Determination of Ketchup Sensory Texture Acceptability and Examination of Determining Factors as a Basis for Product Optimization

Abstract

The aim of this study was to examine possible usage of instrumental determination of selected texture parameters to estimate sensory texture acceptability of ketchups. Likewise, the authors focused on examination whether the consumer’s perception of texture acceptability is connected to the selected factors related to the ingredients and production technology. Yet another aim was to describe the selected textural parameters of ketchups available in the European market. Within the frame of this study, 29 samples of ketchups were analyzed by means of instrumental and sensory texture analysis. Further, a survey research was carried out with the results showing that the texture was the second most significant parameter for the consumer’s choice of a particular product. The average evaluation of sensory texture acceptability reached the values from 3.78 to 7.67. Analysis of the evaluation results of textural parameters based on the values of Pearson’s correlation coefficient confirmed a high correlation (p < 0.05) between instrumental and sensory determined viscosity and density. However, none of these partial textural parameters affected the sensory acceptability of the texture separately. Thus, using the instrumental analysis for estimation of the sensory texture acceptability of ketchups is not possible. For the optimization and control of ketchup texture in practice, sensory analysis is essential.

Keywords:

• Consistency
• Ketchup
• Sensory analysis
• Texture
• Viscosity

INTRODUCTION

Ketchup is a commonly consumed and popular foodstuff that has been more often discussed in recent years, particularly because of its lycopene content and related beneficial properties. In the European market, products of this type are abundant varying from each other in nutritional and sensory quality. With regard to the fact that the food market is generally saturated, the assertion of food products including ketchups is possible only in case of their systematic innovation. For the desirable innovation, sensory quality is of the primary importance as the majority of consumers are most aware of this quality aspect.

Texture along with taste, smell, and color belong to the organoleptic properties of foodstuffs. Texture assessment is considered a crucial point in the assessment of potential consumer acceptability of various types of foodstuffs.^[1−3] The recent trend to modify food properties with the help of different additives or to enrich foodstuffs using new ingredients gives a rise to the necessity of examination of their influence on final product texture.^[4−6] The texture includes several properties of which consistency and viscosity are usually considered the key quality aspects of tomato products.^[7,^8] Ketchup consistency (or density) stands for the amount of suspended particles in serum, in other words, the ratio between the liquid phase and the solid phase composed of insoluble particles from tomatoes.^[9] Measuring consistency is a basic detection method used in industrial processing of tomatoes for determining the quality of the product. During the period of storage, separation of serum occurs naturally in tomato products, however, this phenomenon is often considered undesirable by consumers, so the manufacturers use various substances, most commonly in the market is xanthan, to stabilize the texture and limit the separation. Another important quality parameter of tomato products is their viscosity. Knowledge of this rheological property of liquid and semi-solid foods is especially important for setting the flow parameters during production. Reliable and accurate rheological data are necessary to set up and optimize individual operations (intake, mixing, heating, etc.) and to ensure the acceptability of the product, as products with the wrong viscosity can be considered inacceptable, or salable only at a lower price. Consistency and viscosity of ketchups are therefore an important attribute both technologically and from consumer’s perspective—during storage (to determine stability) as well as in the field of quality control in general.^[10,^11]

Manufacturing technology of tomato products, in particular vegetable juices and concentrates, includes mechanical disruption of parenchymatous plant tissues. This results in dispersions which are a combination of the liquid phase, or serum containing pectic polysaccharides and other soluble substances and the dispersed phase (pulp) containing insoluble plant components, such as cell walls.^[12] Textural properties of these products are dependent on the variety, the environmental factors during growing and the stage of tomato ripening to a certain extent,^[13] but they are also affected within the processing and associated with structural changes in pectins, which may be attributed to the activity of pectolytic enzymes of pectinmethylesterase and polygalacturonase. The result of the synergistic effect of these enzymes is a decrease in viscosity. Processing conditions affect the extent of their inactivation and thus the extent of decomposition of pectin. In order to maintain a higher viscosity, it is necessary to control the activity of these enzymes within the course of industrial processing.^[14] Recently, a new method for mechanical disruption of plant material by high pressure homogenization was introduced. This method allows for better use of natural structural potential of different plant sources.^[15] Other factors potentially affecting the viscosity of ketchups include the manner and conditions of homogenization and concentration, which affects the degradation of pectin as well as interactions between pectins and proteins and the contents of pulp.^[16−18] A number of substances, in particular various starches and gums, are used for further adjustment of consistency and the associated viscosity in the production of ketchups.

