3.1. Major Chemical Indicators
The contents of the main chemical components in natural and inoculated sufu are shown in
Figure 2. The increase of moisture content in fermented food will loosen the protein network structure and weaken the bonding between protein molecules, resulting in a softer texture of the product [
19]. As shown in
Figure 2A, the moisture content in both natural and inoculated sufu was stabilized at 60–70% during post-fermentation, which was in accordance with the Chinese standard for locally marked products of sufu (2015) (DB53/T 713-2015) (moisture content ≤75 g/100 g), indicating that the fermentation strains had little effect on the moisture content in sufu (
p < 0.05), which was consistent with Cui [
20].
The main role of salt in the fermentation process of curd is to impart salty taste to the curd, inhibit enzyme activity, inhibit the growth of microorganisms, especially miscellaneous bacteria and pathogenic bacteria, and prevent quality problems caused by excessive fermentation of curd [
1]. The salt content in sufu is shown in
Figure 2B, where the salt content of natural and inoculated sufu was 2.5 g/100 g and 3.0 g/100 g, respectively, at 0 day of post-fermentation, and increased to 8.5 g/100 g and 8.2 g/100 g, respectively, at 90 days of post-fermentation. The main reason for the significant increase in the salt content of sufu is that the salt on the surface of the salt billet and in the soup penetrated into the inside of sufu during the post-fermentation process. The salt content at 180 days of post-fermentation showed less change compared with that at 90 days, indicating that the osmotic effect of salt content inside and outside the sufu billet had basically stabilized at 90 days of post-fermentation.
The total acid in curd mainly comes from amino acids, fatty acids and organic acids generated from protein, fat and carbohydrates under the action of enzymes, which is one of the important indicators of the quality and fermentation degree of curd. As shown in
Figure 2C, the total acid content in sufu increased with increasing fermentation time, which is consistent with the trend of total acid content in curd fermented by
Rhizopus oligosporus and
Rhizopus wagnerii [
20], while the difference between total acid content in sufu fermented naturally and by inoculation was not significant (
p ≥ 0.05).
Proteins in curd are degraded into small molecules of peptides, free amino acids and ammonia through fermentation, which play an important role in the texture and flavor formation of curd [
21,
22], while the water-soluble protein content is closely related to the degree of protein hydrolysis. As shown in
Figure 2D,E, the protein content in both natural and inoculated sufu was stable at about 13 g/100 g during post-fermentation, while the water-soluble protein content increased significantly (
p < 0.05) with the extension of fermentation time. At 0 days of post-fermentation, the water-soluble protein content in naturally fermented and inoculated fermented sufu was not significantly different (
p ≥ 0.05) and was around 2.1 g/100 g. At 90 days of post-fermentation, it increased to 4.25 g/100 g and 3.53 g/100 g, respectively, reaching the standard for locally marked products of sufu (Chinese standard for locally marked products of sufu marked products of sufu (2015) (DB53/T 713-2015)) (water-soluble protein content ≥ 3.5 g/100 g). The increase of water-soluble protein content was mainly due to the proteins in sufu being hydrolyzed into water-soluble small-molecule peptides and amino acids under the action of protease. After 180 days fermentation, due to the consumption of nutrients by microorganisms and the accumulation of metabolites, the increase rate of water-soluble protein content in sufu became slower, and the contents were 5.14 g/100 g and 5.25 g/100 g, respectively.
Amino acid nitrogen refers to the nitrogen element in the form of amino acid, which is the end product of protein hydrolysis. Therefore, the determination of amino acid nitrogen can further illustrate the hydrolysis of protein, and the amino acid nitrogen content (≥0.45 g/100 g) is often taken as the main indicator of curd maturity (Chinese standard for locally marked products of sufu marked products of sufu (2015) (DB53/T 713-2015)). The content of amino acid nitrogen in natural and inoculated sufu gradually increased with the extension of fermentation time
Figure 2F, which is consistent with the results of Cai [
23]. At 90 days of post-fermentation, the amino acid nitrogen content in natural and inoculated sufu reached 0.66 g/100 g and 0.55 g/100 g, respectively, both of which reached the standard for maturation of sufu. After 180 days of post-fermentation, the amino acid nitrogen reached 0.80 g/100 g and 0.74 g/100 g, respectively.
