Development of Food Hydrogels with Andean Purple Corn (Zea mays L.) Extracts and Cushuro (Nostoc sphaericum) Polysaccharide: Rheological Characterization ^†

Abstract

Andean purple corn (Zea mays L.) is an ancient native Peruvian crop that is currently used in Peruvian gastronomy. Cushuro is a cyanobacteria from the Andean lakes of Peru. They have considerable amounts of bioactive compounds that can improve the physicochemical properties of foods. The objective of this research was to characterize the rheological and functional properties of food hydrogels developed with purple corn extracts, red prickly pear fruit pulp, and cushuro polysaccharide (CP). Acid-soluble polysaccharides obtained from the Nostoc sphaericum variety from Ancash, Peru, as well as Peruvian purple corn extracts, were used. Food hydrogels at concentrations ranging from 0.5% to 3.5% (w/v) were elaborated by dispersing the polysaccharides in a 4:1 extract/pulp (v/v) ratio. Likewise, control samples with Tara (Caesalpinia spinosa) gum (TG) were made. The effect of hydrocolloid concentration (0.5; 1.5; 2.5; 3.5%) on the rheological properties was evaluated using a unifactorial design. CP and TG hydrogels exhibited a shear-thinning nature, a concentration-dependent yield point (0.02–29.91 Pa; 2.01–508.39 Pa), and high antioxidant activity and phenolic content. Adding CP revealed slow structural regeneration, while TG showed thixotropy and a symmetric hysteresis loop. CP gels showed a fluid-like structure with viscoelastic properties (G″ > G′) even in the highest concentration evaluated (3.5%), contrary to TG gel that had a more solid (gel-like) structure (G′ > G″) at a low concentration (1.5%). These results showed a suitable rheological profile and desirable properties of the food hydrogels development for the functional food industry and processing.

1. Introduction

Andean purple corn (Zea mays L.) is an ancient native Peruvian crop that is currently used in Peruvian gastronomy. The intense purple color on its pericarp and cob and, consequently, on its extracts is used to prepare drinks like “chicha morada” and desserts like “mazamorra morada”. According to Salvador-Reyes et al. [1], this variety of corn is a potential ingredient for the development of new products as it can provide a variety of colors, flavors, and textures, elevated phenolic content, and high antioxidant activity, contributing to a reduction in the use of additives in the food industry while providing health-promoting effects.

Cushuro (Nostoc sphaericum) is a bluish-green type of cyanobacteria found in the Andean lakes of Peru. Among its beneficial bioactive compounds, such as protein (28.18 ± 0.33%), iron (4.76 ± 0.08 mg/100 g), and calcium (377.80 ± 1.43 mg/100 g), it contains a significant amount of polysaccharides [2] which have thickening and structural properties as well as potential health benefits (e.g., antitumor, anticoagulant) [3]. Tara (Caesalpinia spinosa) gum, also known as Peruvian carob, is a natural hydrocolloid obtained by grinding the endosperm of the seeds of Caesalpinia spinosa. It has been approved as a food additive by the Food Chemicals Codex and primarily functions as a thickener and stabilizer [4]. Polysaccharide hydrogels are commonly used in food products due to their ability to modify the rheology of complex food systems, perform a range of technological functions, and create innovative products to answer consumer’s needs.

The objective of this investigation was to develop food hydrogels with cushuro (Nostoc sphaericum) polysaccharides and Andean purple corn (Zea mays L.) extracts to leverage their multiple properties, gain a clearer understanding, expand their potential applications, and promote the use of ancestral crops and new hydrocolloid sources in the functional food industry and food processing. Therefore, this work evaluates and summarizes the dynamic viscoelastic properties, flow behavior, thixotropy, hysteresis, phenolic content, and antioxidant activity, among other aspects of the rheological and functional profile.

2. Materials and Methods

2.1. Materials and Sample Preparation

The cushuro was obtained from the Cotaparaco district, province of Recuay, Ancash region, Peru. It was dried at 60 °C for 18 h and then pulverized. Cushuro polysaccharide (CP) was extracted following the methodology described by Chasquibol et al. [3] at the Functional Food Laboratory of the University of Lima, Peru. Tara gum (TG) was provided by Molinos Asociados SAC (Lima, Peru). Andean purple corn and red prickly pear fruit were purchased from a local market in Lima, Peru. Purple corn extracts were prepared, taking into consideration the outlines recently reported by Jing et al. [5]. The crushed and milled purple corn cob and whole kernels were added to deionized water (10 mL/g; mL solvent/g sample), treated at 75 °C for 1 h, and then strained to obtain the extract. Red prickly pear fruit was washed, peeled, blended, and strained to obtain the seedless pulp.

