3.1 Effect of different steam explosion conditions on the macroscopic morphology and moisture content of betel nut
The appearance of a food product is an important quality that determines first impression. This is also true for betel nut as a local specialty and chewable product. The results of the transverse diameter and macroscopic morphology of betel nut are shown in Table 1 and Fig. 2. Untreated betel nut was a brighter green, with dense surface folds and tighter fibres. After the steam explosion treatment, the surface folds spread and changed from bright to dark green and showed a slightly orange‒red colour at both ends. When the pressure was 0.55 MPa, the surface folds nearly disappeared. When it reached 0.95 MPa, cracks appeared on the surface, the appearance was damaged, and it was no longer suitable for subsequent processing. A comparison of the cross-section of a betel nut shows that the internal fibres change from white‒green to brown after steam explosion. Decreasing the pressure or increasing the initial moisture content caused the appearance to more closely resemble the untreated betel nut.
The transverse diameter of the betel nut increased significantly with increasing pressure holding time and explosion pressure. The transverse diameter increased by 23.47% and 22.96% after the A-110 s and B-0.95 MPa treatments, respectively. The transverse diameter of the sample steam explosion after soaking was between that of the unsoaked and untreated samples. However, the transverse diameter of the samples did not change as the initial moisture content increased further. Whether due to the moisture in the steam or the moisture added by soaking, the betel nut fibre swells and its volume increases. Moreover, the pressure drops rapidly to atmospheric pressure when steam explodes, and the betel nut expands with the mechanical force generated by this gas expansion. Prolonging the pressure holding time and increasing the explosion pressure promotes further steam intrusion, and an increased initial moisture content can hinder high-pressure steam entry and steam release(Hong et al., 2020).
Colours are the synthetic result of the reflection of visible light by an object, as determined by the substance's chromophores and auxiliary chromophores. The colour of betel nut is mainly determined by pigments such as chlorophyll and lutein in the epidermis as well as lignin, which includes a large number of auxiliary chromophores and chromophores, as well as a few small but essential phenolic compounds and quinones and their oxides(Yazaki, 2015). These substances cause colour change as the steam explosion conditions change. According to the results of the different steam explosion conditions shown in Table 1, The L* value was negatively correlated with both the pressure holding time and the explosion pressure. There was a significant increase in the a* value compared to the untreated betel nut, indicating a reddish colour. A-50 s treatment increased from 12.81 to 16.02, and the a* value increased with pressure holding time and explosion pressure and decreased with increasing initial moisture content. For the b* value, there was a significant increase with increasing moisture content and pressure holding time, generally towards yellow. However, increasing or decreasing the pressure had little effect on the b* value. These findings are consistent with the reports of Gong et al.(2021).
Table 1