3.3. Free Amino Acids
Free amino acids (FAA) are an important taste source in fermented food. They mainly contribute to umami, sweetness and bitterness: Glutamic acid and Aspartic acid contributes to umami; Proline, Alanine, Serine, Lysine, Glycine, Valine and Threonine promote sweetness; while Leucine, Phenylalanine, Tyrosine, Isoleucine, Histidine, Arginase and Methionine contribute to bitterness [
15,
24,
25,
26]. In addition, they are also an important precursor for the formation of volatile flavor components [
27]. The changes of FAA in sufu fermented using both natural and inoculated bacteria are shown in
Table 3. The FAA in both types of sufu increased with the prolongation of fermentation time. Glu, an umami-flavored FAA, was apparently the most abundant of all these FAA, which was consistent with the previous results [
24]. Before fermentation (0 day), the total FAA in two types of tofu were 5.16 and 5.04 mg/g, respectively, and this amount was around 5 and 7.5-fold higher after 90 and 180 days of fermentation, respectively. In the post-fermentation stage, there was no significant difference (
p < 0.05) in the total amount of FAA in both natural and inoculated fermented sufu.
These FAA affected the taste profile when the ratio of concentration and threshold (TAV values) of these compounds was >1. In addition, the taste compounds will also affect the flavor through the synergistic effects of these compounds [
28].
Table 4 indicates the FAA TAV values in both natural and inoculated fermented sufu. There were three amino acids with TAV values > 1 in natural and inoculated fermented sufu before fermentation, namely, Glu, Ala and His, which indicated their greater contribution to the overall taste profile. The TAV values of the other 13 amino acids were all <1, indicating that their contribution to the taste was not prominent. After 90 days post-fermentation, the number of TAV values >1 increased to 10 in both natural and inoculated fermented sufu, with the remaining 6 amino acids <1. After 180 days post-fermentation, the number of FAA with TAV values > 1 was higher in naturally fermented sufu than inoculated fermented sufu. Among all these FFA, Glu with an umami taste had the highest TAV value, followed by Val and Lys with sweet tastes. The synergetic function of these flavor amino acids mostly accounts for the unique flavor of sufu. In general, with the prolongation of fermentation, the content of FAA in sufu increased significantly, resulting in a stronger taste. There was no significant difference (
p < 0.05) in the total amount of FAA between natural and inoculated fermented sufu at the same fermentation stage.
3.4. Textual and Structural Properties
TPA analysis was carried out on both natural and inoculated sufu, and the results are shown in
Table 5. The hardness of both types of sufu decreased during the fermentation, with inoculated fermented sufu higher than naturally fermented sufu, which was consistent with Xia, Li, Zheng, Ran and Kan [
29]. This reduction may result from the enzymatic hydrolysis of the three-dimensional protein network [
30], which also led to increased FAA, as shown in
Table 2. The texture of sufu could be an indicator of sufu ripening [
31,
32]. A similar trend was observed for the changes in adhesiveness, which was consistent with Zhao and Zheng [
33]. However, the type and time of fermentation had no significant effect on the springiness of sufu.
The microstructure of natural and inoculated fermented sufu at 90 and 180 days post-fermentation was observed using SEM (
Figure 3). With the increase of fermentation time, the internal diameter of both types of sufu gradually decreased, and the internal structure became smooth, which was consistent with previous reports [
33,
34]. This may be related to protein hydrolysis. After 90 days post-fermentation, the interior of sufu had a network structure with more large pores, which may be due to the low degree of protein hydrolysis. The colloidal particles of protein were large and the damage to the protein gel structure was limited. After 180 days post-fermentation, both types of sufu had a significantly smaller pore size and were denser and more uniform than at 90 days, which may be related to the greater degree of hydrolysis of protein. At the same fermentation time, the internal gel pore size of naturally fermented sufu was slightly smaller than that of inoculated fermented sufu, indicating that the degree of protein hydrolysis in naturally fermented sufu was slightly higher, which was consistent with data in
Table 1.
