EFFECT OF HYDROCOLLOIDS, STORAGE TEMPERATURE, AND DURATION ON THE CONSISTENCY OF TOMATO KETCHUP

ABSTRACT

The effect of different hydrocolloids viz. guar gum, sodium alginate, pectin, CMC (carboxy methyl cellulose), xanthan gum and gum acacia on the consistency index, serum loss and flow value of tomato ketchup during storage at 5 and 50°C was studied. All hydrocolloids increased consistency of tomato ketchup, however, guar gum and xanthan gum caused maximum increase followed by CMC, sodium alginate, gum acacia and pectin. The consistency of tomato ketchup decreased with the increase in storage duration and the decrease was more pronounced at 50°C as compared at 5°C. Both serum loss and flow value decreased with the addition of all the hydrocolloids and increased with the increase in storage duration and temperature. Xanthan gum and guar gum caused maximum decrease in serum loss and flow value whereas pectin caused the least. Regression analysis was also performed to compute models which can be used to predict the effect of each hydrocolloid on consistency index, serum loss and flow value of tomato ketchup during storage at different temperature. Guar gum followed by CMC and sodium alginate were observed to be the best thickener for tomato ketchup among hydrocolloids studied.

INTRODUCTION

Tomato ketchup is a heterogeneous spiced product made from either cold or hot extracted tomatoes or directly from tomato puree. Consistency of ketchup is an important attribute from the engineering and consumer view 1-2. The flow behaviour of tomato products has been the subject of numerous studies 3-5. Design of equipment for fluid flow and heat transfer operations involved in the manufacture of tomato ketchup require data on the rheological properties of this product. During storage tomato ketchup tends to loose its consistency due to hydrolysis and also there is some syneresis (serum loss), both of which are not liked by the consumer.

Hydrocolloids are polysaccharides composed of simple sugar building units 6-8 and are widely used in the food industry as gelling, stabilizing, thickening and suspending agents. All of them have hydrophilic molecules, which combine with water to form viscous solutions. Hydrocolloids can be added to increase the consistency of the ketchup at a lower total solids content. Studies on the effect of hydrocolloids on the consistency and syneresis of tomato ketchup during storage has not been investigated. The present study was carried out to study the effect of addition of different hydrocolloids on the consistency index, serum loss and flow value of tomato ketchup during storage at different temperatures.

MATERIALS AND METHODS

Preparation of Tomato Ketchup

Fresh ripe tomatoes were obtained from the local market in the month of May 1999. The tomatoes were washed, crushed and then hot pulped (boiled in their own juice for 5 min). They were then passed through a laboratory pulper (Narang & Corp., New Delhi) to get tomato juice. The tomato ketchup was prepared using the ingredients shown in Table 1.

Table 1. Recipe Used for the Preparation of Tomato Ketchup

Ingredients	Quantity
Tomato juice	1 kg
Sugar	80 g
Salt	10 g
Onions	3.0 g
Ginger	6.0 g
Garlic	6.0 g
Mace	0.3 g
Cloves (headless)	0.3 g
Cinnamon	0.3 g
Red chilli powder	1.5 g
Black pepper powder	0.002 g
Acetic acid (99.5%)	2.5 mL

The tomato juice having a total soluble solids (TSS) content of 4.5% was put in an open pan and the spices were wrapped in a muslin cloth and dipped into the juice. Onion, ginger and garlic were also pulped and added to the juice. The ketchup was heated on a low flame with constant stirring till a final TSS of 28% was obtained. Then sodium benzoate (750 ppm) was added as a preservative. TSS of tomato juice and ketchup was measured with the help of Abbe Refractometer.

Six different hydrocolloids guar gum, xanthan gum, sodium alginate, pectin, gum acacia and carboxy methyl cellulose (CDH, Pvt. Ltd. Bombay) were added to the ketchup at levels of 0.5% just before the end point. The hydrocolloids were preblended with the sugar and salt and then added to the ketchup during the final stages of cooking. The ketchup was filled hot in glass bottles, sealed with crown corks and stored at 5°C and 50°C in incubators for 120 days.

