Influence of Ultrasound on the Rheological Properties, Color, Carotenoid Content, and Other Physical Characteristics of Carrot Puree

Featured Application

Ultrasound is one of the emerging technologies developed to minimize processing, maximize quality, and ensure the safety of food products. In the food industry, sonication is used for the viscosity of juice/puree “creation”. In this study, the comparison of the physical properties of carrot puree before and after sonication is presented. This information can be used in food industries for further design. Ultrasound treatment also enhances the homogenization of the puree, making it easier to control its viscosity and ensure a consistent “texture” that meets industry standards for applications in sauces, baby foods, and soups. Furthermore, US treatment reduces the activation energy necessary for heat transfer and enzyme inactivation, enabling processing at lower temperatures.

Abstract

This study aimed to investigate the effect of ultrasonic frequencies (21 and 35 kHz) on the physical properties of carrot puree at different concentrations (9, 12, and 21 °Brix). The viscosity, total soluble content, density, color, and β-carotene content were tested. It was found that the viscosity of the puree, determined with respect to shear rate, concentration, and the use of ultrasonic treatment, indicates that the purees should be defined as shear thinning fluids. Moreover, a decrease in activation energy was observed with the increase in extract and ultrasonic treatment, which may cause changes in the rate of reactions occurring in the tested material. A significant effect of this may be the observed change in the color of the puree after ultrasonic treatment; the increase in frequency from 21 to 35 kHz caused an increase in redness and yellowness and a decrease in lightness, independently of concentration. The most significant color difference was noted in the puree with a 21 °Brix concentration, where a ΔE value of 21 was recorded. In contrast, the ΔE values for the other purees post-treatment remained below 5. The content of carotenoids did not change after sonication, independently of the concentration of carrot puree.

1. Introduction

Thermal pasteurization and sterilization are crucial in ensuring food safety and extending shelf life, primarily due to their ability to destroy enzymes and microorganisms. When designing preservation processes for liquid foods, it is essential to maintain the natural quality of the food. However, the nonspecific effects of heat can impact the nutritional and sensory qualities and alter the foods’ functional properties. Over the years, various research groups have studied how ultrasound may affect microbial inactivation [1] and physicochemical and nutritional parameters in liquid foods [2], such as juices, purées, and pulps of fruits and vegetables [3], e.g., avocado puree [4], strawberry puree [5], and cashew apple bagasse puree [6], and also cow milk [7] and other beverages, e.g., coconut milk [8,9]. Food’s behavior varies based on the food matrix, necessitating the evaluation of each product before marketing [10].

Carrots are widely cultivated in Poland and are available throughout the year due to their long-term storage capabilities. However, the seasonality of plant materials often results in the overproduction of fruits and vegetables, producing carrot juices and purees to manage the surplus. Carrot puree is particularly valuable due to its high content of carotenoids and polyphenols, known for their potential health benefits, including boosting immune response and preventing diseases such as eye diseases, cardiovascular diseases, and cancer. Carrot puree is rich in β-carotene and α-carotene, with steam-blanched puree showing high levels of these nutrients (94.56 µg/g and 86.62 µg/g, respectively), which contribute to its nutritional value and color [11]. It also contains significant amounts of total soluble solids (5.50 °Brix) and specific color values. However, the sensitivity of carotenoids to thermal or physical degradation due to their long conjugated chain of C–C double bonds poses challenges for consumers and producers. Typically, carrot puree used in the food industry contains an extract ranging from 9 to 15%, or 29% in the case of concentrated carrot puree [11].

