Effects of Organic Acid Coagulants on the Textural and Physical–Chemical Properties of Tofu

Abstract

Tofu is obtained by heating soymilk, to which a coagulant, such as calcium sulfate or magnesium chloride, is added to make it curdle. This study aimed to parameterize the effects of the following three alternative organic coagulant types: apple, rice, and white vinegars, used in different proportions. Six treatments were established with three concentrations (1%, 3%, and 5%), evaluating the coagulation time and curd yield. The treatments with the lowest coagulation time were analyzed for texture by TPA, color through the CIEL*a*b scale, protein content, and moisture. The results showed that the rice vinegar + 3% white vinegar (T6C3) treatment showed the lowest coagulation time (0.78 min). The 5% apple vinegar (T1C5) treatment provided the highest curd yield, averaging 23.73%. This treatment’s protein and moisture contents were 3.93% protein and 69.73% moisture, confirming that better texture characteristics are recorded in tofu at lower pH values. The TPA results showed that using apple and rice vinegars as coagulants provided a challenging, less cohesive, more adhesive, and less elastic tofu. White vinegar provided a soft, more cohesive, less adhesive, and more elastic tofu. In the color analysis, it was observed that tofu coagulated with apple vinegar showed a tendency toward a yellow color, and tofu coagulated with rice and white vinegars showed a tendency toward a white color. These findings parameterize the effects of using each type of vinegar as a coagulant. These organic coagulants provide faster coagulation times and desirable texture characteristics, thus offering a practical alternative to traditional coagulants in tofu manufacturing.

1. Introduction

Tofu is a plant-based food made from curdled soymilk. For its production, soybeans are soaked, ground, and boiled to produce soymilk. When the soymilk is hot, a coagulant, such as calcium sulfate or magnesium chloride, is added to make it curdle. The resulting curd is pressed into blocks [1]. Tofu is prized for its flexibility in cooking and its health benefits; it has a high protein content and low calories, is low in saturated fat, and does not contain cholesterol. It is a good source of iron and calcium, and contains all of the essential amino acids [2]. Worldwide, soybeans are used in healthy diets due to their high content of flavones and anthocyanins, protein, and various micronutrients beneficial to human health [3].

Research on soy-based foods, such as tofu, is a trending area that highlights its relevance in food processing [4]. Tofu is recognized for its gelatinous texture and nutritional value [5], comprised of 23.8% total solids, 15.4% protein, 9% fat, 62 mg calcium, and 580 mg phosphorus present per 100 g [2]. The tofu manufacturing process spans a broad history and involves a variety of technologies and treatments [5]. In turn, the protein content of the soybean used directly impacts the yield and quality of the tofu [1].

For instance, coagulation is achieved using salt, enzymes, and acid to create firm, silken, and soft tofu. Calcium sulfate and magnesium chloride are commonly used as salt coagulants; however, residual undissolved calcium sulfate in tofu or the rapid release of magnesium chloride often leads to a coarse texture, which is a sign of poor quality [6]. Microbial transglutaminase is commonly used to enzymatically crosslink soy proteins to create tofu with a low gel hardness, though this method is time-consuming. Additionally, glucono-d-lactone (GDL) as an acidifying agent has been extensively researched to produce tofu with a uniform network. It is important to note that proteins gel at a pH range from 5.6 to 5.8, depending on the soy variety used, irrespective of the GDL concentration [7]. Therefore, selecting coagulants and treatments is crucial, as it affects the nutritional and textural properties [8]. In contrast, various factors during processing, such as soaking, milling, pretreatment, the types of coagulants used, the operating conditions, pressing, packaging, and storage, affect the final quality [9].

Therefore, various techniques have been developed using alternative coagulants, such as Epson salt, which provides beneficial effects on the functional properties and decreases the susceptibility to spoilage [10]; in turn, coagulants obtained from crab shell waste were shown to increase the calcium levels and improve the overall firmness and texture of tofu [11]. Due to the influence of pH and ionic strength on the gelation process, tofu produced from organic acids exhibits remarkable physical characteristics, such as high stiffness, hardness, and a low non-freezable water content [12]. Also, using acid to cause protein precipitation improves the hydrophobicity, emulsification, and gel strength of the protein, as well as its rheological and sensory properties [13,14].