Consistency Index

Rheological properties were measured using Brookfleld Viscometer (Brookfield Engineering Inc. model DV-II). A 500 mL beaker with a diameter of 8.5 cm was filled with the tomato ketchup sample to a height of 8.5 cm and brought to 25°C temperature in the TC-500 water bath (Brookfield Engineering Inc.). Measurements were taken 2 min after the spindle (no. 18) was immersed so as to allow thermal equilibrium in the sample and to eliminate the effect of immediate time dependence. Log-log plots of shear stress v/s shear rate were plotted to determine the consistency index (k) using the power law equation 9-10.

where τ is shear stress (Nm⁻²), k is consistency index (Nsm⁻²), n is flow behaviour index and γ is shear rate (s⁻¹).

Flow Value

The consistency of the ketchup at a temperature of 25°C was also measured using an instrument similar to the USDA consistometer. It consisted of a flow sheet made of glass over which the product was made to flow. The receptacle holding the ketchup was a PVC cylinder having a inner diameter of 4.5 cm and a height of 10 cm. The cylinder was filled with the ketchup and then raised upwards to allow the ketchup to flow over the glass sheet. The distance of flow after 1 min was measured from the center of the cylinder in centimeters. The flow value was taken as the average of the readings at four quadrants of the flow sheet and expressed in centimeters. The readings were taken at the edge of the ketchup and did not include any free serum that exudes from the ketchup.

Serum Loss

The ability of the gums to hold water in the ketchup was also measured. Tomato ketchup (20 g) was taken in a centrifuge tube and then centrifuged at 5000 rpm for 10 min. The supernatant was discarded and the remaining ketchup was weighed.

Sensory Evaluation

A paired comparison preference test was used to determine if the panelist preferred the tomato ketchup containing different hydrocolloids. Twenty four panelists were presented two coded ketchup samples, one without hydrocolloid and other containing a hydrocolloid and were asked to indicate their preference. The statistical significance of the results from the preference test was analyzed using the statistical table for the two sample test [11].

Statistical Analysis

Regression analysis was carried out using Minitab Statistical software (Minitab Inc. USA) to compute second order polynomials. Hydrocolloid concentration, 0 and 0.5% (X ₁), storage temperature, 5 and 50°C (X ₂) and storage duration, 0, 30, 60, 90 and 120 days (X ₃) were taken as independent variables. The polynomials were fitted to measure the consistency index (k), flow value (fv) and serum loss (s ₁). The equation used was as follows.

where Y is either consistency index, flow value or serum loss.

