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Citation: Zhang, Q.; Huang, L.; Li,
H.; Zhao, D.; Cao, J.; Song, Y.; Liu, X.
Mimic Pork Rinds from Plant-Based
Gel: The Influence of Sweet Potato
Starch and Konjac Glucomannan.
Molecules 2022,27, 3103. https://
doi.org/10.3390/molecules27103103
Academic Editor: Adele Papetti
Received: 24 April 2022
Accepted: 10 May 2022
Published: 12 May 2022
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molecules
Article
Mimic Pork Rinds from Plant-Based Gel: The Influence of
Sweet Potato Starch and Konjac Glucomannan
Qibo Zhang 1, Lu Huang 1, He Li 1, * , Di Zhao 1, Jinnuo Cao 2, Yao Song 3and Xinqi Liu 1,*
1National Soybean Processing Industry Technology Innovation Center, Beijing Technology and Business
University (BTBU), Beijing 100048, China; zhangqibo2021@163.com (Q.Z.); huanglulu1119@163.com (L.H.);
zhaodi22121@163.com (D.Z.)
2
Plant Meat (Hangzhou) Health Technology Limited Company, Hangzhou 311121, China; jinnuocao@163.com
3Handan Institute of Innovation, Peking University, Handan 056008, China; 2005songyao@163.com
*Correspondence: lihe@btbu.edu.cn (H.L.); liuxinqi@btbu.edu.cn (X.L.)
Abstract:
This study investigated the effect of sweet potato starch (SPS) and konjac glucomannan
(KGM) on the textural, color, sensory, rheological properties, and microstructures of plant-based
pork rinds. Plant-based gels were prepared using mixtures of soy protein isolate (SPI), soy oil, and
NaHCO
3
supplemented with different SPS and KGM concentrations. The texture profile analysis
(TPA) results indicated that the hardness, cohesiveness, and chewiness of the samples improved
significantly after appropriate SPS and KGM addition. The results obtained via a colorimeter showed
no significant differences were found in lightness (L*) between the samples and natural pork rinds
after adjusting the SPS and KGM concentrations. Furthermore, the rheological results showed that
adding SPS and KGM increased both the storage modulus (G’) and loss modulus (G”), indicating
a firmer gel structure. The images obtained via scanning electron microscopy (SEM) showed that
the SPS and KGM contributed to the formation of a more compact gel structure. A mathematical
model allowed for a more objective sensory evaluation, with the 40% SPS samples and the 0.4%
KGM samples being considered the most similar to natural pork rinds, which provided a comparable
texture, appearance, and mouthfeel. This study proposed a possible schematic model for the gelling
mechanism of plant-based pork rinds: the three-dimensional network structures of the samples
may result from the interaction between SPS, SPI, and soybean oil, while the addition of KGM and
NaHCO3enabled a more stable gel structure.
Keywords: plant-based pork rinds; composite gel; texture; rheology; gelling mechanism
1. Introduction
Pork rinds are popular snacks in many countries and regions, including the USA,
Australia, Europe, and Asia [
1
,
2
]. As the plant-based market has prospered in recent years,
plant-based products have been further subdivided to meet different consumer demands,
expanding into the pork rind field. Compared to traditional pork rinds, plant-based pork
rinds are environmentally friendly, avoiding the environmental pollution and greenhouse
gas emissions produced from farming. The preparation of plant-based pig skins does not
the slaughtering of pigs, which improves animal welfare [
3
]. Furthermore, meat production
requires an additional energy loss level, while developing plant-based pork rinds without
animal ingredients is more resource-efficient and sustainable [
4
]. Since pork rinds are
produced from the part of the pig directly in contact with external pathogens, they are likely
to cause foodborne diseases if they are not thoroughly cleaned during processing [5]. The
residual grease from pork rinds is challenging to clean, and the traditional manufacturing
process necessitates repeated washing with chemicals, which undoubtedly creates a safety
hazard for consumers [
6
]. Therefore, plant-based pork rinds are gaining popularity due to
their benefits for the planet and human well-being.
Molecules 2022,27, 3103. https://doi.org/10.3390/molecules27103103 https://www.mdpi.com/journal/molecules
Molecules 2022,27, 3103 2 of 14
However, the plant-based pork rinds currently available on the market are primarily
available in one form, puffed pork rinds. Outstanding Foods uses rice, sunflower oil, and
pea protein as ingredients to create plant-based puffed pork rinds via grilling (announced
on its official website). Snacklins has launched a plant-based crisp that uses cassava
to achieve a puffed pork rind-like texture and creates a meaty flavor using mushrooms
(announced on its official website). Minimal studies are available involving edible plant-
based pork rinds that accurately mimic the original texture, appearance, and mouthfeel of
natural pork rinds. There is a significant overlap between food science and medicine as a
multidisciplinary field [
7
]. In the medical field, gels are commonly used to simulate skin for
wound management, cosmetic surgery [
8
], and
in vitro
simulation experiments [
9
]. Natural
gels are non-toxic, biocompatible, and biodegradable and can be obtained from various
sources [
10
]. The current commercial skin substitutes, Integra and Matriderm, use natural
hydrogels as biological scaffolds that not only provide a friendly environment for cells and
load biomolecules but also retain water to promote cell migration and proliferation [
11
].