Physical properties of betel nut under different steam explosion conditions
Samples
| Transverse diameter (cm)
| Moisture content (%)
| L*
| a*
| b*
|
---|
BN
| 1.96 ± 0.18d
| 8.51 ± 1.13%b
| 64.72 ± 1.40a
| 12.81 ± 0.85c
| 40.78 ± 0.38c
|
A-0s
| 2.21 ± 0.17c
| 11.92 ± 0.38%a
| 59.51 ± 0.22b
| 15.98 ± 0.51ab
| 41.08 ± 1.21c
|
A-5s
| 2.23 ± 0.14bc
| 12.93 ± 1.12%a
| 60.27 ± 0.91b
| 15.67 ± 0.09b
| 40.45 ± 0.39c
|
A-20s
| 2.25 ± 0.20bc
| 12.75 ± 0.81%a
| 60.58 ± 0.87b
| 16.07 ± 0.40ab
| 40.67 ± 0.25c
|
A-50s
| 2.28 ± 0.17bc
| 12.34 ± 1.57%a
| 57.95 ± 0.79c
| 16.02 ± 0.13ab
| 42.33 ± 0.75b
|
A-80s
| 2.32 ± 0.14ab
| 11.95 ± 1.23%a
| 57.19 ± 0.28c
| 15.99 ± 0.33ab
| 42.33 ± 0.26b
|
A-110s
| 2.42 ± 0.22a
| 13.86 ± 0.65%a
| 57.41 ± 0.83c
| 16.60 ± 0.07a
| 44.31 ± 0.53a
|
BN
| 1.96 ± 0.18e
| 8.51 ± 1.13%d
| 64.72 ± 1.40a
| 12.81 ± 0.85d
| 40.78 ± 0.38bc
|
B-0.15MPa
| 2.17 ± 0.18d
| 13.18 ± 2.18%bc
| 65.10 ± 0.58a
| 12.82 ± 0.40d
| 41.22 ± 0.82b
|
B-0.25MPa
| 2.25 ± 0.14cd
| 12.35 ± 0.83%c
| 59.22 ± 0.54bc
| 16.19 ± 0.13b
| 42.56 ± 0.31a
|
B-0.35MPa
| 2.28 ± 0.17bcd
| 12.34 ± 1.57%c
| 57.95 ± 0.79cd
| 16.02 ± 0.13b
| 42.33 ± 0.75a
|
B-0.55MPa
| 2.32 ± 0.16abc
| 12.72 ± 1.16%c
| 59.10 ± 1.05bc
| 16.77 ± 0.39a
| 39.93 ± 0.48c
|
B-0.75MPa
| 2.40 ± 0.17ab
| 14.86 ± 1.10%b
| 60.00 ± 0.76b
| 15.83 ± 0.11bc
| 40.10 ± 0.62c
|
B-0.95MPa
| 2.41 ± 0.15a
| 17.65 ± 1.75%a
| 55.86 ± 1.21d
| 15.49 ± 0.04c
| 38.88 ± 0.24d
|
BN
| 1.96 ± 0.18c
| 8.51 ± 1.13%f
| 64.72 ± 1.40a
| 12.81 ± 0.85d
| 40.78 ± 0.38c
|
C-0h
| 2.28 ± 0.17a
| 12.34 ± 1.57%e
| 57.95 ± 0.79cd
| 16.02 ± 0.13a
| 42.33 ± 0.75b
|
C-2h
| 2.14 ± 0.13b
| 18.49 ± 2.38%d (20.08 ± 0.88%)
| 56.50 ± 0.94d
| 16.04 ± 0.10a
| 41.88 ± 0.60bc
|
C-5h
| 2.11 ± 0.13b
| 21.87 ± 2.30%c (28.07 ± 1.88%)
| 61.98 ± 0.81b
| 14.38 ± 0.49bc
| 42.58 ± 0.46ab
|
C-8h
| 2.14 ± 0.16b
| 25.60 ± 0.67%b (32.15 ± 0.48%)
| 59.96 ± 0.39c
| 14.95 ± 0.08b
| 42.66 ± 1.32ab
|
C-24h
| 2.10 ± 0.12b
| 30.53 ± 1.51%a (46.53 ± 2.03%)
| 59.68 ± 0.72c
| 13.92 ± 0.41c
| 43.74 ± 0.66a
|
* The parentheses after group C represent the moisture content after soaking. (A-50 s, B-0.35 MPa, and C-0 h are samples with the same treatment conditions.)
|
The moisture content (Table 1) indicated a significant increase in moisture in most of the samples after the steam explosion and a significant increase with increasing explosion pressure, excluding the samples that had absorbed additional moisture due to soaking. This finding was also reported by Elliston et al.(2015). The moisture content reached 17.65% after the B-0.95 MPa treatment, which was 2.07 times greater than that before treatment. However, there was no significant difference in moisture content with increasing pressure holding time. For samples with increased initial moisture content through soaking, a significant moisture decrease was observed after steam explosion. After soaking for 24 h, the betel nut had a moisture content of 46.53%, which became 30.53% after the steam explosion. However, the moisture content of C-24 h was 3.59 and 2.47 times higher than that of the unprocessed and unsoaked samples, respectively. We propose these results are due to the adhesion of steam to the betel nut surface after steam explosion. As the explosion pressure increases, the fibres loosen and can absorb more water. However, the high temperature causes the original water to evaporate, and the reduced water holding capacity of the fibres after steam explosion leads to a loss of excess water after soaking(Zhu et al., 2022).