Different types and concentrations of organic acids as coagulants can give rise to tofu with differences in texture, microstructural and rheological properties, pH levels, and chemical interactions in a gel. In turn, its acidification process can be accelerated, reducing the time available for the orderly aggregation of soy proteins, thus reducing the quality of the tofu [7]. This study aimed to evaluate three coagulants (apple, rice, and white vinegars) and their combinations concerning their effectiveness on the coagulation time and the textural and physical–chemical properties of the resulting tofu. Our study specifically evaluated using apple vinegar, rice vinegar, and white vinegar as coagulants for tofu production. These options were selected due to their natural acidity, availability, and potential to impart unique flavors and nutritional benefits to the tofu. Apple vinegar was chosen for its mild sweetness and potential antioxidant properties, rice vinegar for its subtle flavor and traditional use in Asian cuisine, and white vinegar for its firm acidity and widespread use.

We hypothesized that using these alternative coagulants would enhance the texture and physicochemical properties of the tofu. Specifically, we expected improvements in the firmness, cohesiveness, and water-holding capacity, and desirable changes in the pH levels and nutrient profiles. This evaluation aimed to explore these coagulants’ potential benefits and viability in tofu production, contributing to more diverse and healthful tofu options for consumers.

2. Materials and Methods

2.1. Raw Materials

Soybeans were sourced from a local market in the Daule canton. The beans had a protein content of 38% and a moisture content of 12%. The vinegars (rice, apple, and white) were purchased locally from a market in Guayaquil, Ecuador. The rice vinegar had a protein content of 0.3% and an acidity of 4.5%. The white vinegar had a protein content of 0.0% and an acidity of 5%. The apple vinegar had a protein content of 0.1% and an acidity of 5%.

2.2. Experimental Design

The soybean milk was elaborated with some modifications according to the indications made in [15]. Firstly, 35 kg of soybean grains were soaked for 10 h. The swollen grains were drained and then hulled. Afterward, they were scalded at 90 °C for 10 min. The grains were drained and poured in an industrial blender (Metvisa LQL.811, Bimg Brazil Machinery Industry for Gastronomy, Santa Catarina, Brazil) to be crushed using a 1:1 (w/w) ratio with water; the obtained mixture was filtered with an ultrafine 120-µm nylon mesh sieve (Merck Millipore from Burlington, MA, USA), and the resulting soymilk was pasteurized at 100–110 °C during 10 min. Later, the coagulation process was carried out, where the treatments were applied according to

Table 1. Experimental design, mean values, and standard errors of the coagulation time.

Treatment/Vinegar and Combinations	Denomination (Nomenclature)	Dose % (Concentration)	Coagulation Time Means
T1 Apple	T1C1	1	6.5 a ± 0.51
	T1C3	3	3.99 c ± 0.03
	T1C5	5	2.85 d ± 0.21
T2 Rice	T2C1	1	4.73 b ± 0.31
	T2C3	3	0.99 i ± 0.33
	T2C5	5	1.43 g ± 0.14
T3 White	T3C1	1	2.08 e ± 0.17
	T3C3	3	1.30 gh ± 0.16
	T3C5	5	1.58 fg ± 0.11
T4 Apple + Rice	T4C1	1	3.78 c ± 0.10
	T4C3	3	1.63 fg ± 0.13
	T4C5	5	1.55 fg ± 0.07
T5 Apple + White	T5C1	1	1.65 fg ± 0.08
	T5C3	3	1.21 h ± 0.13
	T5C5	5	0.89 i ± 0.13
T6 Rice + White	T6C1	1	1.73 f ± 0.11
	T6C3	3	0.78 i ±0.09
	T6C5	5	0.93 i ± 0.08

Note: ^a–i means in the same column with different superscripts significantly differ (p < 0.05).

: T1C5 = tofu coagulated with apple vinegar at a 5% concentration; T2C3 = tofu coagulated with rice vinegar at a 3% concentration; T3C3 = tofu coagulated with white vinegar at a 3% concentration; T4C5 = tofu coagulated with apple vinegar + rice vinegar at a 5% concentration; T5C5 = tofu coagulated with apple vinegar + white vinegar at a 5% concentration; T6C3 = tofu coagulated with rice vinegar + white vinegar at a 3% concentration. The analysis of the physical–chemical characteristics, the texture profile analysis, and the color analysis were performed on all of the treatments.