Table 1. Formulations of food hydrogels with Andean purple corn (Zea mays L.) extracts, cushuro (Nostoc sphaericum) polysaccharide, and Tara (Caesalpinia spinosa) gum.

Ingredient	CP0.5	CP1.5	CP2.5	CP3.5	TG0.5	TG1.5	TG2.5	TG3.5
Cushuro polysaccharide (% w/v)	0.5	1.5	2.5	3.5	−	−	−	−
Tara gum (% w/v)	−	−	−	−	0.5	1.5	2.5	3.5
Purple corn extract (% v/v)	80	80	80	80	80	80	80	80
Red prickly pear fruit pulp (% v/v)	20	20	20	20	20	20	20	20

CP: cushuro polysaccharide; TG: Tara gum.

shows eight formulations developed with Andean purple corn (Zea mays L.) extracts, red prickly pear fruit pulp, and two different hydrocolloids: cushuro (Nostoc sphaericum) polysaccharide and Tara (Caesalpinia spinosa) gum. Food hydrogels at concentrations ranging from 0.5% to 3.5% (w/v) were elaborated by dispersing the hydrocolloids in Andean purple corn extracts with continuous stirring for 1 h at room temperature (approx. 25 °C). The solutions were then heated in a water bath at 80 °C with continuous stirring until complete dissolution and homogenization. After cooling the solutions to 50 °C, prickly pear fruit pulp was added. The hydrogels were strained to remove lumps and stored at 4 °C in 50 mL tubes for further analysis. Purple corn extracts and red prickly pear fruit pulp were combined in a 4:1 (extract/pulp v/v) ratio.

2.2. Rheological Measurements of Food Hydrogels

2.2.1. Steady Shear Properties: Flow Behavior, Thixotropy, and Hysteresis Tests

Rheological measurements were performed 24 h after sample preparation using anMCR92 (Anton Paar GmbH, Graz, Austria) rheometer equipped with a parallel plate geometry (50 mm diameter) and a 1 mm measuring gap. The RheoCompass software (version 1.13) integrated into the rheometer was used for data collection, basic analysis, and visualization of the results. For flow behavior tests, the samples were continuously sheared at rates ranging from 0.1 to 100 s⁻¹. The shear rate-dependent Viscosity Ratio (VR) was determined by calculating the ratio between the viscosity of a sample at a high (ηB) and at a low (ηA) rotational speed (Equation (1)):

VR = ηB/ηA

(1)

If the value of VR = 1, the sample shows Newtonian flow behavior. If VR < 1, the sample shows speed-dependent shear-thinning flow behavior, and if VR > 1, the sample shows speed-dependent shear-thickening flow behavior [6].

A 3-interval thixotropy test (3ITT) was conducted to simulate the conditions of application processes. The shear rate was maintained at 0.25 s⁻¹ for 90 s (1st interval), at 1000 s⁻¹ for 60 s (2nd interval), and then gradually decreased from 1000 to 0.25 s⁻¹ over 100 s (3rd interval).

Hydrogels were also subjected to a hysteresis loop test to evaluate their response to increase–decline cycles and assess the reversibility of viscosity. The shear rate increased from 5 to 131 s⁻¹ over 90 s and gradually declined back to 5 s⁻¹ after being held at 131 s⁻¹ for 3 min. All steady-state tests were performed at 25 °C.

2.2.2. Dynamic Viscoelastic Properties: Amplitude and Frequency Sweeps

An oscillatory amplitude sweep test was conducted at an angular frequency of ω = 10 rad/s (1.6 Hz) with strain ranging from 1% to 100% to establish the linear viscoelastic (LVE) region. Subsequently, frequency sweeps from 0.1 to 100 Hz (0.63 to 628.3 rad/s) were performed using an appropriate strain within the LVE region to measure the storage modulus (G′), loss modulus (G″), and complex viscosity (η*). All dynamic tests were conducted at 25 °C.