3.2. Effect of steam explosion on the microscopic morphology of betel nut
Optical and scanning electron microscopy observations showed significant modifications to the betel nut micromorphology under different steam explosion conditions. All of these modifications are attributed to the effects of high temperature, high pressure, and high humidity and the mechanical damage generated by steam explosion. As seen in the cross-section of the betel nut under optical microscopy in Fig. 3, the betel nut fibres before steam explosion were densely connected; after steam explosion, voids existed within and between the betel nut fibres; the epidermis was everted or burst and fell off. This variation shows a positive correlation with explosion strength. As shown in the betel nut under scanning electron microscopy in Fig. 3, the surface was compact, and the surface folds were densely intact under magnification. After steam explosion, the surface showed various cracks. With magnification, the grooves on the surface showed varying degrees of destruction and even disappeared completely, and holes and fragments appeared. The increased initial moisture content and reduced explosion pressure microscopic morphology of betel nut was closer to that of untreated betel nut. Similar changes in the microscopic morphology of steam explosion-treated soybean hulls were described by Zhu et al.(2022).
3.3 Effect of different steam explosion conditions on the texture and fibre tensile strength of betel nut
Analysis of texture data is primarily based on the relationship between distance, force, and time; the universal tester measures the multiple mechanical properties. For fibre hardness measurements, the sample was cut into small pieces to produce a flatter sample and minimize the detrimental effect of the betel nut's internal cavity. The hardness is determined by the maximum force applied under the experimental parameters. As the probe is pressed down, the greater the counter force generated by the betel nut fibres, the progressively higher the hardness value measured by the device. As shown in Fig.
4, whether the betel nut was stretched transversely or the fibres were stretched, the force or load very rapidly fell toward zero after a fracture.
The texture of betel nuts directly affects their quality and is a judgement indicator of softening effectiveness. To compare the effects of the different steam explosion conditions on the texture and fibres of betel nuts, the results of betel nut fibre hardness, Shore hardness, transverse tensile strength, coefficient of friction and fibre tensile strength are shown in Table 2. All betel fibre hardness values significantly decreased after the steam explosion treatment. After the A-50 s treatment, hardness decreased by 61.20% from 10018.14 g to 3886.83 g. While the fibre hardness values decreased significantly with increasing explosion pressure and initial moisture content, there was no significant change in the effect on fibre hardness with increasing pressure holding time. This trend was also evident in the results for Shore hardness. However, the overall decrease ratio was not as great as that of the fibre hardness. The C-5 h treatment decreased the most, from 90.05 to 68.00, by 43%. A significant decrease in transverse tensile strength was observed with increasing pressure holding time, explosion pressure, and initial moisture content. Tensile strength decreased from 603.12 g to 449.06 g after A-50s treatment, a 25.54% decrease. A proposed explanation for this result is that when the steam explosion takes place, high-pressure steam is instantly released at atmospheric pressure, gas expands, and thermal energy is converted into mechanical energy to cause the explosion effect; this causes cracks and numerous small gaps to appear on the surface of the betel nut and internal fibres, reducing the strength of the interconnected fibre network(Tu et al., 2019). This phenomenon was also supported by observing the microstructure of the betel nut. Moreover, high temperature, high pressure and high humidity caused lignin, hemicellulose and a small amount of pectin between the fibres to degrade, broke hydrogen bonds, and resulted in inter-fibre bonds loosening. The increased moisture of samples also caused betel nut fibre softening, reducing the mechanical bond strength(Chang et al., 2012).