The tofu was elaborated with soymilk (72 L) with 3.54% protein, which was taken to a temperature of 38 °C to be divided into 1-L samples, which were placed in transparent recipients; the coagulants were then added at concentrations of 1%, 3%, and 5%, taking readings of the coagulation times with the aid of a digital clock with chronometers (AL-613A, Takshun Electronic Co., Ltd., Putian, China) until visible curd formation. After the curd was formed, perforated recipients eliminated the residual whey. The curd yield was expressed as g of the obtained tofu per 1 L of soymilk, which was later expressed as a percentage. The treatments that achieved a shorter clotting time had all of the analyses performed. The tofu varieties obtained per treatment were labeled as follows: T1, apple vinegar; T2, rice vinegar; T3, white vinegar; T4, apple vinegar + rice vinegar; T5, apple vinegar + white vinegar; T6, rice vinegar + white vinegar, accompanied by C1, C3, and C5, which corresponded to the used concentrations (1%, 3%, and 5%).

2.3. Analytical Determinations

2.3.1. Protein Analysis

The protein content in each tofu sample was determined using the Kjeldahl method, according to AOAC 955.04D (using the coefficient 6.25). The protein determinations were the average of ten measurements. This method was used for the protein digestion and total N counting from the acid digestion (with sulfuric acid and catalysts), and required a significant time of up to 10 h.

2.3.2. Moisture Determination

The moisture content in each tofu sample was determined according to the AOAC method 925.10. Based on the weight loss that the sample underwent by heating, a constant weight was obtained.

2.3.3. Ash Determination

The ash content of the tofu produced with different coagulants was measured because the ash content indicates the amount of minerals, which can vary depending on the type of vinegar used in the coagulation. The amount of minerals allows for the assessment of its influence on the nutrition. It also allows for a comparison of the effects of the coagulants. Due to their unique compositions, different types of vinegar (apple cider, rice vinegar, and white vinegar) can contribute different mineral profiles to the tofu. For example, apple cider vinegar and rice vinegar may contain traces of specific minerals not present in white vinegar.

Finally, it allows one to understand the variations in the texture and the physical properties. Minerals can influence the structure and texture of the tofu. A higher mineral content could result in a firmer, more cohesive texture.

By evaluating the ash content, we could determine how each type of vinegar affected not only the physicochemical properties of the tofu but also its nutritional value. It provided valuable information for optimizing the production and creating healthier and more attractive consumers products.

The mineral content in the tofu was determined by applying calcination to the ash at 550 °C for 4 h. The basic concept of incineration involves the combustion of organic matter and the evaluation of inorganic waste. This process was carried out in two phases: initially, to remove the moisture and then to completely carbonize the sample.

2.3.4. Texture Profile Analysis (TPA)

The textural properties in the tofu were evaluated through a texture profile analysis (TPA), according to the previous indications [16,17], using a CT3 Brookfield texture analyzer with a TA4/1000 cylindrical probe (AMETEK Brookfield America Centre of Excellence, Middleborough, MA, USA). Each tofu sample was cut into 30-mm cubic forms, and a 50% compression force was applied at a speed of 1.0 mm/s. The curve of the texture profile analysis was recorded in the Texture Pro CT V1.8 Build 31 program (AMETEK Brookfield America Centre of Excellence).

2.3.5. Color Measurement

Our study measured specific parameters using the CIELAB scale to evaluate the color aspects of the tofu produced with different coagulants. For this, a Minolta colorimeter CR-400 (Illuminant D65, Konica Minolta, Osaka, Japan), which was calibrated with a black glass plate, was used. The CIELAB scale measures color in the following three dimensions:

L*: Represents the lightness of the color, with values ranging from 0 (black) to 100 (white). This parameter indicates how light or dark the tofu is.

a*: Indicates the position on the red–green axis. Positive values of a indicate the presence of red, while negative values indicate the presence of green. This parameter helps to evaluate any reddish or greenish tint in the tofu.

b*: Indicates the position on the yellow–blue axis. Positive values of b indicate the presence of yellow, while negative values indicate the presence of blue. This parameter is used to measure yellow or bluish tones in the tofu.

By measuring these parameters, we can obtain a complete characterization of the tofu color, which is crucial to assess the impact of the different coagulants (apple cider vinegar, rice vinegar, and white vinegar) on the final product’s appearance. Color assessment is essential, as it can influence consumer perception and product acceptance.