The viscosity of carrot puree decreases as the shear rate increases, a behavior known as shear thinning. This behavior can be accurately described by the Ostwald–de Waele (Power Law) model with a flow behavior index of 0.34 at 20 °C [12]. Various preparation methods can affect the consistency of carrot puree. High-pressure homogenization and high-temperature blanching reduce water separation (syneresis), while low-temperature blanching increases it. The order of thermal and mechanical treatments also impacts consistency, with higher consistency achieved when high-temperature treatment is followed by homogenization [12]. Moreover, the apparent viscosity of vegetable juices increases with higher concentrations of carrot puree, with significant changes observed at concentrations above 17.5% [13]. In the literature, the viscosity of carrot puree can be influenced by particle size and the presence of pectin. In blended carrot purees, water-soluble pectin (30.3%), chelator-soluble pectin fraction (42.6%), and sodium-carbonate-soluble pectin fraction (19.9%) were observed in non-heated samples [14]. Homogenization, commonly used in the food industry, tends to reduce particle size, resulting in a smoother consistency, while low-temperature blanching can increase consistency due to enhanced intercellular adhesion [12,13,15]. These rheological properties are important for various processes, such as pumping, mixing, pasteurization, and evaporation. Additionally, the physicochemical properties of carrot puree serve as indicators of the product’s quality. Furthermore, applying new methods, such as sonication, can enhance the quality of the product [16].

Ultrasounds (USs) are sound waves with frequencies starting from 20 kHz, and their mechanical waves propagate in solids and liquids. Ultrasound is an emerging technology developed to minimize processing time, maximize quality, and ensure the safety of food products [16,17]. In the food industry, sonication is used for inactivating microorganisms [18], creating the viscosity of juice through the emulsification process [19], and as a pre-step before freezing, drying, and extraction processes [17,20,21] in food products such as fruit juices, mayonnaise, and tomato ketchup [22,23].

Ultrasound techniques have emerged as an energy-efficient and straightforward technology in food processing. These are classified into two main categories: low-power ultrasound (high-frequency) and high-energy ultrasound (high-power and high-intensity) [17,24,25]. Low-power ultrasound is used for the non-destructive quality control of fresh fruits and vegetables during the pre- and post-harvest stages and in the processing of cheese, fat-based products, and aerated and frozen foods. In contrast, high-energy ultrasound can disrupt foods’ physical, mechanical, or chemical properties, thereby enhancing their processing, preservation, and safety [26,27,28]. The process primarily relies on acoustic cavitation, whereby ultrasound waves create and collapse bubbles in liquids. This generates mechanical forces, localized heating beneficial for sterilization, and highly reactive radicals that facilitate chemical reactions such as pesticide degradation and enzymatic cross-linking [17,24,25].

Several studies have explored the impact of ultrasound on various fruit and vegetable juices. While some research suggests a minimal effect on their physical properties, others have identified significant changes in specific parameters. Extensive research has been conducted on tomato juice and its concentrated puree, revealing that ultrasonic treatment can alter their viscosity and consistency. Additionally, studies have shown that ultrasound treatment can affect the viscosity of carrot puree. One study indicated that the viscosity of carrot juice gradually increased with longer ultrasound treatment time [16]. However, another study suggested that the viscosity of carrot juice remained relatively unchanged under different treatment conditions (changed US power), indicating that the impact of ultrasound on viscosity may depend on specific treatment parameters [29]. Ultrasound treatment significantly influences the color stability of carrot puree, particularly after longer treatment durations, resulting in observable changes in the lightness parameters of carrot tissue [30]. Moreover, ultrasound treatment has been found to increase the turbidity of apple juice, potentially affecting its visual appearance and color stability [31]. Furthermore, ultrasound treatment has been shown to impact the nutritional content of carrot puree. Studies have revealed that ultrasound treatment could enhance the total carotenoid content, total soluble solids, total sugars, and ascorbic acid contents of carrot products, indicating a positive effect on nutritional quality [16,29,32]. However, the impact of ultrasound treatment on carotenoid content was found to be variable, with some studies reporting a decrease in carotenoid content under specific treatment conditions [29].

Current research on carrot puree’s rheological and physical properties is somewhat limited, and there appears to be a gap in data concerning carrot purees treated with ultrasound. As a result, this study seeks to examine the effects of two ultrasonic frequencies (21 and 35 kHz) on carrot purees’ rheological and physicochemical properties. This study assumes that a product with repeatable and designed functional and nutritional features will be obtained due to sonication. The scope of this study includes an analysis of the effect of sonication and its frequency on the rheological properties and activation energy of the tested carrot puree, which were considered functional features. Additionally, it examines color and carotenoid content, which are functional properties that can characterize the nutritional value of the tested samples. To verify the hypothesis, selected rheological parameters that determine the functional properties of the purees were evaluated. The activation energy, which influences the rate of reactions affecting the storage life of carrot purees, was measured. Furthermore, the color and carotenoid content were assessed, as these factors can serve as criteria for evaluating the nutritional value of the carrot purees under different sonication conditions.