Zhang et al. mimicked the viscoelastic properties of porcine skin using agarose gels [
12
].
Although skin simulation gels are primarily used for medical purposes at this stage, they
show significant potential for extrapolation to the food field.
By utilizing the synergy between the ingredients, composite gels can compensate
for the limitations of single-ingredient gels to better imitate pork rinds [
13
–
15
]. Soybeans
represent a high-quality plant source that contains proteins and lipids with high nutritional
benefits [
16
], as well as excellent functional properties like emulsification and gelation [
17
].
Sweet potatoes are rich in dietary fiber containing more than ten micronutrients and little
cholesterol or lipids and are considered an ideal natural food by nutritionists [
18
]. Sweet
potato starch (SPS) is the main product of sweet potatoes and forms a complete three-
dimensional gel structure to ensure its firmness [
19
]. Konjac glucomannan (KGM) is a
natural polymer polysaccharide extracted from konjac, which is popular for its positive
health effects, such as anti-obesity, antioxidant, and hypoglycemic activity [
20
]. KGM is
widely used in the food industry due to its excellent biocompatibility, thickening, and
gelling properties [
21
]. Studies have shown that KGM displays good synergy with protein
and starch [22,23], exhibiting potential for further developing composite gel products.
This research aims to prepare plant-based gels that can better mimic the texture,
appearance, and mouthfeel of natural pork rinds and optimize these properties by ad-
justing the SPS and KGM concentrations. Furthermore, the interaction mechanisms and
microstructures of the plant-based gels were examined via rheometry and scanning electron
microscopy (SEM).
2. Results and Discussion
2.1. Textural Analysis
The textural properties were crucial for measuring the similarity of the samples to
natural pork rinds. The textural parameters (hardness, cohesiveness, springiness, and
chewiness) of the plant-based pork rinds containing different SPS and KGM concentrations
are presented in Table 1, except for the10% SPS samples which were too soft to form the
specified shape.
The hardness, cohesiveness, and chewiness of the samples in the SPS group increased
at higher concentrations while those in the KGM group were initially higher, followed by a
decline as the concentrations increased. Studies have shown that amylose concentration
is closely related to gel strength [
24
]. Therefore, the addition of SPS resulted in higher
amylose content and firmer gels. Schwartz et al. reported that low KGM concentrations
affected gelatinization and starch retrogradation, but this effect is barely noticeable at high
KGM concentrations [
25
], which is consistent with the experimental results. The 0.2% KGM
samples were softer than those without KGM, possibly because KGM forms a complex with
leached amylose that cannot fill the gel network structure well [
26
]. However, increasing
the KGM concentration from 0.2% to 0.8% increases the number and volume of complexes,
which are more successful in filling and strengthening the gel structure [27].
Molecules 2022,27, 3103 3 of 14
Table 1.
The Texture profile analysis (TPA) of the plant-based pork rinds containing different SPS
and KGM concentrations.
Type and Proportion Hardness (g) Cohesiveness Springiness Chewiness (mJ)
SPS
10% none none none none
20% 312.67 ±28.57 d0.66 ±0.03 c0.93 ±0.01 a5.67 ±0.71 d
30% 1941.67 ±42.72 c0.55 ±0.02 d0.91 ±0.01 a28.63 ±0.38 c
40% 3098.67 ±60.19 b0.64 ±0.03 c0.92 ±0.03 a53.59 ±0.61 b
50% 6681.33 ±122.27 a0.76 ±0.02 b0.93 ±0.02 a138.67 ±2.38 a
Natural pork rinds
(boiled for 30 min) 2078.67 ±55.81 c0.93 ±0.02 a0.92 ±0.03 a52.27 ±1.64 b
KGM
0% 2181.67 ±122.52 b0.60 ±0.03 c0.89 ±0.02 a34.82 ±1.23 c
0.2% 1941.67 ±42.72 c0.52 ±0.02 d0.91 ±0.01 a26.70 ±0.46 e
0.4% 2134.33 ±143.40 b0.54 ±0.01 d0.91 ±0.04 a30.04 ±1.44 d
0.6% 2138.33 ±25.38 b0.63 ±0.02 c0.88 ±0.01 a34.98 ±0.82 c
0.8% 2453.67 ±69.92 a0.73 ±0.03 b0.89 ±0.03 a46.79 ±1.45 b
Natural pork rinds
(boiled for 30 min) 2078.67 ±55.81 b,c 0.93 ±0.02 a0.92 ±0.03 a52.27 ±1.64 a
Different lowercase superscript letters (a–e) in the same column denote significant differences (p< 0.05). All data
are presented as mean values ±standard error (n= 3).
