Instrumental Textural Characteristics of Restructured Carrot Cubes

Abstract

Studies were carried out on the instrumental textural evaluation of restructured carrot cubes. The experiment was conducted by incorporating different levels of alginate, glucono delta lactone (GDL), and calcium salt to the carrot pulp. Investigations showed that as pulp level increased from 0 to 90%, there was a corresponding decrease in failure stress, failure strain, and deformability modulus. Instrumental textural profile analysis (TPA) parameters viz. hardness, springiness, gumminess, cohesiveness, chewiness, and resilience also showed a similar trend. Effect of formulation variables, i.e., alginate, GDL, and calcium salt on hardness (response variable) were evaluated by the application of response surface methodology. All the three ingredients showed a significant (P < 0.05) influence on hardness of carrot gel. Heat treatment of restructured carrot samples resulted in an increased hardness, cohesiveness, gumminess, and chewiness while springiness, cohesiveness, and resilience decreased. The data indicated that the shrinkage during thermal treatment may be responsible for the change in textural attributes. The authors concluded that a thermally stable restructured product with appreciable textural integrity can be obtained from carrot pulp.

Keywords:

• Alginate gels
• Calcium salt
• Glucono-δ-lactone
• Texture profile analysis
• Carrot
• Mechanical properties
• Response surface methodology

INTRODUCTION

Texture in addition to appearance and flavor is the most important quality factor which determines the acceptance of processed food products. It is of special significance to structured/fabricated gelled products where texture is imparted to otherwise liquid material. Most of the publications on gelled texturized products utilizing alginate deal with mechanical strength/texture of the material.[1,2,3,4] Rheological profile analysis related to texture of a mixture of sago-wheat gel was studied by Zaidul et al.[5] Texture could be measured in three ways: fundamental, imitative, and emperical.[6] Objective measurement of food texture predominantly involves an analysis of mechanical behavior of food materials and includes measurement of load distance characteristics using a mechanical device and assessment of subjective characteristics using a suitable texture profiling method.

Considering the importance of carrot in human nutrition, the experiment was carried out on the development of restructured carrot gel, and the evaluation of its instrumental textural characteristics. Reconstitution of carrot/alginate particles suitable for aseptic processing and an evaluation of its physical properties, including texture, was calculated by Marcotte et al.[7] They optimized the formulation of reconstituted carrot/alginate particles on the basis of various levels of alginate concentration, calcium chloride concentration, and water content. Hardness was considered as one of the quality parameters for optimization.

Apart from pulp, three constituents: sodium alginate, glucono-δ-lactone, and calcium salt are considered to be important for alginate texturization of fruit and vegetable pulps. Alginate gels are made with calcium salts to give a calcium alginate gels. The usual procedure is to use a controlled reaction in which a soluble alginate is reacted with a sparingly soluble calcium salt under conditions of concentration and suitable pH. The rate of reaction and selection of proper calcium salt is extremely important.[8] An acid pH obtained through the use of glucono-δ-lactone, which has the property of being slowly hydrolyzed and gradual pH reduction improves product firmness, microbiological stability and sensory qualities.[3] Kaletunc et al.[2] investigated the effect of pulp or juice concentration on the mechanical strength of texturized apple pulp and grape fruit juice. However, the effect of pulp concentration on the mechanical strength of texturized carrot gel, as well as the effect of different combinations of constituents required for texturization on hardness of restructured carrot, has not been reported so far.

The present investigation was undertaken to study the effect of pulp concentration and functional ingredients as well as thermal treatment on the instrumental textural characteristics of restructured carrot.

MATERIALS AND METHODS

Carrots procured from the local market at Mysore, India were thoroughly washed, peeled, cut into small pieces, diced, and cooked in a pressure cooker at 34.5 kPa (5 psi) for 10 minutes. Cooked carrot was pulped to puree in a blender. Total soluble solids content of the puree was adjusted to 10° Brix with sugar. Sodium alginate (Across Organics, New Jersey, USA) and calcium hydrogen orthophosphate (CaHPO₄) (s-d fine chemicals, India) were mixed and added together to the puree. The concentration of carrot puree was varied between 0–90%. After mixing thoroughly, glucono-δ-lactone (Sigma-Aldrich, USA) was added on the basis of gel solution. To evaluate the effect of pulp concentration on the mechanical strength of the product, concentrations of three additives were alginate 2%, CaHPO₄ 1% and glucono-δ-lactone 1.5% by weight. The levels of three additives were determined on the basis of preliminary experiments.