2. Materials and Methods

2.1. Materials and Sample Preparations

This study used carrot puree obtained from a company and characterized by 12 °Brix. The total soluble content in the study was 9, 12, and 21 °Brix concentrations. To achieve the 21 °Brix concentration, the 12 °Brix puree was processed in the laboratory using a pressure evaporator (Büchi Labortechnik AG, Flawil, Switzerland) at 55 hPa, maintaining the carrot puree at a maximum 35 °C. The 9 °Brix was obtained by adding water to the 12 °Brix puree. Selected lower and higher concentrations were based on the most common low- and highly concentrated carrot products available on the market. Moreover, the 21 °Brix concentration represented the highest achievable level, rendering the puree a fluid substance.

Table 1. Description of material.

Concentration (°Brix)	Carbohydrates (g/100 mL)	Protein (g/100 mL)	Fat (g/100 mL)	Mineral Content (g/100 mL)	Water (g/100 mL)
9 *	8.79	0.22	0.04	0.30	90.65
12	11.80	0.30	0.05	0.40	87.45
21 *	19.92	0.51	0.08	0.68	78.81

* The authors calculated the data for the carrot purees based on the total soluble content.

presents the primary information about the carrot purees used.

The experimental design described above is also shown in Figure 1. The tests shown in Figure 1 for the 12 °Brix concentration are the same as those conducted for the 9 and 21 °Brix concentrations.

2.2. Sonication

Carrot puree, regardless of concentration, underwent a 30 min sonication process at 21 kHz using the Ultrasonic MKD-3 device (Stary Konik, Poland, internal dimensions 240 × 135 × 100 mm) with 300 W of power. Sonication with a frequency of 35 kHz was carried out in the Inter Sonic model IS-3 device (Olsztyn, Poland, internal dimensions 240 × 135 × 100 mm) with 300 W of power. The 200 g (accurate to 0.01 g) carrot puree was placed in glass containers immersed in 1.5 L of distilled water to submerge the entire carrot puree completely. Temperature measurements were taken using thermocouples (PT 100, CZAKI Thermo-Product, Raszyn, Poland) connected to a computer at 30 s intervals during the ultrasonication treatment. The initial temperature was recorded as 20 ± 3 °C, and it increased by 5 °C over 30 min. The experiments were conducted in five repetitions.

2.3. Carrot Puree Analysis

This study examined the properties of carrot puree with and without US treatment, including total soluble content, density, apparent viscosity (η), color, and total carotenoid content. Five replicates were performed.

2.3.1. Total Soluble Content and Density

The total soluble content (°Brix) was measured at a temperature of 20 °C using a pocket refractometer PAL-1 (Atago Co., Ltd., Tokyo, Japan). Density (kg/m³) was measured using an oscillating portable densitometer Densito 30PX (Mettler Toledo, Greifensee, Switzerland).

2.3.2. Viscosity and Activation Energy

The apparent viscosity of the carrot puree (η) was tested using a Brookfield viscometer (model RVDV-III, Middleboro, MA, USA) with 11 and 12 mL of carrot puree for spindle numbers SC4-27 and -28, respectively. Viscosity tests were performed at temperatures of 20, 30, 40, and 50 °C with a shear rate range of 20 to 100 rpm. The Rheocalc V 3.3 software is dedicated to the rheometer; it is a program that collects data and controls the viscometer. Activation energy (Ea) was calculated using the Arrhenius equation from Kobus et al.’s [33] article.

The equation for viscosity behavior was Ostwald–de Waele’s power equation:

$$\mathsf{\sigma }={\mathrm{K}·\dot{\mathsf{\gamma }}}^{\mathrm{n}}$$

(1)

where

σ—Shear stress (N/m²);

γ—Shear rate (1/s);

K—Consistency factor (Pa sⁿ);

n—Flow behavior index (-).