The static friction coefficient of betel nut is related to its texture, frictional damage. The typical inclined slope method is designed according to Newton's motion theorem, which is based on the fact that the component forces of gravity and friction along an inclined plane are equal when the object is about to slide; hence, µ = tanα. The results of the coefficient of friction on the betel nut surface showed a certain increase after steam explosion, and all betel nuts showed coefficients of friction greater than that between the experimental planks of wood (0.461 ± 0.042). In addition, the coefficient of friction increased significantly with increasing pressure holding time, explosion pressure and initial moisture content. After the C-24 h treatment, the coefficient of friction increased by 47.37% from 0.513 to 0.756. Based on previous studies, the coefficient of friction has been proven to have a positive correlation with moisture content(Fu et al., 2021). Thus, we presume that one reason for the increase in the coefficient of friction is the increase in moisture, which is supported by a similar trend in the results of moisture measurement. Another possible explanation is that the steam explosion treatment leads the otherwise smooth and firm epidermis to become rough and loose.
The betel nut fibre tensile strength per unit linear density fracture decreased after steam explosion treatment. However, the differences upon changing the steam explosion conditions were not statistically significant. After the A-50 s treatment, the fibre tensile strength decreased by 17.64% from 23.19 N to 19.10 N. This finding is consistent with Kim et al.(Kim & Fujii, 2009), who found a comparable reduction in tensile strength of bamboo fibres treated by steam explosion. These changes are attributed to the thermal degradation of cellulose as a result of the steam explosion process's high temperature, pressure and humidity as well as mechanical damage to the fibres due to gas expansion during pressure release. However, excess water blocks steam entry during the explosion process, and water's high specific heat capacity causes temperature to rise slowly, reducing the overall effect of the steam explosion(Sun et al., 2015).
Table 2
The betel nut texture and fibre tensile strength under different steam explosion conditions
Samples
| Fibre hardness (g)
| Shore hardness
| Transverse stretching (g)
| Coefficient of static friction
| Fibre tensile strength (N)
|
---|
BN
| 10018.14 ± 3879.62a
| 90.05 ± 7.12a
| 603.12 ± 110.80a
| 0.513 ± 0.039b
| 23.19 ± 3.92a
|
A-0s
| 3831.94 ± 1042.93b
| 82.77 ± 5.65b
| 480.57 ± 77.89ab
| 0.578 ± 0.068ab
| 20.83 ± 3.58ab
|
A-5s
| 4127.60 ± 1219.61b
| 81.50 ± 7.51b
| 474.80 ± 81.43ab
| 0.555 ± 0.106ab
| 18.63 ± 2.02b
|
A-20s
| 3672.67 ± 971.41b
| 81.82 ± 7.09b
| 484.20 ± 78.78ab
| 0.570 ± 0.060ab
| 18.01 ± 2.65b
|
A-50s
| 3886.83 ± 843.82b
| 81.97 ± 5.83b
| 449.06 ± 52.99ab
| 0.559 ± 0.076ab
| 19.10 ± 3.47b
|
A-80s
| 3683.25 ± 976.67b
| 78.94 ± 6.32bc
| 415.4 ± 76.71b
| 0.572 ± 0.083ab
| 18.91 ± 1.71b
|
A-110s
| 3620.66 ± 1384.99b
| 75.17 ± 7.58c
| 404.88 ± 124.12b
| 0.619 ± 0.084a
| 18.65 ± 1.65b
|
BN
| 10018.14 ± 3879.62a
| 90.05 ± 7.12a
| 603.12 ± 110.80a
| 0.513 ± 0.039b
| 23.19 ± 3.92a
|
B-0.15MPa
| 4390.89 ± 1787.47b
| 81.27 ± 6.00bc
| 491.84 ± 142.53ab
| 0.567 ± 0.063ab
| 19.64 ± 3.70ab
|
B-0.25MPa
| 3980.25 ± 1161.67b
| 83.72 ± 7.70b
| 412.00 ± 66.18ab
| 0.563 ± 0.093ab
| 19.10 ± 3.47b
|
B-0.35MPa
| 3886.83 ± 843.82b
| 81.97 ± 5.83bc
| 425.60 ± 81.55ab
| 0.559 ± 0.076ab
