Effect of roasting conditions on color development and Fourier transform infrared spectroscopy (FTIR-ATR) analysis of Malaysian-grown tropical almond nuts (Terminalia catappa L.)
© Ng et al.; licensee Chemistry Central Ltd. 2014
Received: 19 June 2014
Accepted: 22 August 2014
Published: 7 September 2014
Proper roasting is crucial to flavor, color, and texture development in the final product. In recent years, several research studies have been carried out to establish the best optimum roasting conditions for some common edible nuts such as; hazelnut, peanut, and pistachio nut. Although roasting is an important process for nuts and oilseeds, there is little or no information on the development of color, aroma, and textural changes in Terminalia catappa nuts during roasting.
Results showed that color formation and browning index were significantly (P < 0.05) influenced by the roasting temperature and time of roasting. However, the fracturability of nuts was significantly (P < 0.05) affected by both temperature of roasting and time as well as pH. The optimized results showed that the best response was reached when the roasting time was 29.9 min, roasting temperature 174.5°C, and pH 6.08, respectively. Moreover, the 3400–15603400–1560°Cm-1 carbonyl region for carboxylic acid, alkenes, esters, and amines was found to provide a flavor-print of the roasted tropical almond nut. While, increase in temperature did not produce new carbonyl compounds, it however led to higher concentration of compounds. Scanning electron microscopy of the almond nuts showed that the starch granules were embedded in tissues.
On a global basis, almonds ranked number two after cashew nuts in tree nut production with 2,560,000 metric tons in 2010 . According to FAO  report, the global consumption of the edible nut was reported as 2.1 kg per person per production. Terminalia catappa Linn is a tropical almond nut and a member of the Combretaceae family . The dried nuts are sometimes consumed naturally or in most cases they can be thermally processed prior to consumption.
Hot air roasting is a common practice to which nuts are subjected to before being used as a snack item or before being incorporated into food ,. Roasting is one of the methods used in developing sensorial properties of nuts. It also deactivates enzymes that accelerates nutrient destructions and eliminates unwanted microorganisms, and food contaminants -.
For roasted products, brown color plays an important role in consumer's acceptability and preferences. In recent years, there have been limited literatures on the kinetic studies of color development in roasted products. Therefore, establishing an optimum brown color for the roasted products is a major objective of roasting ,. Color development has been shown to dependent on factors such as; raw sample's pH, and the roasting conditions (i.e. roasting temperature and roasting time) . For example, studies have revealed that the optimum roasting conditions for macadamia nuts and peanuts were 135°C for 20 min; and 180°C for 45 min ,, respectively.
Besides brown color, texture and aroma are also some of the major characteristics that contribute to the quality of roasted products. The development of brown color and aroma are phenomena that results from the Maillard reaction. Generally, analyses of aroma compounds in roasted nuts have been carried out qualitatively and quantitatively by using gas chromatography–mass spectrometry (GC-MS), and gas chromatography-olfactometry (GC-O) -. These approaches are time-consuming. Recently, the Fourier Transform Infrared (FTIR)-attenuated total reflection (ATR) spectroscopy has been employed. This is a simple, rapid, high sensitivity and easy to monitor technique. This technique has been used for discriminating different genotypes and origins of roasted coffee, and the degree of roasting temperature , cashew nut shell , pistachio-nut shell , brewed coffee  and almond oil  respectively.
Proper roasting is crucial to flavor, color, and texture development in the final product. In recent years, several research studies have been carried out to establish the best optimum roasting conditions for some common edible nuts such as; hazelnut , peanut , and pistachio nut . Although roasting is an important process for nuts and oilseeds, however, there is little or no information on the development of color, aroma, and textural changes in Terminalia catappa nuts during roasting. Therefore, this research study is aimed at characterizing the roasting conditions that would produce almond nuts with desirable color, browning index, and fracturability; meanwhile, changes in the functional groups and flavor-print of nuts during roasting were evaluated using Fourier transform infrared (FTIR-ATR) analysis.
Results and discussion
Regression coefficients and ANOVA for color (L, a, b), browning index and fracturability
b 1 2
b 2 2
b 3 2
R 2 (adj)
Regression (P value)
Lack of fit (F value)
Lack of fit (P value)
ANOVA and regression coefficients of the first- and second-order polynomial models
X 1 2
X 2 2
X 3 2
X 1 X 2
X 1 X 3
X 2 X 3
Effect of different roasting variables on color, browning index and fracturability of almond nuts (Terminalia catappa)
Colors L, a, and b
Unlike color L, color a, and b values increased as roasting temperature and time were increased (Table 1). The color changes were most probably not due to enzymatic browning because enzymes associated with enzymatic browning had been destroyed at temperatures ≥100°C. Thus, changes in colors a, and b might be due to internal browning. From Eq. 2 and 3, quadratic term was fitted for predicting the color a value; whereas linear term was fitted for color b. The increase in color a value denotes a redder Chroma, which is indicative of browning reaction . According to Hodge , redness of roasted products increases as temperature increases for all yellow materials and this color changes might be associated with Maillard reactions.
Apart from the influence of roasting temperature and roasting time, colors L and b were not significantly (p > 0.05) influenced by the pH of the almond nuts; however, pH had significant influence on the color a. Higher pH produces a darker color (i.e. decrease in color L) .
The extension of brown color is recognized as browning index (BI) of the products . As observed in Eq. (4), a squared term was fitted for predicting the BI value. Results revealed that the BI value was increased as roasting temperature increased. BI value was insignificantly (p > 0.05) influenced by linear effect of roasting temperature; however it was significantly (p < 0.05) affected by the squared terms of roasting temperature (Table 1). During roasting, chemical reactions of phospholipid compounds of the nuts enhance the development of brown pigments which give the roasted products darker color, and therefore BI value increased . The optimum BI (25.85) was predicted to be obtained when roasting time was 29.9 min, the roasting temperature was 174.5°C and pH 6.08 respectively.
As revealed in Table 1, texture (fracturability) of roasted almond nuts was significantly (p < 0.05) affected by both the linear and quadratic terms of roasting temperature, time and pH. A surface plot displays the effect of roasting temperature, roasting time and pH on the fracturability (Figure 1b). As observed in Eq. (5), a full quadratic term was fitted for predicting the fracturability value. The 3D surface plot revealed that increasing roasting temperature up to 150°C resulted in increase in the fracturability. However, further increase in temperature led to a gradual decrease in fracturability. Similar trend was observed with time of roasting. Increase in the pH led to significant increases in fracturability. The optimum fracturability (1107.62 g/s) was predicted to be obtained when roasting time was 29.9 min; the roasting temperature was 174.5°C and pH 6.08 respectively.
Optimization and validation procedures
Both multiple graphical and numerical optimization were established to determine the exact optimum point of the different roasting conditions on the color, browning index, and fracturability of almond nuts leading to the desirable response goals. The final reduced models were expressed as three-dimensional (3D) response surface plot to obtain a better visualization and understanding of the interaction effect of main roasting conditions on the color, browning index and fracturability of almond nuts. The optimum roasting process performed at 29.9 min, 174.5°C and pH 6.08 were recommended for producing roasted almond nuts with optimum quality. The predicted response values for color L, color a, color b, browning index and fracturability were found to be 44.93, 3.64, 8.98, 25.85 and 1107.62, respectively. All response models were verified theoretically. The experimental data were compared with fitted values to verify adequacy of final reduced models by using 2-sample-t-test (results not shown). There was insignificant differences (p > 0.05) between experimental and predicted values. This indicates that the predicted models are able to describe the response variables satisfactorily.