All the samples could better simulate the springiness of natural pork rinds boiled for
30 min. In the SPS group, no significant differences were evident between the chewiness of
the samples with 40% SPS and natural pork rinds. Although the hardness of the samples
in the KGM group was exceedingly similar to natural pork rinds, the chewiness was
significantly different from natural skin even when the KGM concentration was increased
to 0.8%. This may be because the KGM group was based on 30% SPS, indicating that the
SPS concentrations more significantly affected the textural properties of plant-based pork
rinds than KGM.
2.2. Color Analysis
Color plays a crucial role in food acceptability and consumer preference [
28
]. De-
termining the lightness (L*), redness/greenness (a*), and yellowness/blueness (b*) color
parameters of the samples reflected the similarities in appearance between the plant-based
pork rinds and natural pork rinds. The 10% SPS samples that were too soft to form the
specified shape were not involved in the determination.
As shown in Figure 1, the addition of starch significantly changed the color parameters,
especially L*. Although the L* value decreased as the SPS concentration increased from 20%
to 40%, it dramatically increased when the added SPS exceeded 40% (Figure 1A). This may
be because the starch is too dry to sufficiently gelatinize with a low moisture content [
29
],
resulting in white lumps in some parts of the samples. Contrary to this, as shown in
Figure 1B, KGM abruptly increased the L* value, followed by a gradual decrease, while
exhibiting a negligible effect on a* and b* values. The color of the plant-based pork rinds
was generally more similar to natural pork rinds boiled for 30 min than raw pork rinds.
Molecules 2022,27, 3103 4 of 14
Molecules 2022, 27, x FOR PEER REVIEW 4 of 14
Figure 1. The color parameters of the plant-based pork rinds containing different SPS (A) and KGM
(B) concentrations. Different lowercase letters (a–f) in the same color group denote significant dif-
ferences (p < 0.05).
2.3. Sensory Evaluation
The fuzzy mathematics sensory evaluation can objectively and precisely distinguish
the merit levels of different products and select the best product from several samples
according to the grade membership degree and membership function theory [30].
Based on the preliminary sensory results (Table 2), the fuzzy matrix R was formed
via the fuzzy mathematical method. Matrix multiplication was used to calculate the eval-
uation matrix Y, while the degrees of membership for each grade in Y were assigned and
accumulated to obtain the sensory comprehensive score (SCS) of the samples (Figure 2A).
Table 2. The preliminary sensory score of the plant-based pork rinds containing different SPS and
KGM concentrations (v1, v2, v3, and v4 represented very good, good, medium, and bad, respectively).
Group Appearance Chewiness Stickiness Cohesiveness
SPS v1 v2 v3 v4 v1 v2 v3 v4 v1 v2 v3 v4 v1 v2 v3 v4
10% 0 0 0 10 0 0 1 9 0 0 0 10 0 0 2 8
20% 0 4 4 2 0 2 5 3 0 1 6 3 1 4 5 0
30% 2 6 2 0 3 4 2 0 4 5 1 0 1 7 2 0
40% 2 7 1 0 5 5 0 0 7 3 0 0 1 8 1 0
50% 0 2 5 3 0 5 5 0 9 1 0 0 3 4 3 0
KGM v1 v2 v3 v4 v1 v2 v3 v4 v1 v2 v3 v4 v1 v2 v3 v4
0% 0 1 4 5 1 4 5 0 0 0 5 5 1 5 4 0
0.2% 1 4 5 0 0 3 7 0 1 6 2 1 0 7 3 0
0.4% 4 6 0 0 2 4 4 0 3 7 0 0 1 7 2 0
0.6% 0 6 4 0 1 6 3 0 7 3 0 0 1 8 1 0
0.8% 0 3 6 1 1 5 4 0 9 1 0 0 2 7 1 0
Figure 1.
The color parameters of the plant-based pork rinds containing different SPS (
A
) and
KGM (
B
) concentrations. Different lowercase letters (a–f) in the same color group denote significant
differences (p< 0.05).
2.3. Sensory Evaluation
The fuzzy mathematics sensory evaluation can objectively and precisely distinguish
the merit levels of different products and select the best product from several samples
according to the grade membership degree and membership function theory [30].
Based on the preliminary sensory results (Table 2), the fuzzy matrix R was formed
via the fuzzy mathematical method. Matrix multiplication was used to calculate the
evaluation matrix Y, while the degrees of membership for each grade in Ywere assigned and
accumulated to obtain the sensory comprehensive score (SCS) of the samples (Figure 2A).
The SCS of the SPS and KGM groups increased initially, followed by a decrease. As
shown in Figure 2B, the 10% SPS gels were soft and sticky with a pasty texture, which was
difficult to associate with natural pork rinds. The SCS of the samples increased at higher
SPS concentrations, peaking at 40% SPS due to improved appearance and texture. Without
KGM, the samples were difficult to shape during steaming, while a water film covered
the surface, resulting in high viscosity. The addition of KGM reduced the stickiness of the
samples, improved the gel strength, and caused the formation of KGM-SPS complexes.