2.4. Statistical Analysis

Factorial ANOVA analysis was carried out in the statistical program InfoStat version 20.0, comparing the physical–chemical parameters, textural properties, and color. Tukey’s test was performed with a significance level of p < 0.05 to determine the significant differences.

3. Results and Discussion

3.1. Coagulation Time in the Elaboration and Yield of Tofu and Compositional Analysis in Tofu Elaboration

The coagulation time for tofu is significantly influenced by the type and concentration of vinegar used as a coagulant. Table 1 shows the coagulation times with concentrations of 1%, 3%, and 5%; the results of each treatment are represented as the mean time in minutes, obtaining intervals from 3.99 to 0.78 min to show the presence of curd. In treatments T1, T4, and T5, it is observed that, at a 5% coagulant concentration, there was a decrease in the time required for curd formation. In contrast, treatments T2, T3, and T6 showed that the lowest coagulation times were obtained with a 3% coagulant concentration.

Differences in the coagulation time compared to the experiments conducted by other researchers were related by applying scalding as a pretreatment. Scalding accelerates the coagulation because heating soymilk denatures protein particles, mainly glycinin and β-conglycinin, exposing hydrophobic groups. There is a specific relation between thermal induction and coagulation process changes, as glycinin and β-conglycinin have different denaturation temperatures [18,19]. Other factors affecting the coagulation time include the temperature and coagulant concentration, which are proportional to the quantity of applied coagulant [20]. The results indicate that the 3% and 5% concentrations significantly accelerated the coagulation time, with the lowest time being 0.78 min, showing that the amount of coagulant is crucial.

Table 2. Mean yield, protein, moisture, and ash values in the treatments.

Treatment	Yield (%)	Protein (%)	Moisture (%)	Ash (%)
T1C5 1	23.73 a ± 0.43	3.93 a ± 0.13	69.74 b ± 0.23	0.68 a ± 0.25
T2C3 2	23.45 a ± 0.87	3.25 a ± 0.07	73.19 a ± 1.40	0.65 a ± 0.04
T3C3 3	22.48 a ± 1.28	2.7 a ± 0.42	76.03 a ± 1.93	0.70 a ± 0.10
T4C5 4	22.08 a ± 1.96	1.95 a ± 0.78	73.29 a ± 0.63	0.73 a ± 0.12
T5C5 5	19.18 b ± 0.46	2.45 a ± 0.92	74.05 a ± 1.21	0.58 a ± 0.07
T6C3 6	15.73 c ± 1.11	2.85 a ± 0.35	74.50 a ± 1.33	0.55 a ± 0.03

¹ T1C5 = tofu coagulated with apple vinegar at a 5% concentration; ² T2C3 = tofu coagulated with rice vinegar at a 3% concentration; ³ T3C3 = tofu coagulated with white vinegar at a 3% concentration; ⁴ T4C5 = tofu coagulated with apple vinegar + rice vinegar at a 5% concentration; ⁵ T5C5 = tofu coagulated with apple vinegar + white vinegar at a 5% concentration; ⁶ T6C3 = tofu coagulated with rice vinegar + white vinegar at a 3% concentration. ^a–c means in the same column with different superscripts significantly differ (p < 0.05).

shows the tofu yields, indicating that the yield increases with longer coagulation times. The results indicated that, while there were significant differences in the tofu yield, no significant differences were found for the protein, moisture, and ash contents across the treatments (p > 0.05). The mean values and standard deviations are presented in Table 2, demonstrating that treatment T1C5 had the highest yield (23.73%), followed by T2C3 (23.45%), and T6C3 had the lowest yield (15.73%). Despite the variations in the yield, the compositional analysis showed consistent protein, moisture, and ash contents across all treatments.

Treatment T1C5, which required the most time for curd formation, achieved a 23.73% yield. However, there were no statistical differences among the treatments. The tofu yield is linked to the protein types present; the T1C5 (5% apple vinegar) treatment showed a higher curd yield despite taking longer to coagulate compared to the T6C3 (3% rice vinegar + white vinegar) treatment, which was considered the best treatment for its significantly reduced coagulation time. The T6C3 treatment achieved a pH of 4.7, promoting the precipitation of reserve proteins (globulins 11S and 7S) at their isoelectric pH (4.5 to 4.8), accelerating the coagulation but leaving protein remnants in the whey [21].