FTIR functional group composition
Frequency range (cm-1)
Class of compound
C = O stretching
The presence of carboxylic acid existing in almond nuts are indicated by the broad absorbance peak of O-H stretching vibration between 3400 and 2400°Cm-1. This observation certainly indicates that the compound is a carboxylic acid because the O-H stretch appeared in the spectrum as a very broad band which centers on 3000°Cm-1 and partially obscures the C-H stretching bands . The presence of alkanes is indicated by the strong absorbance peak of C-H vibrations between 3000 and 2800°Cm-1 and the C-H deformation vibrations between 1475 and 1350°Cm-1. Absorption bands at 2922°Cm-1 and 2857°Cm-1 correspond to asymmetric and symmetric stretching vibrations of methyl (CH3) groups, respectively. The sharp and narrow band observed at 1743°Cm-1 is assigned to C = O stretching vibration of ester groups in triacylglycerol -.
The absorbance peaks around 1743°Cm-1 represent the C = O stretching vibration indicating the presence of ester. Although some ester carbonyl groups may appear in the same general area as ketones, ketones can be eliminated in this study by observing the strong and broad C-O stretching vibrations that appeared in region 1300-1000°Cm-1 where ketonic absorptions appeared as weaker and narrower bands. Two bands appeared for the C-O stretching vibrations in esters in the range from1300-1000°Cm-1 (Table 3). The aliphatic secondary amines absorbed near 1500°Cm-1 (about 1535°Cm-1), the N-H bending vibrations was very weak and usually not observed.
The major differences in the percentage of transmittance (%T) from light roast to medium roast can be found from three compounds: ester, carboxylic acid and amines. The increase in %T was observed in ester (at 1000-1300°Cm-1); while decreases were observed in the carboxylic acid (around 3289°Cm-1), ester (around 1743°Cm-1) and primary and secondary amines (around 1637 and 1535°Cm-1, respectively). The development of the ester and acid flavor compounds seems to give medium-roasted nuts a more desirable, enhanced and stronger nutty-roasted aroma and flavor compared to light-roasted nuts. Similar results were obtained by Donald et al. on the analysis of brewed coffee. The increase in %T in ester might be due to the release of volatiles during roasting of nuts whereas the decrease of %T in primary and secondary amines might be due to the Maillard reaction and the formation of color and aroma. Results of the `medium roast' to `dark roast' revealed major alterations in the transmittance of the carbonyl compounds. There were increases in the %T of carboxylic acid (at 2400-3400°Cm-1), esters (around 1047°Cm-1), primary amines (around 1636°Cm-1) and secondary amines (around 1533°Cm-1) respectively. There were also decreases in the amount of esters (around 1743°Cm-1 and at 1236-1240°Cm-1). These changes are compatible with the sensory panelists' evaluations of a stronger aroma, taste and aftertaste of nuts. For dark roast, the longer heating time promotes caramelization of sugar. Overall, there was significant increase in %T from raw nuts to roasted nuts in terms of the carboxylic acid, primary amines and secondary amines; however, there was significant decrease in %T for esters formation from raw nuts to roasted nuts.
Scanning electron microscopy (SEM)
The observations revealed that the almond nut is mainly composed of globular structures, where oval-shaped starch granules were distributed throughout the cell. However, this distribution could only be visualized in raw nuts and low-level roasted nuts (100°C for 5 min). The globular structures were possibly starch granules because the starch granules were larger than protein bodies recorded by previous studies -. Theoretically, protein bodies should be observed from the images captured since nuts are rich in protein (~17%) (Ng, Lasekan, Muhammad, Sulaiman, Hussain: Physicochemical properties of Malaysian-grown almond nuts; forthcoming). However, SEM had limitation in the capability to assess the differences.