Mechanical Testing

The force-deformation rate was measured using texturized sample of 225 mm² sectional area and 10 mm height, cut from the gel set for 24 hours at 5°C. All measurements were obtained using a Texture Analyser, Model TA-HDi (Stable Microsystems, UK) with a loadcell of 5 kg. The data were recorded through a control console and transferred to a PC AT for analysis using a software Texture Expert Version 1.22. For the measurement of compressive stress, the product specimen was compressed to 70% of its original height between parallel lubricated plates using a 75 mm compression plateau. Pre-test, test and post-test speeds were fixed at 1 mm/sec, 0.5 mm/sec, and 5 mm/sec respectively. The rheological parameters were calculated by assuming the gels as incompressible and expressed in true stress-strain terms using the following equation for corrected stress and Hencky's strain.[2]

(1)

(2)

Where F(t) is the force at time t; A_o and H_o are the initial cross sectional area and length respectively; ΔH(t) is the momentary absolute deformation. The deformability modulus (modulus of deformability) E_D is a term suggested to replace modulus of elasticity in the system which is not purely elastic and is derived from the slope of initial straight line of the stress-strain curve. This was suggested by Mohsenin and Millal[9] to avoid the confusion of taking it as an indication of degree of elasticity of food material.

Instrumental Texture Profile Analysis (TPA)

The product specimen of height 20 mm and area 225 mm² were compressed to 40% strain (of original height) with a pretest, test and post-test speeds of 1 mm/sec, 0.5 mm/sec, and 0.5 mm/sec, respectively. Force time curve obtained were analyzed for six textural parameters, viz. hardness, resilience, springiness, cohesiveness, gumminess, and chewiness.[10] Effect of thermal treatment was studied by preparing gel at three different concentration of alginate, cutting into 2 cm³ cubes, packing in polypropylene pouches (300 G, 50 g) and subjecting them to heat treatment by keeping in boiling water for 25 minutes without monitoring the central temperature of the cube. Both thermally treated and untreated samples were subjected to instrumental texture profile analysis.

Experimental Design

The experiment was based on three independent variables and one response was measured to study the effect of functional ingredients. Independent variables were the concentrations of alginate, calcium salt and glucono-δ-lactone (GDL). A central composite rotatable design with 3 variables (five levels each) was employed.[11] The response function was hardness of the carrot gel. The experimental design in coded (X) and uncoded (actual, x) levels of variables has been presented in the Table 1. A quadratic polynomial regression model was assumed for predicting response variable. The proposed model was:

Table 1 Experimental design used to obtain different combinations of sodium alginate (x₁), calcium hydrogen phosphate (x₂), and glucono-δ-lactone (x₃) for restructuring carrot and experimental data for response variables.

Assay number	Variables						Response Variable
	Coded			Actual
Sl.No.	X1	X2	X3	Sod. alginate (%)	Ca Salt (%)	Glucono-δ-lactone (%)	Y(Hardness) (N)
1	−1	−1	−1	1.0	0.5	1.0	3.814
2	−1	−1	+1	1.0	0.5	2.0	3.966
3	−1	+1	−1	1.0	1.5	1.0	4.240
4	−1	+1	+1	1.0	1.5	2.0	5.822
5	+1	−1	−1	2.0	0.5	1.0	5.991
6	+1	−1	+1	2.0	0.5	2.0	6.173
7	+1	+1	−1	2.0	1.5	1.0	7.347
8	+1	+1	+1	2.0	1.5	2.0	10.130
9	+1.682	0	0	2.341	1.0	1.5	9.383
10	−1.682	0	0	0.659	1.0	1.5	3.285
11	0	+1.682	0	1.5	1.841	1.5	6.251
12	0	−1.682	0	1.5	0.159	1.5	2.070
13	0	0	+1.682	1.5	1.0	2.341	7.254
14	0	0	−1.682	1.5	1.0	0.659	3.758
15	0	0	0	1.5	1.0	0	6.260
16	0	0	0	1.5	1.0	0	6.585
17	0	0	0	1.5	1.0	0	6.216
18	0	0	0	1.5	1.0	0	6.461
19	0	0	0	1.5	1.0	0	6.486
20	0	0	0	1.5	1.0	0	6.363

Where B₀, B₁, B₂ etc. are coefficients of regression and Xs are the coded independent variable. All the data were subjected to analysis of variance to determine the effects of concentration of alginate (X₁), calcium salt (X₂), and GDL (X₃) on response variable (Y), hardness. Microsoft Excel (Windows 98) was used to fit the experimental data to the 2^nd order polynomial equations, and obtain coefficients of the equation, for regression analysis as well as analysis of variance.