The T1C5 treatment obtained a pH of 4.9, delaying its coagulation time but resulting in higher protein accumulation and yield, with a more transparent whey, indicating better protein utilization. The pKa (negative logarithm of the acid dissociation constant (Ka) of an acid) of the acids influences the yield; lower pKa acids tend to ionize more, promoting protein flocculation. In the T6C3 treatment, acetic acid (pKa = 4.79) was prevalent, while the T1C5 treatment contained malic acid (pKa = 3.45). Stronger acids lead to better protein flocculation and higher yields.

In another study [22], lower yield values were observed due to the longer agitation times required to dissolve the salts, which can destroy the gel structure, producing higher tofu yields and a thicker texture [23]. This study’s results align with those of other authors, who reported tofu yields of 42.33%, 37.69%, and 39.70% for regular, germinated, and combined tofu, respectively. The variation in soybean varieties and treatment processes explains the differences in the tofu yield percentages [24].

Table 2 also shows the results of the different moisture, ash, and protein contents in the treatments. The highest protein content in the studied product is attributed to the lower precipitation of storage globulins caused by the coagulants, leading to higher retention of soybean proteins during the protein network formation in the tofu. This result is consistent with the recorded moisture, protein, and yield values for treatments T1C5 and T2C3, which had the lowest moisture, highest protein, and highest yield values.

3.2. Texture Profile Analysis (TPA)

No differences in hardness and tractability are noted in the evaluated textural properties (

Table 3. Texture profile analysis of the tofu in each evaluated treatment.

Treatment	Hardness (N)	Fracturability (N)	Adimensional Cohesiveness < 1	Adhesiveness (g/cm)	Adimensional Elasticity < 1
T1C5 1	31.70 b ± 3.18	32.22 b ± 3.00	0.31 a ± 0.04	25.00 b ± 5.37	0.45 a ± 0.05
T2C3 2	64.16 a ± 1.20	63.44 a ± 0.99	0.20 c ± 0.04	30.18 a ± 1.56	0.30 c ± 0.07
T3C3 3	11.95 d ± 0.03	11.96 d ± 0.05	0.25 b ± 0.03	16.04 c ± 0.93	0.38 b ± 0.05
T4C5 4	15.60 c ± 0.12	15.54 c ± 0.10	0.24 bc± 0.05	14.50 c ± 1.74	0.35 b± 0.07
T5C5 5	15.23 c ± 0.27	15.40 c ± 0.21	0.34 a ± 0.05	12.63 c ± 0.52	0.48 a ± 0.07
T6C3 6	9.52 e ± 0.41	9.64 e ± 0.06	0.30 ab ± 0.02	13.13 c ± 0.36	0.35 b ± 0.07

¹ T1C5 = tofu coagulated with apple vinegar at a 5% concentration; ² T2C3 = tofu coagulated with rice vinegar at a 3% concentration; ³ T3C3 = tofu coagulated with white vinegar at a 3% concentration; ⁴ T4C5 = tofu coagulated with apple vinegar + rice vinegar at a 5% concentration; ⁵ T5C5 = tofu coagulated with apple vinegar + white vinegar at a 5% concentration; ⁶ T6C3 = tofu coagulated with rice vinegar + white vinegar at a 3% concentration. ^a–e means in the same column with different superscripts significantly differ (p < 0.05).

). In addition, the coagulant type does influence these properties, because the lowest indices in strength required to deform the foodstuff are in the treatments where white vinegar has been applied as the coagulant. In the case of the cohesiveness, it is influenced by the coagulant because the highest values are reached when the apple vinegar and white vinegar are applied, thus causing the tofu to acquire a higher cohesiveness; that is, the limit point to deform the sample and the necessary work to overcome the internal bond in it is increased in the tofu samples on which these to coagulants have been applied.