| 19.06 ± 2.53b
|
B-0.55MPa
| 3816.54 ± 1578.37b
| 79.67 ± 7.50bcd
| 374.3 ± 77.14b
| 0.547 ± 0.072ab
| 17.89 ± 3.64b
|
B-0.75MPa
| 2129.83 ± 613.54c
| 75.00 ± 8.88d
| 388.4 ± 67.30b
| 0.602 ± 0.088a
| 18.02 ± 2.28b
|
B-0.95MPa
| 1767.90 ± 512.41c
| 77.35 ± 7.67cd
| 349.6 ± 61.24b
| 0.602 ± 0.084a
| 17.48 ± 2.47b
|
BN
| 10018.14 ± 3879.62a
| 90.05 ± 7.12a
| 603.12 ± 110.80a
| 0.513 ± 0.039d
| 23.19 ± 3.92a
|
C-0h
| 3886.83 ± 843.82b
| 81.97 ± 5.83b
| 449.06 ± 52.99ab
| 0.559 ± 0.076c
| 19.10 ± 3.47b
|
C-2h
| 3104.01 ± 1175.45b
| 82.33 ± 6.05b
| 375.40 ± 65.98bc
| 0.562 ± 0.066c
| 18.10 ± 3.43b
|
C-5h
| 2458.88 ± 699.99bc
| 68.00 ± 10.86c
| 317.80 ± 71.98cd
| 0.699 ± 0.065b
| 18.30 ± 2.89b
|
C-8h
| 2436.24 ± 943.16c
| 70.06 ± 6.45c
| 339.80 ± 61.09c
| 0.695 ± 0.068b
| 18.06 ± 1.68b
|
C-24h
| 1940.78 ± 482.52c
| 70.03 ± 7.65c
| 236 .89 ± 75.53d
| 0.756 ± 0.047a
| 21.98 ± 1.81ab
|
3.4. Effect of different steam explosion conditions on lignin, cellulose and hemicellulose of betel nut
Cellulose, hemicellulose and lignin are the main constituents of plant fibre. To compare the effects of steam explosion on these major chemical constituents of betel nut under different conditions, cellulose content was determined using the nitric acid-ethanol method, hemicellulose by the hydrochloric acid-DNS reducing sugar method and lignin by the concentrated sulfuric acid method (Klason's method). The results are shown in Table 3. For hemicellulose, the standard curve for glucose was as follows: Y = 0.7901X + 0.0428 (where X is the glucose concentration (mg/mL), Y is the absorbance value, R2 = 0.9990, n = 8). The cellulose content significantly increased with increasing pressure holding time and explosion pressure. After A-110 s and B-0.95 MPa treatment, the cellulose content increased from 27.12–29.87% and 30.49%, respectively. Unfortunately, the differences were not significant when comparing the soaked samples to the untreated samples. The lignin and hemicellulose contents decreased significantly with increasing pressure holding time and explosion pressure, and were negatively correlated with the initial moisture content. After the A-110s treatment, hemicellulose and lignin decreased from 45.43% and 25.89–41.38% and 23.38%, respectively, but those of other substances increased. This finding is consistent with that of Sun et al.(2015). Under steam explosion at high temperature, high pressure and high humidity, studies have shown that hemicellulose, a heterogeneous multimer consisting of many different monosaccharides, is the most unstable and prone to thermal hydrolysis, producing xylose, arabinose, glucose, galactose, furfural or hydroxymethylfurfural, etc., and hydrolysis of acetyl groups generates organic acids, which also accelerates the decomposition of other substances(Jacquet et al., 2015; Sun et al., 2015). As phenolic polymers, lignin undergoes a series of cleavages, rearrangements and breakdowns that can produce phenolics or phenolic acids and benzene derivatives(Dong et al., 2020). Additionally, cellulose undergoes thermal degradation in which the glycosidic bonds are cleaved to form monosaccharides, and the breaking of the C-C bond in furanose forms small molecules, as similarly occurs in hemicellulose, thereby reducing the absolute cellulose content. However, hemicellulose and lignin degraded more than cellulose, such that the relative content of cellulose increased after the steam explosion. These effects were closely related to the intensity of the steam explosion(Jacquet et al., 2011).