RESULTS AND DISCUSSION

Mechanical Strength

The failure stress (compressive strength), strain at failure and deformability modulus values of the texturized carrot as a function of pulp concentration are represented in Table 2. As shown in Table 2, all the three fundamental textural parameters, i.e., failure stress, failure strain, and deformability modulus decreased considerably as the pulp level increased from 0 to 90%. Almost similar observations were made by Kaletunc et al[2] from their work on the texturization of apple pulp. Nussinovitch et al.[4] observed that fruit pulp weakens the gel down to a minimum point by interfering with matrix formation. However, the gel system may regain its strength if the pulp is capable of forming its own structure.

Table 2 Mechanical properties of restructured carrot as a function of pulp concentration.

Pulp level (%)	Stress at failure (kPa)	Strain at failure	Deformability modulus (kPa)
0	115.80 ± 1.02	1.077 ± 0.012	109.88 ± 0.54
20	74.10 ± 1.30	1.005 ± 0.007	76.94 ± 0.22
40	58.69 ± 0.66	0.963 ± 0.007	63.19 ± 0.70
60	45.17 ± 0.70	0.883 ± 0.008	52.68 ± 0.70
80	26.36 ± 0.46	0.671 ± 0.003	40.65 ± 0.55
90	15.76 ± 0.27	0.591 ± 0.011	32.64 ± 0.41
Mean ± SD (n = 5).

Instrumental texture profile analysis data for the specimen samples (Table 3) showed a considerable decrease in hardness, gumminess and chewiness as the pulp level increased from 0 to 90 %. Joshi et al.[12] while discussing instrumental textural properties of processed cheese reported that hardness of cheese made from high protein milk was more than that made from low protein one. A typical force-time curve of instrumental texture profile is shown in Fig. 1. However the reduction in springiness, cohesiveness and resilience were found to be moderate. Bourne[13] observed increase of ripening pears that both the parameters gumminess and chewiness correlates closely with hardness. In the present study, the same relation was observed among hardness, gumminess, and chewiness. Although no correlation was observed between the fundamental objective measurement of stress strain relations and imitative tests of instrumental texture profile analysis, it may be concluded fairly that the instrumental textural properties depend on the concentration of vegetable pulp. Sanderson et al.[14] investigated the TPA parameters of gellan gum gels to evaluate the influence of gellan gum and calcium ion concentration on textural properties. Instrumental texture profile (TPA) of date flesh as a function of moisture content was reported by Rahman and Al-Farsi.[15] They observed that hardness, chewiness and resilience increased exponentially with decrease in moisture content.

Table 3 Instrumental texture profile of restructured carrot as a function of pulp concentration.

Pulp level (%)	Hardness (N)	Springiness (mm)	Cohesiveness	Gumminess (N)	Chewiness (Nmm)	Resilience
0	12.488 ± 0.288	7.130 ± 0.022	0.736 ± 0.001	9.196 ± 0.208	65.56 ± 1.27	0.448 ± 0.004
20	10.450 ± 0.109	7.104 ± 0.042	0.700 ± 0.003	7.315 ± 0.075	51.96 ± 0.54	0.411 ± 0.004
40	8.745 ± 0.112	7.045 ± 0.028	0.666 ± 0.004	5.823 ± 0.101	41.02 ± 0.68	0.371 ± 0.003
60	7.672 ± 0.224	6.968 ± 0.028	0.608 ± 0.002	4.665 ± 0.146	32.50 ± 0.97	0.310 ± 0.005
80	6.362 ± 0.194	6.514 ± 0.029	0.562 ± 0.003	3.576 ± 0.110	23.29 ± 0.67	0.259 ± 0.005
90	4.782 ± 0.194	6.056 ± 0.031	0.490 ± 0.012	2.343 ± 0.109	14.19 ± 0.71	0.219 ± 0.006
Mean ± SD (n = 5).

Figure 1 Typical Force-time Curve of Instrumental Texture Profile Analysis (TPA) of Carrot/alginate Gel.

Effect of Formulation Variables

The main effect of formulation variables i.e. alginate, calcium salt and glucono-δ-lactone concentrations on the response (hardness) has been represented in the form of three dimensional response surfaces [Fig. 2, (A, B, C)]. It revealed that the increase in the concentration of glucono-δ-lactone, alginate and calcium salt caused a simultaneous increase in hardness. Response surface for the effect of glucono-δ-lactone and alginate concentration on the texturized product at optimum calcium salt concentration (Fig. 2A) and the effect of calcium salt and glucono-δ-lactone concentration at optimum alginate concentration (Fig. 2B) showed that at low level of calcium salt, the hardness remained almost the same in spite of increase in glucono-δ-lactone. However, as the concentration of calcium salt was increased, hardness increased with glucono-δ-lactone concentration. It was also observed that at a certain glucono-δ-lactone level the increase in level of calcium salt beyond a point resulted in decreased hardness. Effect of GDL and alginate concentrations on hardness at constant calcium salt concentration (Fig. 2C) showed that as the concentration of GDL and alginate increased there was a steep rise in the hardness of the product. Similarly Fig. 2C, showed that the hardness increased steeply with the increase in concentration of alginate and calcium salt. However, at the highest level of calcium salt, hardness reached a maxima and decreased thereafter. Similar results were reported by Sime,[16] i.e., up to a certain level of calcium, the product increased in gel strength. In general Figs. 2, (A–C) showed a strong influence of all the three functional ingredients on the gel strength of the texturized carrot.

Figure 2 Response surface for the hardness of restructured carrot as a function of: (A) glucono-δ-lactone and alginate concentration at constant calcium salt concentration; (B) glucono-δ-lactone and calcium salt concentration at constant alginate concentration; (C) calcium salt and alginate concentration at constant glucono-δ-lactone concentration.

The regression coefficients of the second order polynomial model (Table 4) showed that the hardness of the carrot gel depended significantly on linear terms of concentrations of sodium alginate (X₁), calcium hydrogen phosphate (X₂), and GDL (X₃); quadratic terms of calcium salt concentration (X₂ ²) and interaction term of calcium salt and GDL (X₂X₃):

The regression model explained 95.8% of the total variation in hardness of restructured carrot.

Table 4 Regression coefficients and analyses of variance of the second order polynomial models, showing the relationships among response and independent variables.

Coefficient	Hardness Y
Intercept
Bo	6.374
Linear
B1	1.613*
B2	1.069*
B3	0.777*
Quadratic
B11	0.120
B22	−0.649*
B33	−0.173
Interaction
B12	0.383
B13	0.150
B23	0.500*
R^2	0.958
F	25.25
P≤	0.05

Thermal Stability of Restructured Alginate Carrot

The effect of thermal treatment on the texture profile of the textured product prepared with different levels of alginate has been presented in Table 5. Results showed that all the six parameters increased considerably with the increase of alginate level. However in case of heat treated sample hardness, gumminess, and chewiness increased whereas springiness, cohesiveness, and resilience decreased considerably at all the levels of alginate. Increase in hardness and, decrease in springiness and resilience showed loss of elasticity, which may be due to synersis of gel structure during heat treatment making it more dense. Slight accumulation of water in the pouch was noticed but not measured. Marcotte et al.[6] observed that the hardness of reconstituted carrot/alginate particles remained almost unchanged during heating at 100°C as well as at 120°C. They used more than 4% alginate in their experiment and since the gel strength was so high, the increase in thermal treatment might not have affected the hardness.

Table 5 Effect of alginate level and heat treatment on the texture profile of carrot pulp.

Alginatelevel	Heat treatment	Hardness (N)	Springiness (mm)	Cohesiveness	Gumminess(N)	Chewiness (Nmm)	Resilience
1.5%	UT	6.978 ± 0.062	6.760 ± 0.005	0.607 ± 0.003	4.236 ± 0.027	28.63 ± 0.17	0.315 ± 0.004
1.5%	T	8.074 ± 0.111	6.440 ± 0.005	0.570 ± 0.006	4.602 ± 0.090	29.64 ± 0.57	0.285 ± 0.004
2.0%	UT	8.795 ± 0.016	6.968 ± 0.007	0.666 ± 0.005	5.857 ± 0.043	40.81 ± 0.32	0.371 ± 0.005
2.0%	T	9.574 ± 0.014	6.872 ± 0.007	0.630 ± 0.004	6.032 ± 0.048	41.45 ± 0.36	0.341 ± 0.003
2.5%	UT	10.164 ± 0.138	7.368 ± 0.003	0.688 ± 0.005	6.993 ± 0.093	51.52 ± 0.69	0.409 ± 0.006
2.5%	T	11.664 ± 0.210	7.048 ± 0.035	0.649 ± 0.005	7.570 ± 0.149	53.35 ± 0.81	0.383 ± 0.004
Mean ± SD (n = 5); UT = without thermal treatment; T = with thermal treatment in boiling water for 25 min.

CONCLUSION

Results showed that the mechanical strength of restructured carrot gel decreased as the pulp level increased. However, the product showed an appreciable mechanical integrity at higher level of pulp (80%). Effect of ingredients used for the formulation, on the hardness of gel was found to be significant. Thermal processing resulted in a product with increased mechanical strength and less elasticity at all the levels of alginate at 1.5% GDL and 1% Ca salts.

ACKNOWLEDGMENTS

The authors thank Dr. A.S. Bawa, Director, Defence Food Research Laboratory, Mysore for his keen interest and encouragement during the research work and also for permission to publish this paper.