RESULTS AND DISCUSSION

Consistency Index

The statistical analysis in Table 2 revealed a highly significant effect of hydrocolloid concentration and storage duration on consistency index of tomato ketchup. The tomato ketchup had a consistency index of 13.98 Pa s and a flow behaviour index of 0.251 confirming its pseudoplastic nature. Similar results for flow behaviour index have been reported [12]. The addition of different hydrocolloids led to a significant increase in the consistency index of the tomato ketchup. The consistency index increased from 13.98 to 14.93, 22.67, 16.92, 32.43, 17.13, and 32.32 Pa s with the addition of pectin, CMC, gum acacia, guar gum, sodium alginate and xanthan gum, respectively. The flow behaviour index of the tomato ketchup containing the different hydrocolloids remained below 1 and varied between 0.22–0.45 confirming that the ketchup remained pseudoplastic in nature even when hydrocolloids were added to it. The increase in consistency index was highest with guar gum (131.97%) followed by xanthan gum (131.18%), CMC (62.16%), sodium alginate (22.54%), gum acacia (21.03%) and least with pectin (6.79%). The consistency index of the tomato ketchup dropped from 13.98 to 7.62 and 6.99 Pa s during storage for 120 days at 5°C and 50°C, respectively. These changes can be attributed to the hydrolysis of the ketchup constituents because of its low pH (3.0) and the hydrolysis was observed to be storage duration and temperature dependent. Guar gum has a straight chain of D galactose-D-mannoglycan with many single galactose branches, therefore it combines the properties of linear and branched polysaccharides and has a higher molecular weight [13]. This explains the highest consistency imparted by Guar gum. Xanthan gum also caused a significant increase in the consistency index of the tomato ketchup. It is an extracellular polysaccharide secreted by a bacteria of the genus Xanthomonas. Its primary structure consists of β-(l-4)-D-glucan backbone (cellulose) substituted at C-3 on alternate glucose residues with a trisaccharide side chain. The stiff xanthan gum molecules are extended in solutions and thus have higher viscosity and are highly pseudoplastic. The storage duration and temperature also showed significant effect on consistency index of tomato ketchup containing different hydrocolloids. During storage for 120 days at 5°C there was a drop in the consistency index of the tomato ketchup (Fig. 1). A highest drop in consistency index was observed in tomato ketchup containing pectin, followed by those containing guar gum, xanthan gum, sodium alginate, gum acacia and CMC, respectively. Storage of tomato ketchup for 120 days at 50°C resulted into greater decrease in the consistency index (Figure 1). The decrease in consistency index at 50°C during storage was highest in the tomato ketchup containing pectin followed by guar gum, sodium alginate, xanthan gum, gum acacia and lowest in the tomato ketchup containing CMC. Pectin is a polysaccharide of galacturonic acid or of its methyl ester. It is not efficient as a thickener compared to other water soluble polymers. The rheological properties of pectin solutions has been reported to depend on the presence of salts of calcium, pH, degree of esterification and molecular weight. Sodium chloride or other salts of monovalent cations reduce the viscosity of pectin solutions because charge effects are reduced at high ionic strength 14-15. This could have resulted in the low consistency index provided by the pectin in tomato ketchup. Pectin has been reported to be slowly degraded by depolymerization as well as by deesterification when the pH is close to 4 [16]. This could have led to maximum decrease in consistency index of tomato ketchup containing pectin during storage.

Table 2. Coefficient for Regression Models for Consistency Index of Tomato Ketchup Containing Different Hydrocolloids

Term	Guar Gum	Sodium Alginate	Gum Acacia	Pectin	CMC	Xanthan Gum
Constant (a)	13.893	13.893	13.893	13.893	13.893	13.893
X 1 (b 1)	36.141****	6.632****	5.806****	2.134**	17.777****	37.025****
X 2 (b 2)	−0.0019	−0.0019	−0.001	−0.001	−0.001	−0.0019
X 3 (b 3)	−0.051****	−0.051****	−0.051****	−0.051****	−0.0517****	−0.051****
X 1 X 2 (c 2)	−0.0075	−0.002	−0.0013	−0.002	−0.0066	−0.012
X 1 X 3 (c 3)	−0.0814***	0.439****	0.047****	0.0003	0.032****	−0.022*
X 2 X 3 (d)	−0.0001	−0.0001*	−0.0001	−0.0001	−0.0001	−0.0001
X 1 X 2 X 3 (e)	−0.00049	−0.00023	−3×10^−6	−0.0003	−9.6×10^−5	−9.2×10^−5
R^2	0.997	0.998	0.996	0.992	0.999	0.999

X ₁ hydrocolloid concentration (%), X ₂ storage temperature (°C), X ₃ storage duration (days). *p<0.l, **p<0.01, ***p<0.001, ****p<0.0001.

Figure 1. Effect of storage duration and temperature on the consistency index of tomato ketchup.

Flow Value

Flow value indicates the resistance of a fluid to flow. The statistical analysis in Table 3 revealed a highly significant effect of hydrocolloid concentration and storage duration on flow value of tomato ketchup. Addition of hydrocolloids led to a decrease in the flow value, this could be attributed to the binding of water by the hydrocolloid molecules leading to an increase in the resistance to flow. Addition of xanthan gum in tomato ketchup caused maximum decrease in flow value followed by CMC, guar gum, sodium alginate, gum acacia and pectin, respectively. The flow value of the tomato ketchup was 7.85 cm which increased during storage and increase was more at 50°C as compared at 5°C (Fig. 2). The increase in flow value of tomato ketchup at 5°C after 120 days of storage was 12.92% with CMC against an increase of 9.695%, 7.7%, 7.47%, 7.07% and 6.4%, respectively with gum acacia, guar gum, sodium alginate, xanthan gum and pectin. Similarly an increase in flow value of 17.35%, 12.48%, 10.4%. 10.03%, 9.54% and 9.52%, respectively with CMC, gum acacia, xanthan gum, guar gum, sodium alginate and pectin was observed at 50°C after 120 days. The changes in flow value in tomato ketchup containing CMC may be attributed to the hydrolysis of CMC. The hydrolysis of CMC under acidic conditions has been reported earlier [17]. Below a pH of 4 the free acid is substantially produced, which can cause precipitation of the polymer [18]. Gum acacia is a complex, highly branched and globular molecule which is closely packed which may have caused its lower contribution to the viscosity of tomato ketchup [19]. Gum acacia solutions at low concentrations are essentially newtonian in behaviour and have very low viscosity compared to other polysaccharides of similar molecular mass [20].