Table 3
Cellulose, hemicellulose and lignin contents of betel nut under different steam explosion conditions
Samples
| Cellulose (%)
| Hemicellulose (%)
| Lignin (%)
|
---|
BN
| 27.12 ± 0.55c
| 45.43 ± 1.11a
| 25.89 ± 0.96a
|
A-0s
| 27.90 ± 0.86bc
| 44.52 ± 1.31ab
| 24.96 ± 0.55ab
|
A-5s
| 27.55 ± 0.84bc
| 43.00 ± 0.91abc
| 24.55 ± 0.47abc
|
A-20s
| 28.84 ± 0.69ab
| 42.90 ± 0.64bc
| 24.16 ± 0.80bc
|
A-50s
| 28.37 ± 0.55abc
| 43.05 ± 0.93abc
| 24.18 ± 0.81bc
|
A-80s
| 29.78 ± 1.01a
| 41.51 ± 0.86c
| 24.34 ± 0.70bc
|
A-110s
| 29.87 ± 0.98a
| 41.38 ± 1.18c
| 23.38 ± 0.61c
|
BN
| 27.12 ± 0.55c
| 45.43 ± 1.11a
| 25.89 ± 0.96a
|
B-0.15MPa
| 27.88 ± 0.91c
| 45.14 ± 1.18ab
| 25.02 ± 1.16ab
|
B-0.25MPa
| 28.71 ± 0.51c
| 43.28 ± 1.17ab
| 24.05 ± 0.85bc
|
B-0.35MPa
| 28.37 ± 0.55bc
| 43.05 ± 0.93ab
| 24.18 ± 0.81bc
|
B-0.55MPa
| 27.92 ± 0.97c
| 42.73 ± 1.20b
| 23.62 ± 0.46bc
|
B-0.75MPa
| 29.88 ± 1.10ab
| 43.87 ± 0.78ab
| 24.00 ± 0.99bc
|
B-0.95MPa
| 30.49 ± 1.33a
| 42.61 ± 0.37b
| 22.97 ± 0.90c
|
BN
| 27.12 ± 0.55a
| 45.43 ± 1.11a
| 25.89 ± 0.96a
|
C-0h
| 28.37 ± 0.55a
| 43.05 ± 0.93b
| 24.18 ± 0.81a
|
C-2h
| 28.60 ± 0.52a
| 44.03 ± 0.48ab
| 24.59 ± 1.07a
|
C-5h
| 27.83 ± 0.80a
| 44.18 ± 1.37ab
| 24.29 ± 1.03a
|
C-8h
| 27.09 ± 1.28a
| 45.17 ± 0.76a
| 25.10 ± 0.72a
|
C-24h
| 27.31 ± 1.08a
| 44.54 ± 0.97ab
| 25.92 ± 0.90a
|
3.5. Effect of different steam explosion conditions on free and bound phenols of betel nut
Several reports have shown that polyphenols, as important active substances, are widely present in plant flowers, fruits, leaves, and bark and can exert antioxidant activity by inactivating lipid radicals or preventing the decomposition of hydroperoxides(Pitchaon et al., 2007). The results of the Folin-Phenol method for the free and bound phenols of betel nut are shown in Fig. 5. The gallic acid standard curve is Y = 0.0067X + 0.0054 (where X is the concentration of gallic acid (µg/mL), Y is the absorbance value R2 = 0.9991, n = 5). As seen from the results, the free phenol content decreased significantly with increasing pressure holding time and explosion pressure; interestingly, the free phenol content increased and then decreased with increasing initial moisture content compared to the unsoaked sample. The largest decrease was observed after the A-110s treatment, from 34.32 mg (GAE)/g of the untreated sample to 16.77 mg (GAE)/g, a decrease of more than half. The greatest increase in free phenol content, compared to the unsoaked samples, was observed after D-5 h treatment, from 21.58 mg (GAE)/g to 29.79 mg (GAE)/g. However, the free phenol content was still significantly lower than that of the untreated samples. The bound phenol content in betel nut was only 22.12% of the total phenol (free phenol + bound phenol) of 44.07 mg (GAE)/g, which was only 9.75 mg (GAE)/g. The greatest decrease in total phenol content was observed with the A-110s treatment at only 25.68 mg (GAE)/g. These results corroborate the findings of previous work in heat treatment that resulted in a reduction in the phenolic content. This may be attributed to the fact that phenolic compounds are prone to degradation or polymerization in steam explosion treatment, as determined by the aromatic ring type and orientation(Kim & Mai, 2020). However, it is surprising that this result differs somewhat from Gong et al.(2012), who assumed that the phenol content increased under certain conditions due to the steam explosion treatment inactivating some oxidases and breaking some bonds, facilitating phenolic release.