Textural properties of ketchups may be determined by instrumental techniques as well as sensory analysis, whereas, both methods have their advantages and disadvantages and complement each other. Instrumental methods used to describe sensory properties are objective, repeatable, and in the case of texture measurements also quite time-saving. Sensory analysis is more time-consuming and thus more expensive, but it allows determining the most acceptable nature of product for the consumer. In some food matrices, after verifying the correlation of the selected instrumental method with sensory analysis results, it is possible to describe the range of instrumentally measured values that will be best accepted by consumers.^[19,^20] A high correlation between the results of instrumental texture measurements and sensory evaluation of textural properties could then enable using instrumental analysis for fast and inexpensive prediction of sensory texture and the overall acceptability of the product. A similar approach was described in a study focused on cheese age and quality estimation from instrumentally determined textural parameters.^[21]

Depending on the selected analysis, the view on individual properties and their description somewhat differs. Texture of foodstuffs is generally the sensory and functional manifestation of their structural, mechanical, and surface properties detected through the senses of vision, hearing, touch, and kinesthetic.^[22] Viscosity, in terms of physical properties measurable instrumentally, describes the flow rate generated by the unit of force. In the case of sensory analysis, viscosity is defined as the force required to transfer the liquid from a spoon to one’s tongue. As for consistency, the exact definition is more complicated, as it is often perceived as a general term describing the texture. The consistency of tomato puree and ketchups is usually measured instrumentally by Bostwick method based on rapid outpouring of a defined amount of the product and measuring how far the product runs.^[14] In the case of instrumental measurement of consistency by backward extrusion, it is described as the curve profile demonstrating firmness. Degree of firmness is then described as the force needed to achieve a given deformation. In other words, it is a degree of resistance of the product to the applied compressive forces.^[22] In various dictionaries, consistency is explained as the degree of density, or firmness. In the case of sensory analysis, consistency perception of consumers might be simply marked as density.^[23]

The aim of this study was to examine the possibility to apply instrumental determination of selected textural parameters to estimate the sensory texture acceptability of ketchups from the perspective of consumers. In the case of a confirmed relation between the sensory texture acceptability and partial instrumentally detected parameters, the authors aimed to determine the interval of values of these parameters in ketchups best evaluated within the sensory analysis. Likewise, the authors wanted to examine whether the consumer’s perception of texture acceptability is related to the amount of tomatoes used for ketchup production, or possibly to the usage of hydrocolloids as thickening agents. Another partial aim was to describe textural parameters of ketchups available in the European market.

MATERIALS AND METHODS

Materials

29 samples of ketchups (out of that 3 organic products) came from the Czech Republic, Slovakia, Poland, Austria, Germany, the Netherlands, and Croatia. They are characterized by a rich variety in terms of the contents of tomato raw material indicated as the contents of tomatoes (130–230 g per 100 g of ketchup), although the vast majority of ketchups were produced from tomato paste. Some samples did not contain any thickening agents, other samples were thickened with modified starch, yet in other cases the texture was stabilized by adding xanthan as well. Prior to the measurement, the samples were stored in a refrigerator at 5°C.

Survey Research

Preferences for ketchups, in particular with regard to the criteria for selection and negative factors discouraging from ketchup consumption, were investigated in respondents in form of a brief survey. Respondents were first of all asked to specify the priority of ketchup selection criteria (composition, taste, color, price, advertising, package appearance, texture), afterwards they were asked to specify the priority of negative factors (objectionable taste, objectionable texture, objectionable color) discouraging from ketchup consumption. Two hundred respondents aged 20 to 40 took part in the survey. The obtained results were then statistically processed using Microsoft Excel (2010).

Sensory Analysis

Sensory panel was composed of ten trained assessors (seven women and three men) who had had previous experience with sensory analysis of foodstuffs. The panel composition was chosen on purpose in response to the fact that women go shopping for food in most households. The assessors had not used any perfume, smoked, or consumed any food or beverages that might influence their perceptions for a period of one hour before the analysis. The analysis was carried out in a sensory laboratory with separate boxes for assessors based on the prepared forms for sensory evaluation of ketchups. The samples were evaluated for the overall pleasantness, partial textural properties which included viscosity and density as well as texture acceptability, using a nine-point scale, where nine corresponded to the highest overall pleasantness/viscosity/density/texture acceptability. Table 1 shows a description of five groups that included individual classes of texture acceptability, by which it was possible to detect even subtle differences between samples. In order to create groups of samples evaluated by individual assessors, a statistical model of balanced incomplete block was applied, which allowed reducing the number of samples evaluated by one assessor.^[24] Each assessor evaluated eight samples, whereas, each sample was evaluated in triplicate by three different assessors. The order of samples in the evaluation of individual assessors differed for reducing its impact on the evaluation. Samples were served in plastic cups of 50 ml volume identified by numerical codes. Stainless spoons were available for manipulation with the samples. Clean still water was available as a neutralizer.