As the number of complexes increased, the appearance of plant-based pork rinds became
rougher (Figure 2C), presenting a distinct grainy mouthfeel, especially in the 0.6–0.8%
KGM samples, which exhibited decreased SCS. Therefore, the 40% SPS and the 0.4% KGM
samples were considered the closest to the natural pork rinds in texture, appearance,
and mouthfeel.
Molecules 2022,27, 3103 5 of 14
Table 2.
The preliminary sensory score of the plant-based pork rinds containing different SPS and
KGM concentrations (v
1
, v
2
, v
3
, and v
4
represented very good, good, medium, and bad, respectively).
Group Appearance Chewiness Stickiness Cohesiveness
SPS v1v2v3v4v1v2v3v4v1v2v3v4v1v2v3v4
10% 0 0 0 10 0 0 1 9 0 0 0 10 0 0 2 8
20%0442025301631450
30%2620342045101720
40%2710550073001810
50%0253055091003430
KGM v1v2v3v4v1v2v3v4v1v2v3v4v1v2v3v4
0%0145145000551540
0.2%1450037016210730
0.4%4600244037001720
0.6%0640163073001810
0.8%0361154091002710
Molecules 2022, 27, x FOR PEER REVIEW 5 of 14
Figure 2. The SCS (A) and photographic images (B,C) of the plant-based pork rinds containing dif-
ferent SPS and KGM concentrations.
The SCS of the SPS and KGM groups increased initially, followed by a decrease. As
shown in Figure 2B, the 10% SPS gels were soft and sticky with a pasty texture, which was
difficult to associate with natural pork rinds. The SCS of the samples increased at higher
SPS concentrations, peaking at 40% SPS due to improved appearance and texture. Without
KGM, the samples were difficult to shape during steaming, while a water film covered
the surface, resulting in high viscosity. The addition of KGM reduced the stickiness of the
samples, improved the gel strength, and caused the formation of KGM-SPS complexes.
As the number of complexes increased, the appearance of plant-based pork rinds became
rougher (Figure 2C), presenting a distinct grainy mouthfeel, especially in the 0.6–0.8%
KGM samples, which exhibited decreased SCS. Therefore, the 40% SPS and the 0.4% KGM
samples were considered the closest to the natural pork rinds in texture, appearance, and
mouthfeel.
2.4. Rheological Analysis
2.4.1. Frequency Scanning
As shown in Figure 3, the samples displayed higher storage modulus (G’) and loss
modulus (G’’) as the frequency increased, while each curve showed varying degrees of
frequency dependence. The addition of SPS significantly increased the two moduli (Figure
3A,B). This may be because a higher SPS concentration promotes the interaction and cross-
linking between starch molecules, making the network structure more compact [31]. The
G’ of the 50% SPS samples was lower than the 40% SPS samples, while the G’’ of the for-
mer was higher than the latter. Therefore, it is assumed that the low moisture in the 50%
SPS samples inhibits starch gelatinization, resulting in less elastic components and more
viscous components in the gel system [29,32]. Figure 3C,D showed that the G’ and G’’
increased with the increasing KGM concentration, indicating that KGM contributed to a
stronger network structure. Similar results were observed by Ning et al. [26]. These find-
ings were also consistent with those of Ma et al., who revealed that KGM molecules com-
bined with leached amylose to form a stronger gel structure [33].
Figure 2.
The SCS (
A
) and photographic images (
B
,
C
) of the plant-based pork rinds containing
different SPS and KGM concentrations.
2.4. Rheological Analysis
2.4.1. Frequency Scanning
As shown in Figure 3, the samples displayed higher storage modulus (G’) and loss
modulus (G”) as the frequency increased, while each curve showed varying degrees
of frequency dependence. The addition of SPS significantly increased the two moduli
(Figure 3A,B). This may be because a higher SPS concentration promotes the interaction and
cross-linking between starch molecules, making the network structure more compact [
31
].
The G’ of the 50% SPS samples was lower than the 40% SPS samples, while the G” of the
former was higher than the latter. Therefore, it is assumed that the low moisture in the
50% SPS samples inhibits starch gelatinization, resulting in less elastic components and
more viscous components in the gel system [
29
,
32
]. Figure 3C,D showed that the G’ and
G” increased with the increasing KGM concentration, indicating that KGM contributed
to a stronger network structure. Similar results were observed by Ning et al. [
26
]. These
findings were also consistent with those of Ma et al., who revealed that KGM molecules
combined with leached amylose to form a stronger gel structure [33].
Molecules 2022,27, 3103 6 of 14
Molecules 2022, 27, x FOR PEER REVIEW 6 of 14
Figure 3. The G’ and G’’ of the plant-based pork rinds (frequency scanning) containing different SPS
(A,B) and KGM (C,D) concentrations.
2.4.2. Temperature Scanning
Similar to the frequency scanning results, the G’ and G’’ increased at higher SPS and
KGM concentrations (Figure 4), suggesting a promotional effect on the gel structure. The
G’ and G’’ generally showed a downward trend throughout the heating process. At 25–
75 °C, the two moduli of the samples decreased gradually. At 75 °C, the moduli of the 30%
SPS samples and the high KGM concentration groups (0.4%, 0.6%, and 0.8%) showed a
brief rise, which could be attributed to the interaction between SPS, KGM, and NaHCO3.