2.3. Physicochemical Characterization

2.3.1. Determination of Antioxidant Activity via Radical 2,2-Diphenyl-1-picrylhydrazyl (DPPH)

The antioxidant activity of samples was determined by the DPPH method, with some modifications, at 517 nm with a spectrophotometer (UV-1280 UV-VIS Spectrophotometer Shimadzu, Kyoto, Japan). The results were expressed as mg of Trolox equivalent (TE)/g sample [3].

2.3.2. Total Phenolic Content (TPC)

The total phenolic content (TPC) of the samples was determined using the Folin–Ciocalteu method at 760 nm with a spectrophotometer (UV-1280 UV-VIS Spectrophotometer Shimadzu, Kyoto, Japan). The results were expressed as mg of gallic acid equivalent (GAE)/g sample [3].

2.4. Statistical Analysis

Physicochemical analyses were performed in triplicate, and results were expressed as mean values ± standard deviation. A one-way analysis of variance (ANOVA) was conducted to analyze data at a 95% significance level using Minitab 19.0 Software (Minitab Inc., State College, Palo Alto, CA, USA).

3. Results and Discussion

3.1. Rheological Evaluation

3.1.1. Flow and Viscosity Behavior

Table 2. Steady shear properties (Yield point [Pa], Viscosity [mPa·s] at 7.5 s⁻¹ and 100 s⁻¹ shear rates and Viscosity Ratio [dimensionless]) of food hydrogels with Andean purple corn (Zea mays L.) extracts, cushuro (Nostoc sphaericum) polysaccharide, and Tara (Caesalpinia spinosa) gum.

Parameter	CP0.5	CP1.5	CP2.5	CP3.5	TG0.5	TG1.5	TG2.5	TG3.5
Yield point [Pa] a	0.026	0.336	6.322	29.914	2.007	91.979	268	508.39
Viscosity [mPa·s] (shear rate 7.5 s−1)	7.52	75.07	883.81	7060.4	303.33	8885.9	41,017	77,550
Viscosity [mPa·s] (shear rate 100 s−1)	6.76	47.93	289.49	1071.9	108.27	1233.7	3880.5	4663.6
Viscosity Ratio b	0.90	0.64	0.33	0.15	0.36	0.14	0.09	0.06

CP: cushuro polysaccharide; TG: Tara gum. ^a According to the Bingham mathematical regression model. ^b Obtained by dividing viscosity at a shear rate of 100 s⁻¹ by viscosity at a shear rate of 7.5 s⁻¹.

and Figure 1 present the steady shear flow behavior of CP and TG food hydrogels. For each polysaccharide, all tested solutions (0.5%, 1.5%, 2.5%, 3.5%) exhibited a shear-thinning behavior with a Viscosity Ratio (VR) consistently less than 1 across shear rates ranging from 0 to 100 s⁻¹. All solutions presented a concentration dependence of viscosity, which increased with higher hydrocolloid concentrations.

3.1.2. Yield Point Determination

Typical analysis procedures based on the Bingham mathematical regression model were used to determine the yield point, which is the minimum force required to initiate flow in a sample. The results indicated that CP hydrogels have a relatively low stress threshold before they start to deform (0.026–29.91 Pa), whereas TG hydrogels exhibit higher mechanical strength (2.007–508.39 Pa) even at low concentrations. In application environments (e.g., transport, storage, mixing), CP gels, with their lower yield points, may be more suitable where softer and more flexible materials are needed due to the nature of their internal structural forces. In contrast, TG hydrogels, with their higher yield points, are more appropriate for scenarios requiring greater stress resistance or robust mechanical properties.

3.1.3. Thixotropy Testing and Hysteresis Loop Test

Figure 2 shows the thixotropic properties and hysteresis curves of food hydrogels. In food testing, evaluating structural regeneration is crucial for determining whether the product meets quality standards after processing (e.g., spreading, pouring, or squeezing out of a container). After being subjected to a shear rate of 1000 s⁻¹ for 1 min, TG hydrogels showed rapid recovery and structural regeneration. These results are consistent with those reported by Wu et al. [3]. As for TG1.5 gel, the sample achieved complete regeneration of viscosity (100%)—compared to the structural strength at the end of the first interval—90 s after the high shear load interval ended. In contrast, CP3.5 gel remained thinner than its initial state and exhibited considerably slower structural regeneration (32.7%), with regeneration not yet complete even 90 s after the end of the high shear load interval.