A consistency coefficient for each temperature was used for the calculation of activation energy from the equation described by Quek et al. [34]:

$$\mathrm{K}={\mathrm{K}}_0·\mathrm{e}\mathrm{x}\mathrm{p}\left({\displaystyle \frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{R}\mathrm{T}}}\right)$$

(2)

Ea—Activation energy (J/mol);

R—Universal gas constant 8.314 (J/mol K);

T—Temperature (K);

K₀—Exponential of the y-intercept; frequency factor (Pa sⁿ).

2.3.3. Instrumental Color

The color components of the carrot puree were measured using a Minolta CR-A70 colorimeter (Minolta, Ramsey, NJ, USA) in the CIE 1976 system, calibrated for a white pattern (L* 92.49, a* 1.25, b* −1.92). The measurement was performed at the mid-height of the measuring cell with standard illumination C. To measure the color, 50 mL of juice was placed in a glass cuvette with a wall thickness of 1 mm.

The absolute difference in color between carrot purees without and with US treatment was calculated using the L*, a*, and b* color components based on the equation presented by CIE [35].

$$∆\mathrm{E}=\sqrt{{({\mathrm{L}}^{\mathrm{*}}-{\mathrm{L}}_{\mathrm{p}}^{\mathrm{*}})}^2+{({\mathrm{a}}^{\mathrm{*}}-{\mathrm{a}}_{\mathrm{p}}^{\mathrm{*}})}^2+{({\mathrm{b}}^{\mathrm{*}}-{\mathrm{b}}_{\mathrm{p}}^{\mathrm{*}})}^2}$$

(3)

where

L*, a*, b*—Lightness, redness, and yellowness of carrot puree without US treatment;

L_p*, a_p*, b_p*—Lightness, redness, and yellowness of carrot puree treated by US.

2.3.4. Carotenoid Content

The total carotenoid content of the carrot puree was determined according to the method of Scott et al. [36] with some modifications. The procedure was as follows: 1.5 mL of carrot puree (V1) was combined with 20 mL of water and 1 mL of Carrez I and Carrez II solution in a centrifuge tube. Subsequently, the supernatant solution was decanted after centrifugation at 2000× g for 5 min, and the precipitate was treated with 20 mL of acetone. Subsequently, following the centrifugation and removal of the yellow acetone solution, 25 mL of petroleum ether was added to the sediment. The organic phase was separated and rinsed with 30 mL of water, and then centrifuged with anhydrous sodium sulfate. The resulting ether phase was transferred to a volumetric flask and brought up to the mark with petroleum ether. The absorbance at 450 nm was measured against the petroleum ether used as a blank reference sample. The carotenoid content (expressed as beta-carotene) was calculated using a specific formula presented by Janiszewska-Turak and Witrowa-Rajchert [37].

$$\mathrm{C}\left({\mathrm{C}}_{40}{\mathrm{H}}_{56}\right)={\displaystyle \frac{{\mathrm{A}}_{450}·4.00·\frac{{\mathrm{m}}_2}{{\mathrm{m}}_1}}{\mathrm{d}.\mathrm{m}.}}$$

(4)

where

C(C₄₀H₅₆)—Total carotenoid content (mg/kg d.m.);

A₄₅₀—Absorbance;

4.00—Average conversion factor, based on a ringed test taking into account the average rate of absorption of β-carotene in petroleum ether and dilution made during the analysis;

m₁—Mass of the sample (g);

m₂—Mass of the etheric solution (50 g);

d.m.—Dry matter (g/100 g).

2.4. Statistical Analysis

The obtained results underwent analysis of variance (ANOVA) using Statgraphics Plus 5.1 (StatPoint Technologies, Inc., Warrenton, VA, USA). In instances where ANOVA yielded a statistically significant result, Tukey’s test was subsequently conducted to perform multiple comparisons of means (α = 0.005). Graphs were created in the programs MS Excel 2019 and R Studio 2024.09.0.

3. Results

3.1. Physical Properties of Carrot Puree

To evaluate the viscosity behavior of carrot puree, this study focused on density and extract as the primary parameters (

Table 2. Selected physical properties of carrot purees before and after US treatment.