Table 3. Coefficient for Regression Models for Flow Value of Tomato Ketchup Containing Different Hydrocolloids

Term	Guar Gum	Sodium Alginate	Gum Acacia	Pectin	CMC	Xanthan Gum
Constant (a)	7.86	7.86	7.86	7.86	7.8606	7.86
X 1 (b 1)	−2.517****	−1.895****	−1.505****	−0.921**	−2.701****	−3.144****
X 2 (b 2)	0.003	0.003	0.003	0.003	0.003	0.003
X 3 (b 3)	0.0071****	0.0071****	0.007****	0.007****	0.0071***	0.0071****
X 1 X 2 (c 2)	−0.006	−0.0072	−0.005	−0.007	−0.0021	−0.0048
X 1 X 3 (c 3)	−0.005	−0.0058	−0.002	−0.006*	−0.0021	−0.0071*
X 2 X 3 (d)	5.7×10^−5	5.7×10^−5	5.7×10^−5	5.7×10^−5	5.7×10^−5	5.7×10^−5
X 1 X 2 X 3 (e)	6.4×10^−5	−4.9×10^−5	−4.7×10^−5	1.6×10^−5	−2.5×10^−5	−4.7×10^−5
R^2	0.991	0.987	0.980	0.977	0.985	0.994

X ₁ hydrocolloid concentration (%), X ₂ storage temperature (°C), X ₃ storage duration (days). *p<0.1, **p<0.01, ***p<0.001, ****p<0.0001.

Figure 2. Effect of storage duration and temperature on the flow value of tomato ketchup.

Serum Loss

The serum loss (syneresis) indicates the ability of tomato ketchup to hold water during storage. The statistical analysis in Table 4 revealed a highly significant effect of hydrocolloid concentration and storage duration on serum loss of tomato ketchup. Addition of hydrocolloids led to a decrease in the serum loss. There was no serum loss in the tomato ketchup to which either xanthan gum, guar gum or CMC was added. Sodium alginate, gum acacia and pectin decreased serum loss in tomato ketchup by 82.5%, 54.7% and 36.4%, respectively. The serum loss in tomato ketchup increased with the increase in storage duration. The serum loss of 17.46% in tomato ketchup was observed which increased to 26.7% at 5°C and 58.6% at 50°C during storage for 120 days (Fig. 3). Tomato ketchup containing hydrocolloids also showed an increase in serum loss during storage. This could be attributed to the hydrolysis of the hydrocolloids, as a result they lost their water holding capacity. There was no serum loss in the tomato ketchup containing either xanthan gum or guar gum even after storage of 120 days at 5 or 50°C. This indicates that both xanthan gum and guar gums have excellent water holding capacity and are excellent in preventing the occurrence of syneresis in tomato ketchup. Tomato ketchup containing CMC showed serum loss only after 30 days of storage (Fig. 3). Tomato ketchup containing sodium alginate showed serum loss of 66.26% against 50% and 45.8%, respectively, in the tomato ketchup containing pectin and gum acacia after 120 days of storage at 5°C. The serum loss in tomato ketchup increased with the increase in storage temperature. An increase of 98.32% in serum loss of tomato ketchup containing sodium alginate was observed during storage at 50°C for 120. While a serum loss of 77.4% and 53.84%, respectively, in tomato ketchup containing pectin and gum acacia was observed under similar storage conditions.