3.6. Effect of different steam explosion conditions on betel nut alkaloids
As the nitrogenous alkaline organic compounds present in plants, alkaloids in betel nut mainly include four alkaloids (Arecoline, Arecaine, Guvacoline, Guvacine)(Chen et al., 2021). The chromatograms of the betel nut and the highest concentration of the mixed standards are shown in Fig. 6. According to the results of the determination of betel nut major alkaloids in Table 4 and Fig. 7, it is noteworthy that all four alkaloids decreased significantly with increasing initial moisture content, and D-24h treatment the total alkaloid content decreased from 7.84 mg/g to 5.81 mg/g. This could be attributed to the fact that alkaloids usually have a higher lipid solubility, but the water solubility not being negligible. A similar finding was also reported by Carvajal-Larenas et al.(2014). Arecoline was relatively stable and Guvacoline only showed a significant decrease during high-pressure treatment, while Arecaine and Guvacine showed a significant decrease with increasing pressure holding time and explosion pressure. After A-110s treatment, the contents of Arecaine and Guvacine decreased by 19.31% and 45.09%, respectively. This finding is consistent with that of Shetge et al.(2020), who compared several heat-treated Poppy seeds, which showed different degrees of decrease in alkaloids, and speculated that one of the reasons for this was the promotion of oxidation leading to thermal degradation.
Table 4
Major alkaloids contents in betel nut under different steam explosion conditions
Samples
| Arecoline (mg/g)
| Arecaine (mg/g)
| Guvacoline (mg/g)
| Guvacine (mg/g)
|
---|
BN
| 3.65 ± 0.21a
| 1.45 ± 0.10a
| 0.50 ± 0.02a
| 2.24 ± 0.02a
|
A-0s
| 3.67 ± 0.19a
| 1.21 ± 0.12b
| 0.45 ± 0.04ab
| 1.65 ± 0.10b
|
A-5s
| 3.58 ± 0.18a
| 1.27 ± 0.13ab
| 0.43 ± 0.02b
| 1.62 ± 0.05b
|
A-20s
| 3.71 ± 0.23a
| 1.24 ± 0.02b
| 0.51 ± 0.04a
| 1.60 ± 0.05b
|
A-50s
| 3.56 ± 0.15a
| 1.12 ± 0.03b
| 0.50 ± 0.05a
| 1.32 ± 0.01cd
|
A-80s
| 3.74 ± 0.19a
| 1.21 ± 0.08b
| 0.48 ± 0.04ab
| 1.36 ± 0.03c
|
A-110s
| 3.53 ± 0.08a
| 1.17 ± 0.07b
| 0.46 ± 0.03ab
| 1.23 ± 0.03d
|
BN
| 3.65 ± 0.21ab
| 1.45 ± 0.10ab
| 0.50 ± 0.02ab
| 2.24 ± 0.02a
|
B-0.15MPa
| 3.75 ± 0.13a
| 1.48 ± 0.07a
| 0.52 ± 0.01a
| 2.18 ± 0.05b
|
B-0.25MPa
| 3.73 ± 0.09a
| 1.31 ± 0.11bc
| 0.54 ± 0.04a
| 2.22 ± 0.02ab
|
B-0.35MPa
| 3.56 ± 0.15ab
| 1.12 ± 0.03d
| 0.50 ± 0.05ab
| 1.32 ± 0.01c
|
B-0.55MPa
| 3.57 ± 0.10ab
| 1.22 ± 0.09cd
| 0.51 ± 0.04a
| 1.21 ± 0.04d
|
B-0.75MPa
| 3.67 ± 0.13a
| 1.37 ± 0.04cd
| 0.53 ± 0.02a
| 1.31 ± 0.02c
|
B-0.95MPa
| 3.41 ± 0.12b
| 1.13 ± 0.03d
| 0.45 ± 0.02b
| 1.30 ± 0.01c
|
BN
| 3.65 ± 0.21a
| 1.45 ± 0.10a
| 0.50 ± 0.02a
| 2.24 ± 0.02a
|
C-0h
| 3.56 ± 0.15ab
| 1.12 ± 0.03c
| 0.50 ± 0.05a
| 1.32 ± 0.01d
|
C-2h