For the raw almond nuts (Figure 3(a)), the surface was quite smooth, without any pores, except for some occasional cracks. Heating treatment on the almond nuts from low temperature (100°C) to high temperature (180°C) changed the smooth surface into rougher surfaces (Figure 3b-d). Without roasting, the almond nuts contained moisture content that maintained and supported the structure of the granules, thus the surface was smooth and there were more granules in large sizes compared to those roasted at high temperatures. As roasting temperature increased, the moisture was evaporated and diffused out from the sample. This process caused the larger globules to disintegrate and micropores seem to be developed on the surface indicating the release of volatile matter. In terms of compactability, the granules became more compact after roasting from 100°C to 140°C as shown from Figure 3(e) to (h).
The present study showed that the color formation and browning index in roasted tropical almond nuts were significantly (p < 0.05) influenced by roasting temperature and roasting time; while the fracturability of roasted almond nuts was significantly (p < 0.05) influenced by roasting temperature, roasting time and pH. The optimum roasting process was attained at 29.9 min, 174.5°C, and pH6.08. Moreover, the 3400–1560°Cm-1 carbonyl region for carboxylic acid, alkenes, esters, and amines was found to provide a flavor-print of the roasted tropical almond nut. Increasing the roasting temperature as observed with the different roasts did not produce new carbonyl compounds, but led to increases in concentration as reflected by the percentage of transmittance of compounds. Scanning electron microscopy of the almond nuts showed that the starch granules were embedded in tissues.
The matrix of central composite design (CCD) and experimental data obtained for the response variables studied ( Y 1 - Y 5 ) (mean ± SD)
Temperature (°C),X 1
Time (min),X 2
Colour L,Y 1
Colour a,Y 2
Colour b,Y 3
Browning index,Y 4
Fracturability (g/s),Y 5
53.10 ± 0.54a
3.85 ± 0.05c
8.41 ± 0.17b
22.41 ± 0.23c
1315.48 ± 3.36b
43.85 ± 0.08d
2.19 ± 0.04e
5.32 ± 0.07e
16.50 ± 0.22d
1221.65 ± 3.23c
47.17 ± 0.45c
2.85 ± 0.12d
9.04 ± 0.29a
25.51 ± 0.71b
1546.31 ± 15.09a
48.41 ± 0.39c
2.82 ± 0.07d
7.39 ± 0.07c
20.70 ± 0.19c
1289.27 ± 5.87c
49.12 ± 0.29bc
2.83 ± 0.13d
7.23 ± 0.11c
19.80 ± 0.06d
1419.53 ± 20.03b
48.67 ± 0.08c
4.79 ± 0.09a
8.51 ± 0.08b
26.12 ± 0.50b
1117.77 ± 8.20d
41.71 ± 0.09d
4.97 ± 0.09a
9.76 ± 0.27a
35.18 ± 1.10a
920.68 ± 17.92e
49.40 ± 0.32bc
2.79 ± 0.08d
7.33 ± 0.21c
20.06 ± 0.49c
1247.72 ± 11.96c
50.78 ± 0.42b
3.00 ± 0.06d
7.98 ± 0.08c
21.28 ± 0.11c
1216.48 ± 14.16c
51.21 ± 0.41b
2.22 ± 0.02e
6.79 ± 0.15d
17.29 ± 0.24d
1147.99 ± 17.68d
49.37 ± 0.57bc
2.82 ± 0.05d
6.59 ± 0.11d
18.40 ± 0.20d
1430.61 ± 17.06b
43.82 ± 0.06d
4.41 ± 0.07b
7.02 ± 0.04d
24.68 ± 0.08b
1146.64 ± 8.53d
50.54 ± 0.35b
3.22 ± 0.03d
7.52 ± 0.09c
20.65 ± 0.02c
1251.09 ± 16.27c
36.75 ± 6.37e
3.11 ± 0.14d
4.84 ± 0.15e
20.21 ± 0.56c
981.28 ± 19.17e
49.10 ± 0.32bc
2.93 ± 0.06d
7.27 ± 0.04c
20.27 ± 0.25c
1302.49 ± 16.28b
51.76 ± 1.03b
4.18 ± 0.08b
8.49 ± 0.31b
23.69 ± 0.24b
1215.75 ± 16.54c
45.53 ± 0.49d
2.61 ± 0.03de
7.06 ± 0.21d
20.91 ± 0.33c
991.15 ± 5.14e
50.49 ± 0.54b
1.51 ± 0.03f
7.42 ± 0.12c
24.86 ± 0.12b
1679.96 ± 23.81a
51.06 ± 0.37b
2.37 ± 0.11e
6.74 ± 0.25d
17.46 ± 0.84d
518.60 ± 10.11f
42.88 ± 1.29d
5.11 ± 0.18a
10.06 ± 0.44a
34.93 ± 0.93a
665.62 ± 12.26f
The tropical almond fruits were collected between January and February, 2014 from the forestry Department of the University Putra Malaysia, Serdang. In all, about two thousands almond seeds were obtained.