In a heated alkaline environment, the KGM molecules were prone to deacetylation, form-
ing more hydrogen bonds between the deacetylated KGM and SPS, temporarily facilitat-
ing a firmer gel structure [21]. Similarly, Luo et al. showed that the interaction between
the alkali and KGM rapidly increased the two moduli over 70 °C [34]. However, the hy-
drogen bonds formed at high temperatures were unstable. Further heating ruptured the
hydrogen bonds between the molecules, collapsing the structures and significantly de-
creasing G’ and G’’ [35].
Figure 3.
The G’ and G” of the plant-based pork rinds (frequency scanning) containing different SPS
(A,B) and KGM (C,D) concentrations.
2.4.2. Temperature Scanning
Similar to the frequency scanning results, the G’ and G” increased at higher SPS and
KGM concentrations (Figure 4), suggesting a promotional effect on the gel structure. The G’
and G” generally showed a downward trend throughout the heating process. At 25–75
◦
C,
the two moduli of the samples decreased gradually. At 75
◦
C, the moduli of the 30% SPS
samples and the high KGM concentration groups (0.4%, 0.6%, and 0.8%) showed a brief
rise, which could be attributed to the interaction between SPS, KGM, and NaHCO
3
. In a
heated alkaline environment, the KGM molecules were prone to deacetylation, forming more
hydrogen bonds between the deacetylated KGM and SPS, temporarily facilitating a firmer gel
structure [
21
]. Similarly, Luo et al. showed that the interaction between the alkali and KGM
rapidly increased the two moduli over 70
◦
C [
34
]. However, the hydrogen bonds formed at
high temperatures were unstable. Further heating ruptured the hydrogen bonds between the
molecules, collapsing the structures and significantly decreasing G’ and G” [35].
Molecules 2022,27, 3103 7 of 14
Molecules 2022, 27, x FOR PEER REVIEW 7 of 14
Figure 4. The G’ and G’’ of the plant-based pork rinds (temperature scanning) containing different
SPS (A,B) and KGM (C,D) concentrations.
2.5. Microstructure
The three-dimensional network structure of the samples containing different SPS and
KGM concentrations was observed via SEM (Figure 5A–D). The SPS group exhibited
smaller holes and denser structures due to a higher SPS concentration, forming a firmer
gel, which was consistent with the TPA results (Table 1). In addition, granules were evi-
dent on the surfaces of the 50% SPS samples (Figure 5B), which were likely insufficiently
gelatinized SPS particles. A comparison between Figure 5C,D showed that the addition of
KGM filled the pores with KGM-SPS complexes, improving the hardness and cohesive-
ness of the samples while producing a chewier gel [36].
The microstructures of the natural pork rinds are shown in Figure 5E,F. The grease
in the pores of the raw pork rinds was removed by hydrothermal treatment for 30 min to
obtain a clear collagen pore structure. Since the plant-based pork rinds showed compara-
ble structures to natural pork rinds, with similar pore sizes (10–60 μm) and depths, their
texture and mouthfeel were similar.
Figure 4.
The G’ and G” of the plant-based pork rinds (temperature scanning) containing different
SPS (A,B) and KGM (C,D) concentrations.
2.5. Microstructure
The three-dimensional network structure of the samples containing different SPS
and KGM concentrations was observed via SEM (Figure 5A–D). The SPS group exhibited
smaller holes and denser structures due to a higher SPS concentration, forming a firmer gel,
which was consistent with the TPA results (Table 1). In addition, granules were evident on
the surfaces of the 50% SPS samples (Figure 5B), which were likely insufficiently gelatinized
SPS particles. A comparison between Figure 5C,D showed that the addition of KGM filled
the pores with KGM-SPS complexes, improving the hardness and cohesiveness of the
samples while producing a chewier gel [36].
The microstructures of the natural pork rinds are shown in Figure 5E,F. The grease
in the pores of the raw pork rinds was removed by hydrothermal treatment for 30 min to
obtain a clear collagen pore structure. Since the plant-based pork rinds showed comparable
structures to natural pork rinds, with similar pore sizes (10–60
µ
m) and depths, their texture
and mouthfeel were similar.
Molecules 2022,27, 3103 8 of 14
Molecules 2022, 27, x FOR PEER REVIEW 8 of 14
Figure 5. The microstructures of the plant-based pork rinds containing different SPS (A,B) and KGM
(C,D) concentrations and natural pork rinds (E,F).