Concentration (°Brix)	Ultrasound (kHz)	Density (kg/m3)	Total Soluble Solid Content (°Brix)
9	0	1037 ± 1 ef	9.1 ± 0.2 e
12		1051 ± 0 d	12.2 ± 0.1 d
21		1085 ± 0 b	21.5 ± 0.1 c
9	21	1037 ± 0 f	9.2 ± 0.1 e
12		1051 ± 1 cd	12.3 ± 0.1 d
21		1086 ± 1 b	22.8 ± 0.1 a
9	35	1038 ± 0 e	9.2 ± 0.2 e
12		1051 ± 1 cd	12.3 ± 0.1 d
21		1089 ± 1 a	22.3 ± 0.1 b

a, b, c, d, e, f—Means (±SD) with a different letter in the same column are significantly different at p < 0.05.

). The carrot puree used in this investigation had a concentration of 12° Brix. According to information provided by the company, the puree was not pasteurized before it was donated to us, and the concentration was primarily derived from carbohydrates, reported at 11.8 g/100 mL (Table 1). The measured total soluble content ranged from 12.2 to 12.3 Brix (Table 2), which aligns closely with the carbohydrate values provided by the company. The company did not use pasteurization because it needed to check what changes could occur with US only treatment. In the production line, pasteurization could be a step before packaging.

Sonication (30 min) did not significantly alter the total soluble solids, regardless of the consistency of the carrot puree (Table 2). Similar findings were reported by Adekunte et al. [38], who observed that a 10 min treatment with 20 kHz ultrasound did not affect the extract value of tomato juice measured in °Brix, which remained at 6%. Additionally, Tiwari et al. [39,40] found no significant effect of ultrasound on the soluble components of orange juice, irrespective of the parameters used, such as amplitude, process temperature, and exposure time. These results were consistent for a concentration of approximately 9%. This extract behavior has been recorded in carrot [30], raspberry, and blueberry [41] purees treated by US. The observed effect can be attributed to the enhanced smoothness of consistency and reduction in viscosity, as exemplified by our carrot puree (9 and 12 °Brix). However, for a higher concentration of carrot puree (21 °Brix), a statistically significant increase in the extract was noted following a 30 min ultrasound treatment, irrespective of the ultrasound frequency. The changes in the carrot puree extract may be attributed to the evaporation of water, which is likely to occur during cavitation in liquids when ultrasound is applied [42].

As anticipated, the higher the extract content, the higher the density (Table 2). As the concentration of the extract increased, the density also increased. Similar correlations were observed in guava juices [43]. Additionally, it was noted that the density of the purees did not show a statistically significant difference following sonication at the lower frequency of ultrasounds (21 kHz). However, utilizing higher-frequency sonication increased the density of the carrot puree (Table 2), which could be attributed to the slight evaporation of water due to the local cavitation process.

3.2. Rheological Properties

The relationship between the consistency coefficient (K) and viscosity in carrot juices and purées is well documented. The consistency coefficient measures the thickness or resistance to flow in non-Newtonian fluids, such as carrot purée. It is influenced by factors including mixture concentration and the presence of thickeners [13,44], which were not present in the tested carrot purees (information from the company). Flow curve analysis (Figure 2) characterized carrot puree as a pseudoplastic fluid with shear thinning behavior (n < 1), exhibiting decreased viscosity with increased shear rate. Power equations were used to describe the resulting flow curves for each temperature (

Table 3. Rheological properties of carrot puree according to concentration, US frequency, and measure temperature.