TABLE 1 Classes of sensory texture acceptability

Class of texture acceptability	Class description
1, 2	The sample texture is unsatisfactory, in terms of density and viscosity it ranges in the extreme values (too thin to watery/too thick and too viscous and sticky/lowly viscous, non-cohesive).
3, 4	The sample texture in terms of density and viscosity is less extreme, but it is not evaluated as satisfactory or pleasant.
5, 6	The sample texture in terms of one of the evaluated parameters appears to be satisfactory, however, the other parameter is less pleasant.
7, 8	The sample texture is pleasant, density and viscosity of the sample are acceptable to the bite, the sample is smooth and the texture is also natural, not blancmange-like.
9	The sample texture is very pleasant, leaving a pleasant feeling after ingestion, with a high degree of naturalness evocating high proportion of tomatoes and a low proportion of thickening starches.

Instrumental Analysis

Instrumental analysis of selected textural properties was performed using a texturemeter (TA.XTplus, Stable Micro Systems). A ketchup sample was dosed into the sample container with a diameter of 50 mm and height of 100 mm to approximately 75% of its volume. Dosing into sample containers was carried out immediately after removal from the refrigerator. Since ketchup is one of pseudoplastic fluids, each sample rested for approximately one minute before the measurement due to restoration of the original viscosity. The average temperature of the samples during measurement was about 10°C. Textural properties of each sample were measured in triplicate. Parameters recommended by the manufacturer for measuring the texture of ketchups were applied: Disc for backward extrusion with a diameter of 35 mm, depth of probe penetration of 30 mm, probe velocity over data capture of 1.0 mm·s⁻¹, moment of starting the measurement at the load of 5 g (lower part of the disc is in full contact with the product).

Statistical Analysis

For processing the results of instrumental measurements of viscosity and texture and also the results of sensory evaluation of textural parameters, several statistical methods were applied, namely the T-test, F-test, Kruskal-Wallis test, correlation analysis, regression analysis, general linear model, and cluster analysis. All the analyses were performed using the Minitab statistical software (Minitab Inc., State College, USA).

RESULTS AND DISCUSSION

Survey Research

Two hundred respondents aged 20 to 40 participated in the survey research, male to female respondents’ ratio was 35.2:64.8. Results of the survey research showed that taste is the first and texture is the second most significant parameter out of all listed parameters (composition, taste, color, price, advertising, package appearance, texture) affecting consumers’ choice of a particular product, as shown in Table 2. As confirmed by results of other studies, there is a close correlation between food taste and food texture. The texture significantly determines the overall mouthfeel and influences flavor release, although the rate of the influence depends on the level of viscosity and the type of food matrix or used thickening agent.^[25−27] Sensory unacceptable or objectionable texture was the most significant negative factor discouraging 10% of respondents from purchasing a particular ketchup, the second most significant negative factor discouraging 54% of respondents and the third most significant negative factor discouraging 36% of respondents from purchasing a particular ketchup.

TABLE 2 Ranking of texture significance in survey research

Texture position related with its significance for product selection	Percentage of respondents
1.	6
2.	33
3.	27
4.	21
5.	9
6.	4
7.	0

Sensory Analysis

Examination of the correlation between sensory texture acceptability and overall pleasantness of the samples confirmed the existence of correlation between these two parameters. The sensory texture acceptability values statistically significantly correlated with the values of overall pleasantness of samples (r = 0.744, p < 0.05). The change in textural properties and thus in values of sensory texture acceptability goes hand in hand with the change in overall pleasantness of ketchup. This phenomenon is in the “ketchup” industry universally accepted and legitimized by the fact that for the consumer it is important, how easy/difficult it is to dispense the ketchup from consumer packaging. The requirement for easy dispension in practice determines specific boundaries for textural properties, within which ketchup should ideally range, depending on the type of packaging.^[28] Sensory evaluation of ketchups performed in this study, however, did not evaluate textural properties in relation to dispension out of consumer packaging. Viscosity was evaluated during pouring from a spoon and density only as a perception of resistance in the mouth. Thus it can be concluded that the overall texture affects consumer’s perception, nevertheless, it is not only limited to textural properties of ketchup during pouring or squeezing it out of the container.