37% Hydrochloric acid, 0.01 N NaOH were purchased from Merck.
Sample and preparation
Independent variables and levels established through the central composite design for nuts roasting conditions
Independent variables level
Roasting temperature (°C)
Roasting time (min)
Color and browning index measurement
Texture profile analysis
Fracturability of the roasted almond nuts was analyzed using a Universal Texture Analyzer (CNS, Farnell, UK) equipped with the Texture Pro™ texture analysis software. A 2 mm diameter cylinder probe P/2, with a 20 mm height was used for the measurement of texture. The probe was allowed to penetrate about 3 mm through the sample at 1 mm/s with trigger 5 g. The texture profile analyzer was able to provide the fracturability readings of the sample nuts. Fracturability (g/s) (i.e. first peak of first compression) was used to evaluate the textural properties of the almond nuts. Three replications were performed (i.e. three almonds from each of the roasting levels).
Verification of models
A comparison between the experimental and fitted data predicted by the response regression models was used to check the adequacy of the response surface equation. The experimental response data were shown to be in agreement with the predicted. Closeness with the predicted and experimental data confirmed the adequacy of the corresponding response surface models used to describe the variations of response variables as functions of roasting conditions.
Scanning electron microscopy (SEM)
The shape and surface morphology of raw, and three differently roasted almond nuts samples, were examined by scanning electronic microscopy (SEM). A small amount of dried powders were spread on aluminum stubs. The stub containing the sample was placed in the SEM chamber and coated with palladium with an auto-fine coater for 180 seconds; specimens were viewed with a JEOL JSM 6400 SEM Attached to EDX (Energy Dispersive X-ray) at working distance of 22 mm and an accelerating voltage of 15 kV.
Fourier transform infrared (FTIR) spectroscopy
The IR spectra were collected using PerkinElmer Spectrum 100 Series spectrometer (United Kingdom) facilitated with a mid-infrared detector- DTGS (deuterated triglycine sulphate). Surface functional groups of the nuts were detected using FTIR. The samples were dispersed in potassium bromide pellet and compressed into discs by pressure. Then, the samples were placed in the light path to allow the infrared light to pass through them and the spectrum was obtained. The spectra were recorded in the region of 4,000 to 280°Cm-1, with a resolution of 4°Cm-1.
Five attributes (aroma, color, fracturability, flavor and overall acceptability) of the almond nuts samples were evaluated by 18 trained panelists from the University Putra Malaysia. Four freshly prepared almond nuts (i.e. dried nuts, and three differently roasted nuts) were presented in air-tight containers coded with three-digit numbers, covered and presented to each panelist. Panelists assigned scores to samples using a nine-point hedonic scale for all the attributes (1 = like extremely, 9 = dislike extremely). Hierarchical multiple regression and Pareto charts were established to analyze the outcomes of the sensory evaluation.
The authors are grateful for the extensive financial support of Research University Grant Scheme (RUGS 2, No. 9385800) at the University Putra Malaysia (UPM).
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