The adhesiveness increases when using the apple vinegar and the interactions of the rice vinegar coagulants, for which higher effort was needed to separate the surface of the coagulated samples from the texturometer surface with these kinds of vinegar. Concerning the elasticity property, it is noted that the coagulant type exerts an influence on it; the highest elasticity indices were found in the treatments where the apple vinegar was used, and where the apple vinegar was combined with another vinegar type. The tofu obtained by applying these kinds of vinegars as a coagulant recovered more than 40% of its structure after applying the force for its deformation. Regarding these properties, it was observed that the treatments in which the apple vinegar (T1C5), rice vinegar (T2C3), and their combinations (T4C5) were applied showed the highest hardness and fracturability values, with a required force of up to 64.16 N to achieve its rupture; however, in the treatments in which the white vinegar was applied as a coagulant (T3C3), or in combination (T6C3), soft tofu was obtained, for which the force required to achieve its rupture was very low.

Concerning the cohesiveness property, it was obtained that the highest values were found in the treatments where the apple vinegar and the combination of apple vinegar with white vinegar were applied; thus, achieving the highest cohesiveness was obtained by the T5C5 treatment with a value of 0.34. For the adhesiveness property, it was proven that the treatment with higher adhesiveness (stickier) was the T2C3 (rice vinegar) treatment with 30.18 g/cm, which showed the highest effort required to separate the sample from the texturometer arm. In the elasticity property, it was shown that the treatments that obtained higher elasticity indices were T1C5 with 0.45, T3C3 with 0.38, and T5C5 with 0.48; that is, these treatments recovered more than 40% of the tofu shape after the compression cycle. In a study in which defatted soybean meal was added to the tofu to improve its physical properties by applying the coagulant magnesium chloride and 0.5% glucono delta lactone (GDL), the texture profile of white tofu (without adding defatted soybean meal) was evaluated with TPA in cubic samples of a 2-cm diameter by applying 50% compression and speed of 2.00 mm/s, which obtained the following results: a hardness of 0.52 N, a cohesiveness of 0.60, an adhesiveness of 0.31 g/cm, and an elasticity of 0.40 [16]. Meanwhile, another study evaluated the technological and sensorial properties of a tofu-similar product by applying milk whey to the soybean extract and using the coagulant lactic acid and 0.5% glucono delta lactone (GDL), in which the texture profile was studied by performing TPA on cubic samples of 1 cm diameter, a speed of 1.0 mm/s, and a compression of 75% [17]. They obtained the following results: a hardness of 2.52 N, a cohesiveness of 0.58, an adhesiveness of 1.47 g/cm, and an elasticity of 0.44. Thus, the results reached in this research do not coincide with any cited author; this is because the above-cited authors applied glucono delta lactone (GDL) in their research, and this coagulant has an incidence on textural properties. As mentioned, glucono delta lactone (GDL) was used as an acidity regulator, providing a gradual decrease in the pH, which caused very slow acidification, and thus no flocculation of proteins occurred (as with strong acids); on the contrary, protein gelification was produced, thus obtaining products of soft but fragile textures in the form of gels [25]. Therefore, comparing the obtained results with the ones obtained by the authors mentioned above, it is possible to indicate that the tofu from this research showed different textural properties; that is, the tofu samples with higher hardness values were less cohesive and more adhesive, while the least hard ones showed higher cohesiveness and lower adhesiveness, thus proving that the tofu obtained by using different kinds of vinegar as coagulants had excellent textural properties. These findings can be explained because, within the evaluated matrix for the set-up of the treatments, when applying the coagulant, pH values from 4.7 to 5.0 were managed upon coagulation, as well as applying high temperatures for cooking the soymilk. Then, it was reduced to begin the coagulation process, thus utilizing the textural properties of the reserve globulins (7S and 11S), preserving the proteins in the soybean. The protein content played a crucial role in determining the texture of the tofu, mainly when different types of vinegar were used as coagulants. The most critical proteins in soybean are reserve globulins 7S (β-conglycin) and 11S (glycinin), with a quaternary structure, which has a high presence of amino acids, such as lysine, leucine, and isoleucine; these proteins are essential in forming the tofu’s structure and precipitate at pH from 4.5 to 4.8. These proteins denature and aggregate upon heating, forming a gel that dictates the tofu’s textural properties [26]. The 11S/7S ratio is particularly significant, with higher ratios contributing to the increased hardness, elasticity, and chewability of tofu [27].

Another study mentions that, independently from the coagulant types used, everything depends on the protein subunits of group IIb, which play an essential role in contributing to the tofu firmness. In contrast, the subunit of group IIa negatively affects the hardness and firmness of the tofu [27]. Additionally, it is mentioned that such proteins show SH (thiol) groups and SS (disulfide) bonds, which influence their properties (gelification, adhesion, cohesion, and elasticity); these properties are influenced by the pH, temperature warming, and cooling speed.