Using the methods of cluster analysis, individual ketchups were divided into clusters according to their sensory texture acceptability, these clusters and their similarity is shown in the dendrogram (Fig. 1). Since only the similarity of clusters is displayed in the dendrogram, and not the actual value of sensory texture acceptability of individual ketchup samples, the obtained clusters are displayed using a DotPlot diagram (Fig. 2), while the numbers of clusters match. Graphical display of clusters using the dendrogram and the DotPlot diagram illustratively shows the current status of individual classes of ketchup texture acceptability in the European market. The average values of sample viscosity determined in 29 samples of ketchup by sensory analysis ranged from 3.00 to 8.33, and for density from 2.67 to 7.33. The average evaluation of sensory texture acceptability reached the values from 3.78 to 7.67. Concerning the overall pleasantness, the panel evaluated the spectrum of the samples within the range from 2.00 to 7.00.

FIGURE 1 Dendrogram representing the results of cluster analysis of ketchups organizing the ketchup samples into clusters according to the values of sensory texture acceptability.

FIGURE 2 DotPlot diagram showing ketchup clusters with their actual values of texture acceptability determined by sensory analysis. STA: Sensory texture analysis.

Instrumental Analysis

The repeatability of the method was ascertained by ten-fold determination of one sample and expressed as relative standard deviation 2.06%. In the case of instrumental measurement, values of consistency ranged from 864.26 to 2330.03 g·s. Instrumentally measured viscosity values ranged from –1517. 97 to –352.54 g·s. Quality evaluation of tomato products was also carried out in a Greek study^[14] which, based on viscosity (measured by rotational viscometer) and consistency (measured by Bostwick consistometers), compared heat-treated tomato juice produced in a conventional way of hot break and cold break with juices treated under high pressure. Among these individual samples of tomato juice there were also significant differences and high variability both in viscosity and consistency that was caused by different production technologies in their set of samples.

Statistical Analysis

Prior to each statistical analysis, normality of the measured data in groups was reviewed according to stratification factors listed below. It was assumed that the sensory texture acceptability evaluated by a panel of trained evaluators representing the consumers would depend on the parameters that described individual partial textural characteristics, namely the consistency or density and viscosity. It was also predicted that dependence of sensory texture acceptability on parameters which in terms of technology significantly affect the final texture of ketchup, primarily on the quantity of tomatoes used and potential addition of thickeners or stabilizers. However, statistical analysis based on t-tests comparing mean values of groups of samples with/without the addition of thickeners and also with/without the addition of stabilizers confirmed equality of mean values of both groups of samples classified according to the use/non-use of food additives. Neither of these two observed categorical factors, however, had any statistically significant effect on mean sensory texture acceptability (p < 0.05). F-tests were also used in comparison of variability in individual groups according to the same categorical variables, with the same result—not one of the observed categorical factors had any effect (p < 0.05) on the dispersion values of sensory texture acceptability. The same was the result of non-parametric Kruskal-Wallis test for equality of medians.

Analysis of the results of instrumental and sensory evaluation of textural parameters based on the values of the Pearson’s correlation coefficient confirmed a high correlation (p < 0.05) between instrumentally and sensory determined viscosity and between instrumentally and sensory determined texture (density). The values of the coefficients are presented in Table 3. However, none of these partial textural parameters affected the sensory acceptability of the texture separately. Yet, the situation can vary between different food matrices; this is apparent from the study aimed at cheese where it was possible to estimate the quality of the product from its textural parameters.^[21]

TABLE 3 Correlation between instrumentally and sensory determined textural parameters

	IV_SV	IC_SD	STA_IC	STA_IV	STA_SV	STA_SD
Pearson’s correlation coefficient	−0.824*	0.732	0.265	−0.265*	0.138	0.099
p-value	0.000	0.000	0.164	0.164	0.476	0.611

IV: instrumentally determined viscosity; IC: instrumentally determined consistency; STA: sensory texture acceptability; SV: sensory determined viscosity; SD: sensory determined density;

*This is not a negative correlation, the minus sign is present only because of the mathematical expression of instrumental measurement outcomes in negative territory.