Concentration (°Brix)	US (kHz)	Temperature for Rheology Test (°C)	Apparent Viscosity (η) (Pa∙s)	Consistency Coefficient (K) (Pa∙sn)	Flow Behavior Index (n) (-)	Activation Energy (Ea) (kJ/mol)
9	0	20	7.79 ± 0.01 op	2.14 ± 0.11 o–s	0.33 ± 0.03 b	16.46 ± 0.23 a
		30	7.56 ± 0.31 op	2.02 ± 0.09 p–s
		40	5.70 ± 0.06 p	1.58 ± 0.06 prs
		50	4.17 ± 0.12 p	1.17 ± 0.05 s
	21	20	7.78 ± 1.77 op	2.39 ± 0.07 op	0.34 ± 0.03 b	12.46 ± 0.10 c
		30	7.09 ± 1.42 p	2.14 ± 0.02 prs
		40	5.97 ± 0.55 p	1.68 ± 0.08 prs
		50	5.77 ± 0.07 p	1.54 ± 0.04 rs
	35	20	9.48 ± 0.49 nop	1.90 ± 0.18 prs	0.42 ± 0.04 a	9.77 ± 0.31 e
		30	8.81 ± 0.10 op	1.69 ± 0.07 prs
		40	8.16 ± 0.21 op	1.55 ± 0.15 prs
		50	6.80 ± 0.04 p	1.31 ± 0.05 rs
12	0	20	21.64 ± 0.57 i	5.39 ± 0.44 lm	0.36 ± 0.04 b	14.72 ± 0.13 b
		30	20.25 ± 0.48 ijk	4.87 ± 0.17 m
		40	16.88 ± 2.88 i–m	3.63 ± 0.04 n
		50	14.31 ± 1.79 lmn	3.19 ± 0.03 no
	21	20	20.56 ± 1.10 ij	6.88 ± 0.45 jk	0.27 ± 0.05 c	8.21 ± 0.04 f
		30	19.34 ± 0.31 i–m	6.80 ± 0.40 jk
		40	17.80 ± 0.57 i–m	6.05 ± 0.48 kl
		50	14.96 ± 0.66 k–n	5.04 ± 0.39 m
	35	20	18.53 ± 2.74 i	7.30 ± 0.65 j	0.21 ± 0.03 d	7.74 ± 0.30 g
		30	17.03 ± 0.41 i–m	7.36 ± 0.08 j
		40	15.65 ± 0.65 j–m	6.68 ± 0.44 jk
		50	12.64 ± 0.64 mno	5.36 ± 0.10 m
21	0	20	119.84 ± 0.10 b	40.10 ± 0.45 b	0.28 ± 0.02 c	10.82 ± 0.23 d
		30	106.44 ± 0.10 d	35.57 ± 0.06 c
		40	88.94 ± 1.17 g	30.02 ± 0.49 f
		50	80.56 ± 0.73 h	26.77 ± 0.32 h
	21	20	142.02 ± 0.03 a	42.23 ± 0.57 a	0.31 ± 0.05 bc	9.92 ± 0.10 e
		30	114.12 ± 0.06 c	33.95 ± 0.33 d
		40	100.33 ± 0.46 ef	29.74 ± 0.23 fg
		50	97.09 ± 1.12 f	29.11 ± 0.53 g
	35	20	103.47 ± 1.43 de	33.60 ± 0.79 d	0.29 ± 0.03 c	8.24 ± 0.10 f
		30	100.67 ± 1.29 ef	32.08 ± 0.13 e
		40	87.57 ± 4.85 g	27.06 ± 0.19 h
		50	79.21 ± 1.82 h	25.06 ± 0.15 i

a, b, c…s—Means (±SD) with a different letter in the same column are significantly different at p < 0.05.

). Shear thinning behavior is associated with multiphase materials like vegetable and fruit pulps formed by a dispersion of insoluble components (cellular wall materials) in a water solution [45]. As the concentration of carrot purée in vegetable juices increases, there is a corresponding rise in apparent viscosity and yield stress, as shown in Figure 2 and Table 3. The viscosity of carrot purée decreases with rising temperature, following an Arrhenius-type relationship (Table 3). This relationship validates the observed decrease in viscosity as the puree’s temperature increases [46,47]. This trend is also evident when the carrot purée concentration exceeds 17.5% [13].

The study revealed that ultrasound treatment significantly influenced the viscosity of the puree under investigation (Figure 2, Table 3). A noticeable decrease in viscosity was associated with lower extracted values. Notably, no statistically significant changes were observed in the viscosity of carrot puree with 9% extract, possibly due to the more efficient evaporation of water during sonication at lower concentrations [48,49]. In the literature, it has been observed that ultrasound can both increase the viscosity of avocado puree [50] and decrease the viscosity of orange juices [41]. This effect primarily depends on the concentration of the juice or puree rather than the source, while the presence of small particles can also play a role.