When using different types of vinegar, the acidic environment affects the protein structure and interactions. Vinegar with lower pH values, such as apple vinegar, can enhance the denaturation and aggregation of proteins, resulting in firmer and more elastic tofu. This study shows that the tofu made with apple vinegar (T1C5) and rice vinegar (T2C3) exhibited higher hardness and fracturability values, requiring greater force to rupture. This is because the lower pH of these vinegars promotes better protein flocculation and more robust gel networks [26]. The interactions of these proteins, facilitated by the acidic conditions from the vinegar, involve hydrogen bonds and disulfide bridges, which contribute to the rigidity and elasticity of the tofu [16]. Higher protein content, as seen in treatments T1C5 (3.93%) and T2C3 (3.25%), correlates with higher hardness and improved textural properties. Thus, the type and concentration of vinegar used in coagulation significantly influence the texture of the tofu by affecting the protein interactions and gel formation [26].

Several studies have determined that the gel properties and texture characteristics of tofu are positively related to the hardness, gummosity, and chewability [7,28], for which it is mentioned that the textural properties of tofu are one of the most critical quality traits used to evaluate the acceptability of tofu. Firm tofu often has higher hardness and cohesion, so more power is needed to break its structure; it shows higher elasticity and chewability. Thus, the factors that determine the formation of the tofu structure are closely related to the coagulation process [29]. Regarding the properties of these proteins on the tofu texture, it is stated that glycinin (11S) and β-conglycin (7S), particularly in the hardness, elasticity, cohesion, and chewability of tofu, have a significantly positive relation with the 11S/7S ratio, for which they can form arranged structures, because, in the gelification process generated in the cooking stage, heat is induced at high temperatures (>78 °C) and the exposure of functional groups is achieved, so that, later, in incubation with a reduction in temperature, an interaction among the exposed functional groups occurs, where these groups provide more strength to the tofu structure through hydrogen bonds [30]. The exposure of these proteins to temperatures from 80 °C to 100 °C causes the dissociation of globulin 11S into polypeptides, thus obtaining more rigid gels (tofu); also, at 80 °C, SS bonds (disulfide) occur, which confer elasticity to the tofu. These findings at the level of texture parameters reported in the analyzed samples with different vinegars are, in turn, related to the protein content results reported in Table 2, where the contents in treatments T1C5 (3.93%) and T2C3 (3.25%), the treatments which also reported the highest hardness values (T1C5, 31.7 N; T2C3, 64.16 N), stand out. Likewise, the pH values recorded in the used kinds of vinegar oscillate from 2 to 4.6, recording the lowest values in the apple vinegar. This subsequently promotes the texture properties of the hardness and fracturability mentioned above.

3.3. Color Measurement

The evaluated samples show significant differences in the luminosity and the evaluated chromaticity components (a* and b*), thus showing that the T6C3 treatment had a higher trend toward luminosity L*, for a brilliant tofu with a tendency toward white; the T1C5 treatment showed higher values in b*, for a tofu with a tendency toward yellow; the T3C3 treatment showed tendencies for L* and B*, where the tofu obtained showed a tendency toward white and yellow. Regarding the values obtained in a*, it is observed that the highest tendency was shown by the T5C5 treatment, for which this tofu tended toward green/red; nevertheless, its values in L* and b* increased, showing a tendency toward white and yellow, for which it can be mentioned that the tofu obtained in this treatment showed an opaque tonality. For such reasons,

Table 4. Colorimetry values according to the CIE L*a*b* scale in the treatments.

Treatment	L*	a*	b*
T1C5 1	88.55 c ± 0.67	0.09 d ± 0.19	14.67 a ± 0.72
T2C3 2	91.13 ab ± 0.32	0.57 c ± 0.19	14.54 ab ± 0.30
T3C3 3	91.57 a ± 0.46	0.48 c ± 0.21	14.85 a ± 0.28
T4C5 4	90.82 b ± 0.75	0.95 b ± 0.21	14.45 b ± 0.57
T5C5 5	91.53 ab ± 0.58	1.23 a ± 0.25	13.56 c ± 0.61
T6C3 6	91.63 a ± 0.36	0.86 b ± 0.18	13.99 c ± 0.28