3.5. Aroma Profiles
The aroma compounds in natural and inoculated sufu at 0, 90 and 180 d post-fermentation were obtained using GC-MS (
Table 6 and
Figure 4). Esters, alcohols, acids, aldehydes and alkenes were the main aroma components in the fermented sufu. The total number of aroma compounds increased continuously for both types of sufu with fermentation, while the total number of aroma compounds increased significantly after 90 days and did not change at 180 days. Esters and alcohols were among the most numerous aroma compounds, which was consistent with previous studies [
1,
12]. At the beginning of fermentation, added alcohol could react with acids, such as free fatty acids, through esterification, which resulted in the increase of both the amount and number of esters. After a period of fermentation, the number of esters had been maximized but the amount kept increasing. The content of esters was positively correlated with the added alcohol content [
7]. Most of the esters have a unique aroma, which can give the sufu a sweet smell. Among them, ethyl caprylate, ethyl palmitate, ethyl linoleate, ethyl acetate, 2-phenylethyl propionate and ethyl phenylacetate are associated with fruity, milky and floral flavors. Ethyl benzoate and methyl salicylate contributed wintergreen oil aroma, and ethyl myristate and gamma nonalactone contributed a coconut aroma [
35]. With the increase of fermentation time, the content of aroma compounds in natural sufu was higher than inoculated sufu, while the number of aroma compounds was lower.
Principal component (PC) analysis of aroma compounds of natural and inoculated fermented sufu at 0, 90 and 180 days post-fermentation is shown in
Figure 3D. The sufu with different fermentation strains and fermentation times could be distinguished and the contribution percentages of PC 1 and 2 were 53.9 and 23.4%, respectively. Among them, PC 1 was mainly positively correlated with ES1-ES15, AL1-AL7, AL9, AL10, AL12, AC3, AC4, AC5, AD1, AD2, AD5, AE2-AE4, KT2 and HC1-HC3, while PC 2 was positively correlated with AL8, AL11, AC2, AD3, AD4 and AE1. Sufu fermented with different strains could be distinguished by PC 1, while the sufu with different fermentation times with the same strains were significantly differentiated by PC 2.
3.7. Biogenic Amines Content
The HPLC chromatograms of the biogenic amine standard derivatives and the regression equation tables for the linearity of each biogenic amine derivative are shown in
Figure A1 and
Table A1. The biogenic amine contents of natural and inoculated sufu at post-fermentation 0 days, 90 days and 180 days are shown in
Table 7. The total biogenic amine content in sufu tended to increase gradually with the increase of fermentation time. The main biogenic amines in sufu were putrescine, cadaverine, histamine and spermidine, among which, putrescine and cadaverine were associated with unhygienic environmental conditions of food processing [
36]. Thus, it can be seen that the hygienic conditions of sufu production and processing still need to be improved. The contents of β-phenylethylamine, putrescine, tyramine and total biogenic amines in inoculated sufu were significantly lower than those in natural sufu (
p < 0.05), probably because of the stronger activity of amino acid decarboxylase produced by microorganisms in inoculated sufu. Notably, the histamine content in inoculated sufu was significantly higher than that in natural sufu (
p < 0.05), but all were lower than 100 mg/kg, and Nout [
37] concluded that the histamine content in foods within the range of 50–100 mg/kg could be considered as the production process meeting the requirements of good practice. The production of histamine can be effectively inhibited by inhibiting the activity of histidine decarboxylase in foods, and Kang [
38,
39] found that citric acid, malic acid, sorbic acid, succinic acid and sorbitol significantly inhibited the activity of histidine decarboxylase and suppressed the production of histamine in mackerel.