An increase in the extract content led to a statistically significant rise in the consistency coefficient K. However, the analysis of the flow behavior index n did not reveal any discernible trend during the increase in extract and the use of US treatment (Table 3). Moreover, elevating the measuring temperature resulted in a decrease in the consistency coefficient K across all the presented experiments, irrespective of the carrot puree extract and US treatment. Nonetheless, this decrease was statistically significant only in the case of the highest carrot puree extract. Similar findings were reported by Haminiuk et al. [51] for araçá pulp and Vandresen et al. [47] for carrot juices; however, those authors did not use sonication before testing the viscosity and other descriptive values for purees.

A comparison of the temperature values for each extract and each US treatment revealed a significant increase in the consistency coefficient K in the samples containing 12 °Brix and 21 °Brix. However, for the 9 °Brix extract, the decrease was not significant. Additionally, the tendency for the flow behavior index n to increase with an increase in US frequency was significant only for carrot puree with 12 °Brix. For the 9 °Brix sample, a statistically significant decrease was observed only when using frequency US at 35, while the behavior was not clear for the 21 °Brix sample (Table 3).

An increase in carrot puree concentration caused a decrease in activation energy value (Ea), it was correlated with lower water content in the puree (Table 3). Similar values for vegetable juices were presented in the literature [12,45]; what more, the same correlation was observed by Belibağli and Dalgic [52] in sour cherry concentrates.

The impact of ultrasound on activation energy within a defined carrot juice concentration was evaluated. The findings indicated that an elevated frequency of ultrasound resulted in a reduction in activation energy, regardless of the concentration under examination. The analysis of the increase in US intensity per activation energy demonstrated a downward trend in this parameter for a given concentration of puree. A comparable relationship was observed with ultrasound at 21 kH; however, this trend was not sustained at 35 kH. The juices treated with 35 kH at a concentration of 21 exhibited a higher activation energy result than those at 12 °Brix. Still, this needs to be deeply analyzed for precise conclusions to be obtained.

3.3. Color and β-Carotene Content

The concentration of carrot puree influenced the degree of color variation observed, regardless of the ultrasound treatment employed. An increase in brightness, redness, and yellowness was observed in the samples (Figure 3). The relationships obtained in the presented studies were also given for carrot puree [53], tomato juice [38], and pineapple juice [26], while the opposite results were presented for orange juice (extract 9.23 °Brix) [40]. Such a discrepancy in the results may be related to the different chemical composition of the juice and the presence of other pigments. After sonication, lightness could decrease with an increase in the content of soluble substances in the juice.

The most significant changes in color values a* and b* were observed following ultrasonic treatment at 35 kHz. In contrast, the least significant changes were noted in brightness at the same setting. Similar correlations for low-concentrated juices were obtained for orange juice [40], tomato juice [38], and carrot juice [53].

The application of ultrasound decreased brightness for purees 9 and 21 °Brix, while it increased in a concentration of 12°Brix. Minor changes in the values were observed for the redness parameter (a*), except for the 21 °Brix puree, for which a notable increase was evident. In contrast, yellowness decreased with ultrasound treatment at 9 and 12 °Brix concentrations, while an increase was observed at 21 °Brix.

The yellowness coefficient (b*) values in carrot puree increased as the juice concentration increased. When the low-frequency US was applied, the yellowness coefficient decreased in the 9 °Brix carrot puree (Figure 3a). Statistically significant increases in the b* chromaticity coefficient were observed in carrot puree treated with higher-frequency ultrasonic waves compared to untreated juices (Figure 3a). The sonication of tomato juice at 20 kHz for a period of 10 min resulted in a reduction in the values of the b coordinate [38]. In contrast, an increase in b* coordinate after sonication was observed in orange juice extract with 9.23 °Brix [40] and in pineapple juice [26]. According to Fonteles et al. [27], changes in chromaticity factors a* and b* can be linked to synchronized changes in the luminosity value (L*). They also suggested that during the ultrasound process, the disruption of the carotenoid–protein complex may intensify the orange color of the juice. Costa et al. [26] made a similar observation, suggesting that sonication can release intercellular contents, which may impact the product’s color.