¹ T1C5 = tofu coagulated with apple vinegar at a 5% concentration; ² T2C3 = tofu coagulated with rice vinegar at a 3% concentration; ³ T3C3 = tofu coagulated with white vinegar at a 3% concentration; ⁴ T4C5 = tofu coagulated with apple vinegar + rice vinegar at a 5% concentration; ⁵ T5C5 = tofu coagulated with apple vinegar + white vinegar at a 5% concentration; ⁶ T6C3 = tofu coagulated with rice vinegar + white vinegar at a 3% concentration. ^a–c means in the same column with different superscripts significantly differ (p < 0.05).

shows that each one of the coagulants applied in obtaining the tofu interfered with the tonalities of the final product, but comprised the tonalities of tofu, which were from white to yellow. The color changes in the tofu were due to non-enzymatic browning, specifically the Maillard reaction [31]; thus, the color tendencies will depend on the variation in the soybean, germination time, and thermal treatment. Because of the presence of chlorophyll, all of the soybean varieties were green in the first maturity stages. As they matured, the chlorophyll was lost and flavonoid pigments appeared [32].

The observed differences in the tofu color might result from variations in the soybean varieties and the specific processes used in the production. Previous studies have shown that different soybean varieties and processing conditions, such as heating and agitation, significantly affect the tofu color and texture [24].

The type and concentration of coagulant, and the pH during coagulation, play crucial roles in determining the tofu color. Coagulants with different acidities and buffering capacities can cause variations in the final product’s appearance. For instance, apple vinegar and rice vinegar have different pH levels and buffering capacities, which can influence the Maillard reaction and resulting color [33]. The Maillard reaction, responsible for browning, can vary based on the specific amino acids and sugars present in the soybeans, the processing temperature, and the duration of heating. Different coagulants might accelerate or decelerate this reaction, leading to color differences [31].

Concerning the results obtained in the colorimetry by the CIEL*A*B scale, the tofu samples showed luminosity (L*) values from 88.55 to 91.63, with a tendency toward white, while, in the chromatic values of a* (tendencies from green to red), values from 0.09 to 1.23 were obtained, referring to the fact that certain tofu types can be white but with opaque tonalities. Regarding the chromatic values of b* (a tendency from blue to yellow), values from 13.56 to 14.85 were obtained, specifying that the obtained tofu samples showed the tendency toward yellow, for which it is mentioned that the tofu obtained in this research fulfills the indications made by other authors who mentioned that color is an indicator of tofu quality, showing variations from white to yellow that usually appear in cream-white blocks [33].

4. Conclusions

This study aimed to explore the effects of various vinegar types and concentrations on tofu’s coagulation, yield, and textural properties, contributing valuable insights into optimizing tofu production processes. These findings highlight the benefits of using vinegar as a coagulant, such as faster coagulation times and desirable textural characteristics, thus offering a practical alternative to traditional coagulants in tofu manufacturing. Tofu coagulated with rice vinegar + white vinegar at a 3% concentration achieved coagulation at 0.78 min. This study indicates that higher concentrations of vinegar can decrease the coagulation time compared with the times required by other authors, who, when using inorganic salts, required between 15 and 45 min for curd formation.

Regarding the tofu yield, the treatment with apple vinegar at a 5% concentration showed a yield of 23.73%; this property was influenced by factors such as the isoelectric point of the mixture, the acid concentration, and the pKa with respect to the acids present in the tested vinegar. Additionally, the different types of vinegar influenced the textural properties of the tofu. The apple and white vinegars enhanced the cohesiveness and elasticity, resulting in a tofu that was more cohesive and which could recover a significant portion of its structure after deformation. The evaluated properties in the tofu (the hardness, fracturability, cohesiveness, adhesiveness, and elasticity) were influenced by factors such as the temperature, the warming and cooling speed, and the pH because, when using a matrix in which temperatures of >80 °C and 38 °C, and a pH from 4.7 to 5.0, were managed, the obtained tofu showed excellent hardness and elasticity properties. For the colorimetry, it was shown that the tofu showed tendencies toward yellow and white, and that the vinegar type influenced this characteristic in the tofu; if a tofu with a tendency toward white is desired, then rice or white vinegar should be used as a coagulant; to obtain tofu with a tendency toward yellow, apple vinegar should be used as a coagulant.