Table 4. Coefficient for Regression Models for Serum Loss of Tomato Ketchup Containing Different Hydrocolloids

Term	Guar Gum	Sodium Alginate	Gum Acacia	Pectin	CMC	Xanthan Gum
Constant (a)	17.522	17.54	17.522	17.54	17.522	17.54
X 1 (b 1)	−35.04****	−28.71****	−19.513****	−12.96****	−36.67*****	−35.08****
X 2 (b2)	0.0123	0.012	0.0123	0.012	0.012	0.012
X 3 (b 3)	0.493****	0.048****	0.0493****	0.0487****	0.049****	0.048****
X 1 X 2 (c 2)	−0.024	−0.025	−0.025	−0.017	−0.019	−0.024
X 1 X 3 (c 3)	−0.098****	−0.067***	−0.037*	−0.0091	−0.029	−0.097****
X 2 X 3 (d)	0.0006***	0.00062***	0.00061***	0.00062**	0.0006*	0.00062***
X 1 X 2 X 3 (e)	−0.0012**	−0.00087*	−0.00098*	−0.00019	−0.0009	−0.0012**
R^2	0.999	0.998	0.997	0.994	0.996	0.999

X ₁ hydrocolloid concentration (%), X ₂ storage temperature (°C), X ₃ storage duration (days). *p<0.1, **p<0.01, ***p<0.001, ****p<0.0001.

Figure 3. Effect of storage duration and temperature on the serum loss from tomato ketchup.

Alginate is a linear 1,4-linked polyuronan consisting of β-D-mannuronic and α-L-guluronic acids. The insoluble alginic acid has been reported to precipitate at pH<3.5 [13]. The low consistency imparted by sodium alginate could have been due to the precipitation of the alginic acid molecules at the low pH of tomato ketchup. The higher decrease in consistency at 50°C could be attributed to the presence of a small amount of protein as an integral part of its molecule and the gum undergoes autohydrolysis when heated at high temperatures for prolonged periods resulting in the precipitation of the protein rich fraction [21]. In case of xanthan gum the close alignment of the trisaccharide side chains with the main chain make the molecule a rather stiff rod with extraordinary stability to heat, acid and alkali. The molecular weight of xanthan gum is generally around 2 million. It has excellent thermal stability, solubility and stability in acidic systems. The structural rigidity of its molecule is due to its linear cellulosic backbone that is stiffened and shielded by the trisaccharide side chain.

Sensory Evaluation

The paired comparison preference tests revealed that the tomato ketchup without any hydrocolloid was preferred over the tomato ketchup in which xanthan gum had been added at the 0.1% level of significance. The ketchup containing xanthan gum had a slimy taste which led to its unacceptability. Tomato ketchup containing pectin or guar gum or sodium alginate were not preferred when compared to ketchup without any hydrocolloid at 5% level of significance. The tomato ketchup containing CMC and gum acacia were preferred at 5% and 0.1% level of significance, respectively, over the ketchup without any hydrocolloid. Thus the sensory evaluation revealed that the ketchup containing gum acacia had highest preference followed by CMC. The sensory panel was unable to differentiate between the ketchup without any hydrocolloid and those containing either pectin, guar gum or sodium alginate.

CONCLUSION

The study revealed that guar gum caused highest increase in consistency index of tomato ketchup followed by xanthan gum, CMC, sodium alginate, gum acacia and pectin. All the hydrocolloids decreased the flow value of ketchup. Xanthan gum was observed to have maximum effect followed by guar gum, CMC, sodium alginate, gum acacia and pectin. The tomato ketchup containing xanthan gum or guar gum did not show any serum loss even after 120 days of storage at 50°C. Whereas ketchup containing CMC showed serum loss after 30 days of storage at 50°C. In summary, guar gum followed by CMC and sodium alginate were observed to be the best thickeners for tomato ketchup among hydrocolloids studied. Tomato ketchup containing these hydrocolloids showed highest consistency index and least flow value and serum loss during storage. Xanthan gum was not considered desirable because it imparts a slimy taste to the tomato ketchup and was rejected by the sensory panel.

Acknowledgments