2.6. Schematic Model
Based on the experiments and references mentioned above, this study proposed a
possible schematic model for the gelling mechanism of plant-based pork rinds (Figure
6A). The primary components in the emulsion were soy protein isolate (SPI), soybean oil,
SPS, NaHCO3, and KGM. During heating, the SPS molecules absorbed water, causing
them to swell and even disintegrate to release amylose. The amylose cross-links formed
three-dimensional network structures during cooling [19,29]. Likewise, SPI molecules
were denatured by heating, forming cross-linked aggregates to produce the network
structure [37]. Previous research showed that soybean oil combined with the hydrophobic
groups in SPI, promoting the aggregation of oil and protein [28]. Moreover, the alkaline
environment created by the NaHCO3 increased the size of starch granules and made them
more prone to breakage [38,39]. However, the swelling of the starch granules was re-
stricted in the presence of KGM [39]. On the one hand, KGM competed with starch for
water molecules and reduced the growth of the starch granules [40]. On the other hand,
the acetyl groups were removed from KGM in a heated alkaline environment [21,41]. The
deacetylated KGM molecules displayed stronger interaction with amylose to form KGM-
Figure 5.
The microstructures of the plant-based pork rinds containing different SPS (
A
,
B
) and KGM
(C,D) concentrations and natural pork rinds (E,F).
2.6. Schematic Model
Based on the experiments and references mentioned above, this study proposed a
possible schematic model for the gelling mechanism of plant-based pork rinds (Figure 6A).
The primary components in the emulsion were soy protein isolate (SPI), soybean oil, SPS,
NaHCO
3
, and KGM. During heating, the SPS molecules absorbed water, causing them
to swell and even disintegrate to release amylose. The amylose cross-links formed three-
dimensional network structures during cooling [
19
,
29
]. Likewise, SPI molecules were
denatured by heating, forming cross-linked aggregates to produce the network struc-
ture [
37
]. Previous research showed that soybean oil combined with the hydrophobic
groups in SPI, promoting the aggregation of oil and protein [
28
]. Moreover, the alkaline
environment created by the NaHCO
3
increased the size of starch granules and made them
more prone to breakage [
38
,
39
]. However, the swelling of the starch granules was restricted
in the presence of KGM [
39
]. On the one hand, KGM competed with starch for water
molecules and reduced the growth of the starch granules [
40
]. On the other hand, the
acetyl groups were removed from KGM in a heated alkaline environment [
21
,
41
]. The
deacetylated KGM molecules displayed stronger interaction with amylose to form KGM-
SPS complexes, which were embedded into the network structure during cooling. To sum
Molecules 2022,27, 3103 9 of 14
up, the three-dimensional network structure of the plant-based pork rinds may result from
the interaction between SPS, SPI, and soybean oil, while the addition of KGM and NaHCO
3
increases the stability of the gel structure.
Molecules 2022, 27, x FOR PEER REVIEW 9 of 14
SPS complexes, which were embedded into the network structure during cooling. To sum
up, the three-dimensional network structure of the plant-based pork rinds may result
from the interaction between SPS, SPI, and soybean oil, while the addition of KGM and
NaHCO3 increases the stability of the gel structure.
Figure 6. A schematic model showing the gelling mechanism (A) and internal layout (B) of the plant-
based pork rinds containing different SPS and KGM concentrations.
As shown in Figure 6B, the internal layout of the plant-based pork rinds can be fur-
ther inferred according to the proposed gelling mechanism. The SPS occupies most of the
space, followed by the SPI combined with soybean oil. The KGM that surrounded the
granules could bind to the amylose released from starch, while NaHCO3 could create an
alkaline environment to promote the deacetylation of KGM. Changes in the SPS and KGM
concentrations affect the internal layout of the samples, representing a crucial element of
their macroscopic characteristics.
In the 10% SPS samples, the low starch concentration created significant distances
between the retrograded starch granules and large gaps in the gel structure. Although the
SPI network structure had enough space to stretch, the low content prevented sufficient
gel strength. The excess KGM molecules could not bind to the SPS, consequently entering
Figure 6.
A schematic model showing the gelling mechanism (
A
) and internal layout (
B
) of the
plant-based pork rinds containing different SPS and KGM concentrations.
As shown in Figure 6B, the internal layout of the plant-based pork rinds can be further
inferred according to the proposed gelling mechanism. The SPS occupies most of the
space, followed by the SPI combined with soybean oil. The KGM that surrounded the
granules could bind to the amylose released from starch, while NaHCO
3
could create an
alkaline environment to promote the deacetylation of KGM. Changes in the SPS and KGM
concentrations affect the internal layout of the samples, representing a crucial element of
their macroscopic characteristics.
In the 10% SPS samples, the low starch concentration created significant distances
between the retrograded starch granules and large gaps in the gel structure. Although
the SPI network structure had enough space to stretch, the low content prevented suffi-
cient gel strength. The excess KGM molecules could not bind to the SPS, consequently
entering a free state. Therefore, the 10% SPS samples were paste-like and displayed the
worst textural properties. Contrarily, the 50% SPS samples exhibited the most crowded
Molecules 2022,27, 3103 10 of 14
interiors, to the extent that the SPS compressed the SPI network structure space. The
high starch concentration significantly enhanced the gel structure, as shown by the TPA
results (Table 1). The alkaline environment created by the NaHCO
3
in the absence of the
synergistic KGM effect promoted SPS granule swelling and rupture in the 0% KGM sample,
compromising the gel strength. The sample is also highly sticky due to the transudatory
amylopectin. A high KGM concentration caused the formation of complexes that filled the
pores, strengthened the structure, reduced stickiness, and even allowed the KGM to enter a
free state. The remaining acetyl groups in the KGM increased due to the relatively limited
alkaline environment produced by NaHCO3.