No consistent trend was observed in the changes to the individual color determinants. The total color difference (ΔE) was calculated to assess the overall changes. It was found that, except for the 21 °Brix concentration at 35 kHz, the color difference in juices sonicated at 21 kHz could not be detectable even by a trained expert. At the same time, 35 kHz can be detected by the casual observer. This description is based on the results by Zapotoczny and Zielinska [54], in which the human perception of colors classifies the asymptotic color differences ΔE. It is postulated that absolute color differences between 0 and 2 are unrecognizable, those between 2 and 3.5 are discernible by an uninformed observer, and above 3.5, a distinct color deviation is observed. These small changes confirm the lack of effect of ultrasound on carotenoid pigments, which has also been confirmed by tests in purees with carotenoid content. Regarding ΔE and color coefficients, comparable outcomes have been documented in carrot juice subjected to ultrasound and high-voltage cold plasma treatments [53]. Researchers conducted tests involving non-thermal treatment of carrot juices, including using ultrasounds. Their findings indicated increased a* and b* values and minor changes in ΔE. An exception was carrot puree treated with US at 35 kHz, for which the value was significantly different from the original juice (20.66 ± 0.45). This is associated with significantly increased redness and yellowness values (Figure 3a,b).

The high temperatures during sonication can lead to color degradation due to the isomerization of carotenoids. However, in this study, only a slight increase in temperature (1–5 °C) was observed, regardless of the carrot puree and extract type. Additionally, the total carotenoid content change was insignificant (Figure 4), suggesting that it could not have caused such a considerable difference in ΔE. This indicates that the color change may result from the cavitation process, where rapid particle transfer within the entire volume may cause the disintegration of larger structures [55,56] or may be due to the release of intercellular contents [26].

The concentration of carrot puree extract positively affected the carotenoid content (Figure 4). Following the sonication process, a trend in carotenoid content was noted, although the increase was not statistically significant. Previous research by Sun et al. [57] indicated that the degradation rate of β-carotene decreases with higher ultrasonic power intensity and temperature. In previous studies, an increase in carotenoid content in sonicated carrot juice was observed [53,58]. However, in some research, these changes were not statistically significant [53]. Our study also found similar results, as statistical analysis revealed no significant relationship between changes in carotenoid content and the use of ultrasound treatment.

4. Conclusions

The tested material in the form of carrot puree turned out to be a pseudoplastic fluid with shear thinning properties (n < 1). This behavior defines multiphase materials, the functional properties of which are related to the presence in the structure of the obtained puree of insoluble plant tissue particles dispersed in an aqueous solution, which are components of cell walls, among others. Activation energy decreased with the increase in extract and US treatment. Lower energy demand means that substrate molecules contained in the tested material can reach their transition state faster, and the overall reaction can proceed faster. Therefore, the variability in the extracted content and the use of variable ultrasound treatment parameters can guarantee the assumed physical and rheological properties, often considered in the fruit and vegetable industry as useful in producing high-quality vegetable juices and purees.

Carrot puree extract increased after ultrasound, possibly due to water loss during sonication. Carrot puree density increased with the increase in concentration. Carrot puree density after using US with a 21 kHz frequency remained unchanged. In contrast, the density of the carrot puree treated at the 35 kHz US frequency increased, which could be related to increased water evaporation during the second trial and the increase in temperature during sonication. Increasing the temperature can increase the reaction rate, which also causes a decrease in activation energy and confirms the conclusion regarding the effect of concentration and US treatment. At the same time, changes in activation energy and processing parameters may influence color changes in the tested material. Based on changes in the color parameters, it was found that the thin carrot puree with 9% concentration treated by a lower ultrasound wave (21 kHz) was more stable because US did not affect the color parameters L*, a*, and b*.

Color change in the puree treated by lower US waves was noticeable by the human eye because the value of the absolute color difference was less than 5.0. Color changes were not correlated with the content of carotenoids after the US process. Carotenoid content increased with the increasing concentration of dry matter. After sonication, there were no statistical changes in β-carotene content in the carrot purees, regardless of concentration and US frequency.