| 3.70 ± 0.04a
| 1.26 ± 0.04b
| 0.53 ± 0.03a
| 1.45 ± 0.02c
|
C-5h
| 3.34 ± 0.09bc
| 0.96 ± 0.03d
| 0.41 ± 0.03b
| 1.65 ± 0.03b
|
C-8h
| 3.26 ± 0.08c
| 1.13 ± 0.07c
| 0.39 ± 0.02b
| 1.50 ± 0.03c
|
C-24h
| 3.00 ± 0.11d
| 0.95 ± 0.04d
| 0.39 ± 0.05b
| 1.48 ± 0.04c
|
3.7. Effect of steam explosion on the release behaviour of betel nut
The texture analyser is an instrument for measuring various mechanical properties of foods based on simulated chewing and is widely used in the analysis of food texture; however, there are few reports of using a texture analyser to simulate chewing. To explore the effect of steam explosion on the release behaviour of betel nut, A-50 s was selected as a representative sample after the steam explosion. Comparisons were made with untreated samples under the same release conditions to compare the differences in released substances. As the betel nut is hard and easily exceeds the maximum force range of the texture analyser forces, rendering testing incompatible, the betel nut was first soaked and then the analyser experiment was conducted. And there was no significant difference in the Chemical composition of the solutions after sample soaking. To better simulate chewing, the number of experimental compressions and saliva additions was determined based on the observation of betel nut chewing behaviour and related literature(Chen, 2007).
In general, as shown in Fig. 8, the greatest quantity of substance was released between the 120th and 240th compressions in this simulated chewing condition. One possible explanation for this is that betel nut fibres were not yet loose and experienced less contact with the artificial saliva in the first 120 compressions, releasing a limited amount of substance; subsequently, simulated chews released progressively less material due to a decrease in the total amount of remaining substance. Comparing the release of betel nut before and after steam explosion treatment, the steam explosion promoted the release of soluble solids, total phenols, and total alkaloids, particularly when simulating chewing during the first 240 cycles. Soluble solids, total phenols and total alkaloids increased by up to 56%, 79% and 46%, respectively. There are several explanations for this result: steam explosion treatment disrupts the initially compact structure of betel nut, increases the contact area with artificial saliva, and facilitates material exchange(Adekunle et al., 2017); microstructure disruption also promotes the release of material. Adekunle et al.(2017) similarly found that steam explosion treatment of maize stems, enhanced the release of phenols. In addition, previous studies have shown that steam explosion promotes the release of dietary fibres and lipids, and other substances(Hu et al., 2020).