3. Materials and Methods
3.1. Materials
The SPI (protein = 91.2%, moisture = 5.1%, on a dry basis) was purchased from the
Shandong Yuwang Ecological Food Industry Co., Ltd. (Shandong, China), while the soy oil
was obtained from the China Oil & Foodstuffs Corporation Co., Ltd. (Beijing, China). The
SPS (starch = 90.2%, moisture = 8.6%, on a dry basis) and edible NaHCO
3
were purchased
from the Beijing Shuntian Heng Trading Co., Ltd. (Beijing, China). The konjac powder
(glucomannan = 83.6%, moisture = 10.5%, and ash = 3.8%, on a dry basis) was acquired
from the Hubei Consistent Biotechnology Co., Ltd. (Hubei, China). Fresh pork rinds
(abdomen) were obtained from a local market (Beijing, China). All of the external fat was
removed from the pork rinds, which were stored at 4 ◦C in a fridge before testing.
3.2. The Preparation of the Plant-Based Pork Rind Emulsion
The SPI powder (1%, w/w) was diluted with distilled water and stirred with a magnetic
stirrer until evenly dispersed. The SPI solution was then decanted into an electronic stirrer
(EUROSTAR 40 digital, IKA, Baden-Württemberg, Germany) to continue stirring for 5 min
at 8000 rpm. The soy oil (1.5%, w/w) was added and stirred for 5 min at 8000 rpm to
emulsify. The SPS powder and NaHCO
3
(0.15%, w/w) were dissolved in distilled water,
poured into the electronic stirrer, and blended at 6000 rpm for 5 min. The konjac flour was
gradually added and stirred at 3000 rpm for 5 min to obtain the final plant-based pork rind
emulsion. Two sample groups were used in this study, namely the SPS group: 0.4% KGM
and different SPS concentrations (10%, 20%, 30%, 40%, and 50%, w/w) and the KGM group:
30% SPS and different KGM concentrations (0%, 0.2%, 0.4%, 0.6%, and 0.8%, w/w).
3.3. The Preparation of the Plant-Based Pork Rind Gel
To shape the emulsion, it was decanted into a specific baking mold (a cube, width
3 cm, length 10 cm, and height 4 mm) and steamed for 5 min, ensuring an average thickness
of about 4 mm. Then, the preliminary gel in the mold was packed into a vacuum bag
(width 8.9 cm and length 14.7 cm) and degassed at
−
0.09 MPa for 15 s using a vacuum
machine (Exelway, DZ-300, Quanzhou Liding Mechanical Equipment Co., Ltd., Fujian,
China), boiled at 95
◦
C for 15 min in a thermostat water bath (Jintan Kexi, HH-2 Water bath
pot, Jintan Kexi Instrument Co., Ltd., Jiangsu, China), and finally retrograded in a 40
◦
C
drying oven for 12 h to form a stable gel structure.
3.4. TPA
The textural properties of the samples were determined according to a method described
by Xu et al. with some modifications [
42
]. A texture analyzer (CT3, Brookfield, Middleboro,
MA, USA) equipped with a TA10 probe (diameter 12.7 mm, length 35 mm, and weight 5 g)
was used for TPA. All of the samples were cut into
30 mm ×30 mm ×4 mm
cubes, after
which only the centers were compressed using the probe. The test parameters were set
as follows: a reduced distance of 3 mm, a trigger point load of 5 g, and pre-test, test, and
post-test speeds of 1.0 mm/s. The textural parameters, including hardness, cohesiveness,
springiness, and chewiness, were obtained via the instrument software TexturePro CT.
Molecules 2022,27, 3103 11 of 14
3.5. Color Measurements
A colorimeter (CM-3610, Konica Minolta, Japan) was used to determine L*, a*, and
b* color parameters of the samples. Before testing, the instrument was calibrated with a
whiteboard and a standard blackboard (provided by Konica Minolta), and all samples were
cut into 30 mm
×
30 mm
×
4 mm cubes. Five measurements were performed for each
sample, and the L*, a*, and b* values were recorded.
3.6. Preliminary Sensory Evaluation
The sensory panelists consisted of ten participants with food knowledge and tasting
experience. The sensory panelists were asked to rate the appearance, chewiness, stickiness,
and cohesiveness of the plant-based rinds according to four grades (very good, good,
medium, and bad). The samples were equilibrated at room temperature (26
◦
C) for 1 h
before the sensory evaluation. After each sample evaluation, the panelists were requested
to rinse their mouths with distilled water to avoid the experimental errors caused by the
residual taste of the previous sample.
3.7. Fuzzy Mathematics Sensory Evaluation
A fuzzy mathematics sensory evaluation model was established according to a proce-
dure previously described by Xue et al. to quantify the evaluation [43].