The results of the hysteresis test illustrate the differences in viscosity between the two types of hydrogels and show how the shear stress varies with increasing and, subsequently, decreasing shear rates. TG1.5 gel sample exhibited a nearly symmetrical hysteresis loop, where the upper curve closely overlapped with the downward curve, displaying an area of 39.86 Pa/s between them. In contrast, CP3.5 hydrogel demonstrated significant structural breakdown, with a hysteresis area of 498.93 Pa/s. For CP hydrogels, their structure does not fully recover and remains thinner than initially, while TG hydrogels achieve complete recovery after the defined rest period.

3.1.4. Dynamic Viscoelastic Properties

Frequency sweeps characterize a sample’s time-dependent behavior within the Linear Viscoelastic (LVE) region, where the sample’s structure is not destroyed. By evaluating the curve of the storage modulus (G′), the limit of the LVE region for TG1.5 hydrogel was found to be 8.46% shear strain, beyond which G′ function decreases continuously, indicating a gradual breakdown of the superstructure. Under the applied measurement conditions, CP3.5 hydrogel (with G″ > G′) exhibits a fluid-like structure and can be classified as a viscoelastic liquid, with no distinct transition to a region where the viscoelastic properties change significantly (Figure 3).

Figure 4 shows the mechanical spectra of Tara gum solutions at 25 °C. The results of frequency sweeps for TG and CP food hydrogels indicate a decrease in complex viscosity with increasing frequency (shear-thinning behavior), consistent with the steady shear results. For the TG1.5 gel, the loss modulus (G″) was higher than the storage modulus (G’) at low frequencies, indicating that the viscous component of the viscoelastic behavior dominates and describes the liquid state of the sample. This trend continues until a crossover point (sol/gel transition) at 3.53 rad/s, after which G’ becomes predominant at higher frequencies, demonstrating elastic behavior and strong interaction forces. In contrast, CP3.5 hydrogel exhibited a higher loss modulus than storage modulus (G″ > G′) throughout the frequency range, indicating a viscoelastic behavior characterized by mostly unlinked individual molecules with some degree of entanglement [4].

3.2. Physicochemical Evaluation

The addition of purple corn extracts, prickly pear fruit pulp, and polysaccharides resulted in significant levels of phenolic compounds and substantial antioxidant capacity. CP3.5 food hydrogel exhibited slightly higher total phenolic content (2.61 ± 0.17 mg GAE/g sample) and antioxidant activity (4.28 ± 0.29 mg TE/g sample) compared to TG3.5 food hydrogel, which had 2.43 ± 0.39 mg GAE/g sample and 4.07 ± 0.22 mg TE/g sample. These findings are consistent with reports by Madhujith et al. [7] and Salvador-Reyes et al. [1], suggesting that these ingredients possess excellent antioxidant properties and may offer health-promoting effects. For the specific analysis of this study, the results of the evaluated formulations were obtained through the experimental pairs design. From the rheological point of view, the results obtained for the CP3.5 and TG1.5 samples are very similar and, consequently, present a better dynamic interpretation of the effects of the studied variable. From the physicochemical point of view, analogous samples with the same concentration of CP and TG presented similar results between both hydrocolloids, so it is concluded that the antioxidant activity and phenolic compounds content are directly related to the amount of raw materials used, as seen for CP3.5 and TG3.5 food hydrogels’ evaluation.

4. Conclusions

The hydrocolloid concentrations significantly influenced the rheological properties of the formulated food hydrogels. Specifically, this variable led to increased values of viscosity, pseudoplasticity, and yield point in all food hydrogels. At the highest concentration evaluated, cushuro polysaccharide gels exhibited a dominant viscous component in the viscoelastic behavior, while Tara gum gels displayed elastic behavior and strong interaction forces even at low concentrations. Furthermore, the integration of ancestral crop extracts with novel polysaccharide sources enhanced the total phenolic content and antioxidant activity of the hydrogels. These results highlight the potential for these hydrogels to be effectively used in various applications or to innovate food products leveraging traditional ingredients and bioactive compounds.