Textural properties of the sample best assessed by the panel, namely sample No. 13, are shown in Table 4. This sample is characterized by instrumentally measured high viscosity and consistency, as well as sensory detected high sensory viscosity and density. Sensory worst assessed sample in terms of texture pleasantness was sample No. 20 which, as in the case of sensory as well as instrumental analysis, reached low values of partial textural parameters, as shown in Table 5. Thickeners, but no stabilizers, were used in the production of both of these sensory extremely assessed samples. As regards the quantity of tomatoes used for production of 100 g of ketchup, both samples contained approximately equal amounts, about 130 g of tomatoes, which was the minimum value in the set of samples that we assessed. Concerning other differences in ingredients listed on the product labels, sample No. 20 contained spirit vinegar, sodium benzoate, and acesulfame K—these ingredients were not used in the production of sample No. 13. Because of the absence of statistical relation between the instrumentally measured textural parameters and sensory texture acceptability, the exact values of instrumentally measured textural parameters of the sample best assessed by the panel cannot be used as a recommendation for ketchup industry. Nevertheless, characteristics of the best and the worst sensory assessed ketchup samples suggest that samples with higher values of instrumentally established consistency and viscosity and sensory determined density and viscosity were acceptable for the assessors representing ordinary consumers, which is consistent with the results obtained by Weenen et al.^[29] whose research based on the sensory analysis of thickened sauces, dressings, and similar products confirmed higher sensory acceptability in the case of products with a higher density.

TABLE 4 Textural parameters of sensory best assessed ketchup sample

Sample	IC (g·s)	IV (g·s)	SV	SD	STA	Contents of tomatoes (g)	Modified starch	Xanthan
13	2218.96 ± 50.41	−1401.42 ± 27.43	8.00 ± 0.82	7.00 ± 0.82	7.67 ± 1.25	138	Yes	No

TABLE 5 Textural parameters of sensory worst assessed ketchup sample

Sample	IC (g·s)	IV (g·s)	SV	SD	STA	Contents of tomatoes (g)	Modified starch	Xanthan
20	1076.35 ± 6.98	−542.20 ± 1.06	3.00 ± 2.00	2.50 ± 1.50	2.50 ± 1.50	130	Yes	No

In order to verify the combined effect of the factors of viscosity, density, and the amount of tomatoes per 100 g of product on sensory texture acceptability of ketchup, a regression analysis was carried out with the result that these factors cannot be used to compile a statistically significant model. For the combined effect of categorical factors, an analysis of the general linear model was performed with a similar result. Thus in the set of samples, the clear effect of viscosity and consistency cannot be confirmed as partial textural parameters of the hedonic assessment of texture. A similar conclusion can be drawn from the results published in the study by Sharoba et al.^[27]

Sensory Texture Analysis Design

Since the sensory texture acceptability is independent from instrumentally measured textural parameters, it is not possible to use the instrumental analysis for an estimation of sensory texture acceptability of ketchups. For the control of sensory texture acceptability, it is essential to use sensory analysis. For the optimization and control of ketchup texture in practice, it is recommended to use sensory evaluation based on categorical two-way scales for ketchup viscosity and density with grading sufficiently sensitive to capture slight variations in these textural parameters but easy to interpret and to process statistically. To capture any texture acceptability variations independent on viscosity or density, these evaluations should be accompanied by sensory evaluation of texture acceptability in general, classes with possible verbal description are shown in Table 1.

CONCLUSIONS

Ketchups in the European market are characterized by a wide range of values of both instrumentally measured textural parameters as well as sensory texture acceptability. According to the results of the survey research, texture plays a vital role in consumer’s choice for a particular product. Therefore it is necessary to find a proper measure how to control this parameter. Based on the absence of statistical relation between the instrumentally measured textural parameters and sensory texture acceptability, it is possible to conclude that for the assessors, hence the consumer, the texture is a much more complex property and is probably not perceived solely as viscosity or consistency, although these specific characteristics are usually used for the quality control. Since the sensory texture acceptability is independent on instrumentally measured textural parameters, it is not possible to use the instrumental analysis for the estimation of sensory texture acceptability of ketchups. For the control of the sensory texture acceptability, it is essential to use sensory analysis. Furthermore, sensory evaluation of texture acceptability showed that neither texture nor its acceptability does necessarily depend on the quantity of tomatoes, on the presence or absence of thickeners or stabilizers, but rather on other formulation ingredients or aspects related to technologies. For the optimization and control of ketchup texture in practice, the authors recommend the use of sensory category ordinal scale with sufficiently sensitive grading and verbal description of individual grades. Even though the texture definitely participates in shaping consumer’s opinion on the product, it is not the only factor determining the choice when buying. A further study should be focused on the description and determination of other sensory properties of ketchup important for consumers.

FUNDING

The research was financially supported by the Internal Grant Agency of the University of Veterinary and Pharmaceutical Sciences Brno (Research project IGA 91/2011/FVHE).

Additional information

Funding

The research was financially supported by the Internal Grant Agency of the University of Veterinary and Pharmaceutical Sciences Brno (Research project IGA 91/2011/FVHE).