First, according to the quality attributes and scoring grades, the factor set was ex-
pressed as U= {u
1
, u
2
, u
3
, u
4
}, in which u
1
, u
2
, u
3
, and u
4
represented appearance, chewi-
ness, stickiness, and cohesiveness, while the grade set V= {v
1
, v
2
, v
3
, v
4
}, in which v
1
, v
2
,
v
3
, and v
4
represented very good (10 points), good (7 points), medium (4 points), and bad
(1 point).
Second, the preliminary sensory evaluation results were transformed into a fuzzy
matrix Rusing the fuzzy mathematical method.
Moreover, to determine the weight set A, 15 assessors (five professionals in the field of
plant-based meat and ten researchers with food knowledge) were asked to make one-to-one
comparisons between the importance of each quality attribute. In the three categories
denoting importance, “0:4”, “1:3”, and “2:2” represented “a large difference”, “a little
difference”, and “no difference”, respectively. The scores of each quality attribute were
accumulated and normalized to obtain weight set A= {0.235, 0.338, 0.222, 0.205}, where
0.235, 0.338, 0.222, and 0.205 correspond to the weight of appearance, chewiness, stickiness,
and cohesiveness, respectively.
Finally, the overall sensory comprehensive score (SCS) was calculated using
Equations (1) and (2):
Yj=A×Rj= [y1, y2, y3, y4] (1)
SCSj= 10 ×y1+ 7 ×y2+ 4 ×y3+ 1 ×y1(2)
Ywas the evaluation matrix, in which y1, y2, y3, and y4represented the membership
degrees for the grades of “very good”, “good”, “medium”, and “bad”, respectively, while j
represented the jth sample.
3.8. Rheometry
The rheological experiments were performed according to a method delineated by
Huang et al. with minor modifications [
44
]. The emulsion of the plant-based pork rind
was heated at 95
◦
C and stirred at 4000 rpm for 5 min in a cooking machine (Vorwerk,
TM5, Vorwerk, Wuppertal, Germany) and cooled to room temperature (25
◦
C) before
measurement. A stress-controlled rheometer (DHR-1, TA Instruments, New Castle, DE,
USA) equipped with a Peltier temperature control device was used to determine the
rheological properties of the samples. A sample of approximately 4 g was placed in the
center of the parallel geometric plate (diameter 40 mm), and the gap between the two plates
was set to 1 mm. The specimen excess was removed from the plates using a scraper, while
Molecules 2022,27, 3103 12 of 14
a thin layer of methyl silicone oil was used to prevent evaporation from exposed free edges
of the sample. The sample was equilibrated for 5 min before each measurement.
Frequency scanning (strain 1%, temperature 37
◦
C) was performed to determine the
variation in G’ and G” at different frequencies from 1.0 Hz to 10.0 Hz. Temperature scanning
(angular frequency 10 rad/s, strain 1%) was performed to determine the variation in the G’
and G” at different temperatures from 25 ◦C to 95 ◦C.
3.9. SEM
The samples were cut into 4 mm cubes, pre-frozen in a refrigerator at
−
20
◦
C for 48 h,
and dried in a vacuum freeze dryer (Marin Christ, Beta 1–8 LSC basic, Christ, Osterode,
Germany) for 48 h. The prepared samples were then attached to a metal holder and coated
with gold. The SEM images were obtained using a scanning electron microscope (Zeiss
Gemini 300 SEM, Carl Zeiss, Jena, Germany) at an accelerating voltage of 1.5 kV.
3.10. Statistical Analysis
The statistical analysis was performed using SPSS 25.0, while the graphs were prepared
using GraphPad Prism 9.0.0. An analysis of variance (ANOVA) was used to determine
significant differences between the results at a significance level of 0.05.
4. Conclusions
Based on the above results, we conclude that a plant-based gel can mimic natural pork
rinds while adjusting the SPS and KGM concentrations optimizes the texture, appearance,
and mouthfeel of plant-based pork rinds. The gelling mechanism of the plant-based
pork rinds is hypothesized: the three-dimensional network structures of the samples
result from the interaction between the SPS, SPI, and soybean oil with the KGM-SPS
complexes filling the pores and the NaHCO
3
promoting KGM and SPS interaction. Future
research intends to validate this hypothesis by further investigating the effect of SPI, soy
oil, and NaHCO
3
concentrations on plant-based pork rinds. Moreover, the insight into the
quantitative sensory parameters of plant-based pork rinds can be increased by introducing
oral tribology.
Author Contributions:
Conceptualization, H.L. and X.L.; formal analysis, Q.Z., L.H. and J.C.; in-
vestigation, Q.Z., D.Z. and Y.S.; resources, H.L. and X.L.; writing—original draft preparation, Q.Z.;
writing—review and editing, L.H., H.L. and X.L. All authors have read and agreed to the published
version of the manuscript.
Funding:
The research was supported by the National Key Research and Development Program of
China (2021YFC2101405).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author.
Conflicts of Interest:
J.C. is employed by Plant Meat (Hangzhou) Health Technology Limited Com-
pany. The remaining authors declare that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.
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