch was performed in a Bruker D2 Phaser diffractometer (Karlsruhe, Germany) using CuKα (30 kV and 10 mA) radiation in the range of 5º≤2

The pasting properties of green banana (

Starch gelatinization was evaluated using a differential scanning calorimeter (DSC model Q20; TA Instruments, New Castle, DE, USA), as described by Hart Weber

The morphological properties of granules were evaluated by scanning electron (SEM, JSM-6610; JEOL Ltd., Tokyo, Japan) equipped with energy-dispersive spectroscopy (EDS, EDS NSS 2.3; Thermo Fisher Scientific, Waltham, MA, USA), operating at 5 kV, with acquisition software Scanning Electron Microscope (SEM) Control User Interface v. 2.24 (_{2} and then placed in Desk V apparatus (Denton Vacuum, LLC, Moorestown, NJ, USA) for gold deposition for 2 min, using gold as the conductive metal. These analyses were performed in the High Resolution Microscopy Multiuser Laboratory (LabMic), at the Institute of Physics, UFG (Federal University of Goiás, Goiânia, Brazil).

To analyze the effect of the heat treatment on the starch properties, a central composite rotatable design (CCRD) was used with 19 trials and 15 treatments, and five replicates at the central point (

The extraction yield (19.4%) indicated the industrial potential of green banana as a starch source when compared to the varieties commonly used, such as sweet potato (18%), arrowroots (8–16%), yam (18–23%), and Peruvian carrot (5–23%) (

The effect of the variables time, temperature and moisture during the heat treatment on granule morphology was observed by scanning electron microscopy (

Scanning electron micrographs of green banana (

In the diffractogram of the native starch, well-defined peaks (2

The apparent amylose contents of heat-and-moisture-treated starch were affected by the temperature and moisture used in the treatment. The sample of trial 8 contained 58.78 g/100 g apparent amylose, which was higher than the value found in the native starch (53.70 g/100 g), confirming a direct relationship between temperature and moisture and apparent amylose. When the experimental data for water absorption index and swelling power were evaluated, no changes were observed when lower temperatures and moisture were used, with values similar to native starch (1.81 g/g and 13.66 g/g, respectively). Higher temperatures and moisture levels led to a decrease in solubility compared to native starch (9.85%), reaching a value of 5.56%.

Mathematical models in the statistical analysis were fitted to the experimental data of the apparent amylose content (_{Aap}), water absorption index (WAI), swelling power (SP) and solubility index (SI), with the aim to evaluate the behaviour of these properties as a function of the process variables. After regression analysis, the non-significant terms were eliminated. The models (Eqs. 2–5) demonstrated no significant effect of the variable time for all evaluated properties. Although R^{2} values can be considered low for some models, the non-significant lack of fit (F_{aj}) demonstrated that they are suitable for describing the behaviour of these properties. Graphs were generated from the models to better understand the influence of variables in the process (

Response surfaces of: a) apparent amylose content (Eq. 2), b) water absorption index (Eq. 3), c) swelling power (Eq. 4), and d) solubility index (Eq. 5). • Experimental data

(R^{2}=0.47; non-significant lack of fit)

(R^{2}=0.71; non-significant lack of fit)

(R^{2}=0.57; non-significant lack of fit)

(R^{2}=0.78; non-significant lack of fit)

where x is temperature in °C and _{m} is moisture content.

The modified starch viscosity profile is shown in

Effect of modification on green banana starch pasting profile

Statistical adjustments were made from the experimental data with the purpose of generating mathematical models capable of predicting the behaviour of the response variables maximum viscosity (_{max}), pasting temperature (x_{p}), final viscosity (_{final)}), and setback viscosity (_{setback}) of the hydrothermally modified starch (Eqs. 6–9).

(R^{2}=0.83; non-significant lack of fit)

(R^{2}=0.73; non-significant lack of fit)

(R^{2}=0.34; non-significant lack of fit)

(R^{2}=0.49; non-significant lack of fit)

where x is temperature in °C, y is time in h, and _{m} is moisture content in %.

Response surfaces of the pasting properties measured with rapid visco analyzer: a-c) maximum viscosity (Eq. 6), d-f) pasting temperature (Eq. 7), g) final viscosity (Eq. 8), and h) retrograde of starch (Eq. 9). • Experimental data

A gradual increase in the initial and pasting temperatures was observed in the modified starch, when compared to the native starch at the gelatinization level, with higher values in the sample in trial 8 (83.59 and 87.92 °C, respectively) than in the native starch (69.22 and 74.10 °C, respectively). The final temperature required for gelatinization was affected by the starch characteristics since no change was observed (84.83 °C) under mild moisture and temperature conditions (sample in trial 1) compared to the native starch (85.21 °C). Opposite trend was observed for gelatinization enthalpy, with values inversely proportional to the temperature of the modification process. Melting curves, with endothermic peaks, were observed for all samples, which are associated with the gelatinization process.

When analyzing initial temperature (Eq. 10), pasting temperature (Eq. 11) and final temperature (Eq. 12) in the models, linear behaviour was observed as a function of temperature and moisture content, with moisture having the most significant effect. For the gelatinization enthalpy (Eq. 13), a quadratic term was observed for the variable moisture, and a linear term for temperature, which was the most significant.

(R^{2}=0.65; non-significant lack of fit)

(R^{2}=0.80; non-significant lack of fit)

(R^{2}=0.86; non-significant lack of fit)

(R^{2}=0.60; non-significant lack of fit)

where X_{i}, X_{p} and X_{f} are initial, pasting and final temperature in °C, respectively, y is time in h, Δ_{m} is moisture content in %.

The increase in the initial temperature of gelatinization with an increase in temperature and moisture content (

Response surfaces of the thermal analysis regarding the parameters: a-c) initial temperature (Eq. 10), d-f) pasting temperature, (Eq. 11), g) final temperature (Eq. 12) and h) gelatinization enthalpy Δ

The graph of enthalpy of gelatinization (

The digestibility

A linear model as a function of temperature (x) and moisture (_{m}) was obtained for the non-resistant starch content (_{nrs}) (Eq. 14), and despite the low R^{2} value, the significance of the model associated with lack of adjustment (F_{aj}) shows that the model is adequate and satisfactorily describes the behavior trend for non-resistant starch content.

(R^{2}=0.67; non-significant lack of fit)

The content of non-resistant starch (

Response surface of non-resistant starch (NRS) content modified by heat treatment with low humidity according to the adjusted model (Eq. 14). • Experimental data

Mathematical models describing the behaviour of modified starch properties as a function of the evaluated variables were obtained through statistical analysis. The variables time and temperature had a significant effect on the physicochemical, rheological and digestibility properties of starch. The heat-moisture treatment (HMT) has generated modified starch with very distinct properties ranging from the high tendency to retrograde and high viscosity to the low tendency to retrograde and low viscosity. Reduction of resistant starch content was great in the samples subjected to higher temperature and moisture levels, but it was higher in milder treatments, evidencing that the green banana starch granule is relatively fragile under the most rigorous hydrothermal conditions. The results showed that heat-moisture treatment can be an effective method to improve heat stability and shear stress resistance of banana starch.

The authors are grateful to Coordination for the Improvement of Higher Education Personnel (CAPES) for the financial support and scholarship.

^{®}University Edition, SAS Institute Inc. Cary, NC, USA; 2017. Available from: