Rheological Properties and Texture of Yoghurts When Oat-Maltodextrin is Used as a Fat Substitute

Yoghurts were produced from cow’s milk containing 1, 2 and 3 kg milk fat/100 kg or 1, 2 and 3kg of oat-maltodextrin/100 kg (maltodextrin included 5 kg β-glucan/100 kg). Non-fat yoghurt, without the addition of maltodextrin, was used as a control product. The yoghurts were estimated for sensory properties and using instrumental texture profile analysis (TPA) and rheological investigations, which had to set flow curves and their description by the Ostwald de Waele and Casson models; counting apparent viscosity were also done. Replacing milk fat with maltodextrin in yoghurts does not show significant differences in the sensory analysis, rheological properties and most of the texture parameters. The addition of milk fat in the amount of 1 kg/100 kg or maltodextrin in 2 kg/100 kg caused an increase in apparent viscosity of yoghurts. The addition of oat-maltodextrin caused a decrease in consistency index value and a deviation from the Newtonian flow and yield stress, similar to the effects caused by adding milk fat. No relationship between the results of the sensory evaluation, the instrumental texture parameters and rheological properties, especially in yoghurts containing milk fat, were shown

Keywords:

• Maltodextrin
• Rheology
• Texture
• Yoghurt

INTRODUCTION

Consumers’ interest in non-fat or low-fat dairy products has increased recently. The texture and rheological properties of these products are different from normal fat-level products. In order to improve the rheological properties of these products, different techniques were applied to modify the milk composition used for production. There are attempts to replace milk fat by substitutes such as: modified starch, gelatin, whey proteins, pectin or carrageen. Maltodextrins are also a large group of fat substitutes. Maltodextrins are products of partial starch depolimerization. Maltodextrins characterized by a low degree of starch depolimerization are used as the fat substitutes; they are powdered, white, water soluble products of low bulk density. Their solutions can absorb fat well, making stable complexes as fat-carbohydrate-water. They act as a binding, stabilizing, filling, bulging and dispersing factors. The material used to make these substitutes are starches of different origins: corn, potato, tapioca and oat.[1]

Oat-maltodextrins are obtained by the hydrolysis of flour or oat-bran; they can decrease the level of cholesterol in blood through β-glucan, which is the main component of soluble fraction of the nutritive oat fiber. β-glucan stimulates the secretion of bile into the alimentary canal and absorbs cholesterol, which causes a decrease of its assimilation in the alimentary canal. β-glucans, together with penthosans, create a slimy layer in the stomach and intestine, which delays hydrolysis of starch and the absorbing of glucose, thus can regulate its level in blood.[1,2,3] Technologists are interested in oats and its derivatives. They have tried to develop probiotic fermented dairy products4] and non-dairy fermented products (similar to yoghurt) with the addition of oats and its derivatives.[5]

Yoghurt is a dairy product of high nutritional and health value. Reduction of cholesterol content, anti-cancer effects, and improvement of antimicrobial activity and immunity in the human body are the most important benefits of yoghurt consumption.[6] Rheological properties and texture, besides nutritional value also add to the general sensory quality of this product in a considerable way. Replacing milk fat with maltodextrin in dairy products (for example in yoghurt) can influence the general quality of the product, especially its flavor, rheological properties and texture. The aim of this article was to investigate the changes of rheological properties and the general sensory quality of yoghurts produced with different addition of milk fat or oat-maltodextrin (containing 5kg β-glucan/100 kg) as a fat substitute.

MATERIAL AND METHODS

Cow’s milk for yoghurt production was obtained from the farm and met quality requirements of the Polish Standard PN-86002:1999.[7] The investigations were carried out while cows were feeding in the barn. Milk from the morning milking was cooled down and transported to the laboratory; after initial analysis, it was used for yoghurt production. Milk was centrifuged using the LWG 24E (Spomasz, Gniezno, Poland) and standardized to 11.5 kg of solid non-fat/100 kg milk using skim milk powder. Next the part of milk was standardized to 1, 2 and 3 kg fat/100 kg milk. Oat-maltodextrin was added to non-fat standardized milk in quantities of 1, 2 and 3 kg/100 kg milk. Maltodextrin was obtained in the laboratory by the enzymatic hydrolysis of ground oat-grains. Thus, prepared milk was twice homogenized using homogeniser FT 9 (Armfield, UK) at pressure about 7 MPa, and next was pasteurized at 90°C for 10 minutes. After that, milk was cooled down to 44°C and inoculated with yoghurt culture YC-180 (Chr. Hansen, Denmark) in quantity 2 kg of batch starter/100 kg milk. The thoroughly mixed milk was poured into containers and incubated at 44°C for 4 to 5 hours, to obtain a pH of 4.7. When the suitable pH was reached, the yoghurt was cooled down to 5°C and stored until the next day (about 15 hours). Yoghurt from non-fat milk, without maltodextrins, was used as a control test. The following day, yoghurts were examinated for sensory evaluation, instrumental texture analysis and rheological investigations. The experiment was carried out in three independent replicates and results were described statistically. One factor ANOVA was done, and the differences between averages were assessed with the Duncan test.

Sensory Evaluation

The sensory evaluation of produced yoghurts was done on a 5 point scale (1—the worst; 5—the best). The following quality properties were evaluated: color, flavor, consistency and syneresis. Each of properties had the proper index of importance e.i. color—0,1; flavor—0,5; consistency—0,25; and syneresis—0,15. From the study of the parameters, the overall preference was calculated. For the goat’s milk yoghurt, the model sensory properties should be as follows:

• Color—typical, characteristic, intensive white,

• Taste—sour, characteristic, yoghurt-like,

• Smell—characteristic and intensive,

• Consistency—uniform and compact, creamy not lumpy without syneresis.[8]

Samples of yoghurt for sensory evaluation were presented in glass vessels of a volume of 200 cm³ coded, in the temperature of about 10°C (about half an hour after being taken out of refrigerator). The judges first evaluated the overall look, color, smell and syneresis in the undisturbed gel. Next, with a spoon they gently pressed on the gel in order to assess on the hardness and springiness. Then they mixed the gel with the spoon until they obtained a uniform consistency. After mixing, an oral evaluation was made, testing the taste and mouth feel. The consistency of the product was also evaluated by placing a spoon vertically into the product. Smoothness of the product was observed on the other side of the spoon. The evaluation was completed by a trained panel consisting of 5 persons, whose sensory sensibilities were evaluated. The judges were tested for taste and smell daltonizm and minimum point of taste and smell sensibility. They were instructed about the process of evaluating the different parameters of sensory quality.

Instrumental

Texture Profile Analysis (TPA) was carried out using universal Texture Analyser TA-XT2 (Stable Micro Systems, UK) controlled by a PC computer. The penetrometric test was done using plastic cylinder type SMS P/20 of 20 mm diameter. The depth of penetration was 25 mm, and penetration rate 1 mm/s. Diagrams of force dependence on time, obtained in the test were analysed using the computer program Texture Expert for Windows v. 1.05 (Stable Micro Systems, UK), using Fracture TPA algorithm, which allowed the authors to assign the following texture parameters: hardness, adhesiveness, cohesiveness and gumminess. Hardness is defined as force, which is necessary for obtaining the precise deformation of the probe. Adhesiveness is work necessary for overcoming the force of attraction between the area of foodstuff and other solids coming into contact with them. Cohesiveness is defined as forces of internal bonds, which keep the product as a whole. Gumminess is the energy necessary for crumbling semi-solid consistency product into a state ready for swallowing.[9,10]

The rheological properties of yoghurts were investigated using rotary viscometer Rheotest RV2 (VEB MLW, Medingen, Germany), with controlled the shear rate in coaxial cylinder system s/s₂ in measuring range Ia. The proportion of internal to external radius cylinder was 0.94. The analysis included calculating flow curves for shear rate from 1 to 243 s⁻¹ and from 243 to 1 s⁻¹. At the shear rate γ = 9 s⁻¹ at rising curve, apparent viscosity of yoghurts was counted. The flow curves were described by Ostwald de Waele and Casson models, calculating such parameters as: consistency coefficient K, flow behavior index n, yield stress τ₀, Casson’s viscosity η_c and hysteresis loop area. The analysis was done using computer program US 200 (Physica Messtechnik GmbH, Stuttgart, Germany).

RESULTS AND DISCUSSION

The mean results of sensory evaluation and texture analysis of produced yoghurts are presented in Table 1. The control yoghurts (without fat and maltodextrin) and the yoghurts containing 1 kg fat/100 kg have the lowest scores, whereas yoghurts containing 2 kg fat and 2 kg or 3 kg maltodextrin/100 kg yoghurt have the highest scores. The differences in sensory quality between these yoghurts were significant. The addition level of fat or maltodextrin was significant, too. Differences in sensory quality of yoghurts on the same addition level of fat or maltodextrin were not stated, which can prove that fat is well replaced by maltodextrin. Only with the addition of 3 kg/100 kg, was the yoghurt with maltodextrin (not fat) evaluated higher.

Table 1 Influence of fat or maltodextrin level on the results of the sensory evaluation (overall preference) and instrumental texture parameters of yoghurts from cow’s milk. (The mean values from 3 series ± mean standard error)

	Type of yoghurt
Parameter	Control yoghurt	1 kg fat/100 kgyoghurt	2 kg fat/100 kg yoghurt	3 kg fat/100 kg yoghurt	1 kg md∗/100 kgyoghurt	2 kg md/100 kg yoghurt	3 kg md/100 kgyoghurt
Sensory evaluation [scores]	4.39 ^A, ^a, ^B ± 0.06	4.38 ^C, ^e, ^D ± 0.06	4.70 ^A, ^b, ^C, ^c ± 0.09	4.48 ^b, ^d ± 0.05	4.42 ^c, ^f ± 0.07	4.62 ^a, ^e ± 0.07	4.72 ^B, ^d, ^D, ^f ± 0.06
Hardness TPA [N]	0.82 ^A, ^B, ^C, ^a, ^b ± 0.03	1.10 ^A, ^E ± 0.03	1.10 ^B, ^D ± 0.00	1.05 ^C, ^c ± 0.09	1.00 ^a ± 0.06	0.87 ^D, ^c, ^E ± 0.03	1.00 ^b ± 0.01
Adhesiveness TPA [│N·m│] × 10^−3	3.74 ^A, ^a ± 0.47	5.13 ^c ± 0.39	5.18 ^b ± 0.05	7.03 ^A, ^b, ^c, ^d, ^B, ^C ± 0.93	5.47 ^a, ^d ± 0.66	4.25 ^B ± 0.28	4.67 ^C ± 0.27
Cohesiveness TPA [−]	0.48 ± 0.02	0.49 ± 0.01	0.47 ± 0.01	0.45 ± 0.07	0.47 ± 0.02	0.50 ± 0.03	0.51 ± 0.01
Gumminess TPA [N]	0.40 ± 0.05	0.54 ± 0.01	0.52 ± 0.01	0.47 ± 0.10	0.47 ± 0.08	0.44 ± 0.03	0.50 ± 0.01
^A-E – highly significant differences at p ≤ 0.01 marked with the same letters in rows;
^a-f – significant differences at p ≤ 0.05 marked with the same letter in rows;
∗md – maltodextrin

Texture means physical properties of the product that come from its structural elements, which can be perceptible by human senses. The main texture parameters of yoghurt are: hardness, adhesivness, cohesiveness and gumminess. The addition of milk fat and replacing it with maltodextrin caused a general increase of hardness and adhesiveness of products in comparison to the control yoghurt. When maltodextrin was used, the increase in these parameters (hardness and adhesiveness) was smaller than in yoghurt with milk fat. Yoghurt with the addition of 2 kg maltodextrin/100 kg yoghurt was the only exception, as here, no essential increase of hardness was stated. There were no essential differences in hardness between yoghurts containing different fat contents and also different maltodextrin contents in milk. The yoghurt containing 3 kg fat/100 kg and yoghurt with an addition of 1 kg maltodextrin/100 kg was characterized as the highest, whereas the control yoghurt, the least high adhesiveness. The cohesiveness and gumminess of the control yoghurt and yoghurts containing fat or maltodextrin did not differ significantly. In the case of the addition of 2 kg fat/100 kg yoghurt or 3 kg maltodextrin/100 kg yoghurt the increase of instrumental gel hardness caused higher scores in the sesnsory evaluation. Yoghurt containing 1 kg fat/100 kg, although having comparable instrumental hardness with the yoghurt containing 2 kg fat/100 kg, did not get a significantly higher score in sensory evaluation. Similarly yoghurt with an addition of 1 kg maltodextrin/100 kg, although having a comparable instrumental hardness with the yoghurt containing 3 kg maltodextrin/100 kg, did not show a significantly higher sensory quality. However, in the case of the addition of 2 kg maltodextrin/100 kg yoghurt, despite the lack of instrumental hardness increase, the sensory quality significantly rose. This may mean that between the instrumental hardness and the sensory features of this product there is little or no relation.

Changes of apparent viscosity (at shear rate γ = 9 s⁻¹) of produced yoghurts are presented in Fig. 1. The yoghurt with an addition of 2 kg maltodextrin/100 kg yoghurt was characterized by the highest apparent viscosity while that with an addition of 3 kg fat/100 kg yoghurt by the least viscosity. In comparison to yoghurts made without the the addition of fat and maltodextrin, only yoghurts with an addition of 1 kg fat/100 kg yoghurt and 2 kg maltodextrin/100 kg yoghurt caused an increase in apparent viscosity. In Fig. 2, flow curves of yoghurts produced from milk containing different fat contents (without maltodextrin) are presented, and in Fig. 3, flow curves of yoghurts produced from non-fat milk with different additions of maltodextrin are presented.

Figure 1 Apparent viscosity (at γ = 9 s⁻¹) of yoghurts produced with different content of fat or maltodextrin (md – maltodextrin).

Figure 2 Flow curves of yoghurts produced from milk containing different fat contents.

Figure 3 Flow curves of yoghurts produced from non-fat milk with different additions of oat-maltodextrin.

In both figures (2 and 3), flow curves have the shape of a hysteresis loop. The hysteresis loop area is proportional to energy necessary for destroying the thixotropy structure of yoghurt, and the slant of flow curve may refer to resistance of yoghurt gel to operating shear forces.[11,12] The comparison of the hysteresis loop area shows (Table 2), that the addition of fat caused a decrease, whereas the addition of maltodextrin caused an increase of hysteresis loop area in comparison to hysteresis loop area of control yoghurt (without fat and maltodextrin). This may mean that the addition of fat causes a decrease of the amount of energy needed to destroy yoghurt structure, and lower the degree of shear thinning. The addition of maltodextrin causes the opposite. However the general resistance of yoghurt gel to shear force was not changed as the average slant of flow curve in both cases was similar. At small shear rates on rising curve of yoghurts with the addition of 2 kg and 3 kg fat/100 kg yoghurt, a slower increase of shear stress, together with the increase of shear rate was shown, than for yoghurts without fat and with 1 kg fat/100 kg yoghurt.

Table 2 Rheological parameters of yoghurts produced with different addition of milk fat or maltodextrins

Type of yoghurt	Ostwald de Waele model			Casson model
K [Pa·s^n]	n [−]	R^2	τ0 [Pa]	ηc [mPa·s]	R^2	Hysteresis loop area [W/m^3] × 10^3
Control	24.41	0.18	0.9892	26.25	48.01	0.9116	4.437
1 kg fat/100 kg yoghurt	25.93	0.17	0.9906	27.92	42.94	0.9105	4.396
2 kg fat/100 kg yoghurt	15.53	0.26	0.9911	16.50	93.62	0.9187	3.301
3 kg fat/100 kg yoghurt	14.97	0.28	0.9851	15.84	102.14	0.9005	4.103
1 kg maltodextrin/100 kg yoghurt	15.77	0.28	0.9516	16.66	107.67	0.8130	5.270
2 kg maltodextrin/100 kg yoghurt	21.29	0.22	0.9562	22.90	69.22	0.8230	5.750
3 kg maltodextrin/100 kg yoghurt	19.67	0.23	0.9740	21.21	74.87	0.8615	5.542

In Table 2, rheological parameters from Ostwald de Waele i Casson models are presented, which were used for description of yoghurt’s flow curves. The value of the consistency coefficient K concerned in viscosity of yoghurt, after the fat and maltodextrin addition decreased generally in comparison to the control yoghurt. Only an addition of 1 kg fat/100 kg yoghurt caused an increase of consistency coefficient K, but the addition of 2 kg maltodextrin/100 kg yoghurt caused only a small decrease of it. These tendencies were similar to changes of apparent viscosity of examined yoghurts. The value of exponent n is a measure of deviation from Newtonian flow. For shear thinning fluids n < 1, while for Newtonian fluids n = 1.[11,13] The value of exponent n in the Ostwald model after the addition of both fat and maltodextrin increased, which proves that these yoghurts have shown less deviation from Newtonian flow in comparison to the control yoghurt. Only yoghurt with a content of 1 kg fat/100 kg yoghurt was characterized by a bit smaller value of exponent than control yoghurt.

In all examined yoghurts, yield stress was stated, i.e. such value of shear stress, below which yoghurt behaves as a solid state.[11] The addition of fat and maltodextrin caused a decrease of yield stress in Casson’s model in comparison to yield stress of control yoghurt. Only an addition of 1 kg fat/100 kg yoghurt caused little increase of yield stress value. Casson’s viscosity decreased at addition of 1 kg fat/100 kg yoghurt, but in all other yoghurts, an increase of Casson’s viscosity was stated. The yoghurts with the addition of 3 kg fat and 1 kg maltodextrin/100 kg yoghurt were characterized by the highest Casson’s viscosity. The data obtained from rheological measurements has shown a good fit to the Ostwald de Waele model (R² in range 0.9516–0.9911) and somewhat worse compared to the Casson model (R² in range 0.8130–0.9187).

Both apparent viscosity, as well as calculated rheological parameters, better reflect the results of the sensory evaluation scores of yoghurt with maltodextrin than those with the addition of fat. Yoghurt containing 2 kg maltodextrin/100 kg, characterized by high sensory quality, showed the highest apparent viscosity, a high value of consistency coefficient and yield stress. However, Casson’s viscosity for this yoghurt was lower than for yoghurts containing 1 kg maltodextrin/100 kg and 3 kg maltodextrin/100 kg. In the case of yoghurts with milk fat, opposite tendencies were observed. Yoghurt, which showed the highest sensory quality was characterized by a smaller apparent viscosity, lower consistency coefficient and yield stress than yoghurts of worse sensory quality. Moreover Casson’s viscosity for this yoghurt was higher than for control yoghurt and yoghurt containing 2 kg maltodextrin/100 kg and 3 kg maltodextrin/100 kg, which were characterized by high sensory quality. This may account for the weak relation between sensory features, especially in yoghurts containing milk fat, with instrumental rheological parameters. Additionally, it also accounts for the fact, that in the sensory evaluation of these yoghurts different properties of quality, like taste and smell, were more influential than rheolgical features.

The fat content in milk for yoghurt production has a significant influence on sensory quality and texture of yoghurts.[14,15] When the milk for yoghurt, containing fat, is homogenized, small fat globules, are covered with new membranes, composed of whey proteins and casein submiceles. They react with milk proteins during fermentation and are incorporated in the structure of protein matrix of gel and can increase apparent viscosity in yoghurt and hardness of gel.[14,16,17] It can explain the higher hardness of yoghurts containing fat in comparison to non-fat yoghurt. However, at higher fat content and less effective homogenization, a decrease in viscosity can occur. This effect was obtained by Schkoda et al.,17] which added cream to yoghurt after fermentation.

Different carbohydrate additions such as oat-fiber[18] and other origins,[19] amaranthus seeds,[20] modified starch,[21] also whey protein concentrates (WPC)[22] were applied to the modification of texture and rheological properties in plain yoghurt, non-fat yoghurt or low-fat yoghurt. The use of maltodextrins together with modified starch in production of non-fat yoghurt can give a product a smooth, cream texture and positive mouth feel.[23] Fernandez-Garcia et al.[18] have found that the addition of oat-fiber caused an increase of apparent yoghurt viscosity and an increase in their sensory quality, too. The use of different modified starch caused an increase of sensory quality of fresh yoghurts in comparison to the control yoghurt.[21] Grega et al.[20] have investigated the texture of natural yoghurt and yoghurt with addition of total and ground amaranthus seeds. They have not shown the influence of an amaranthus addition to the sensory quality of fresh products. In investigations on texture profile, they have obtained significantly less values of parameters, both for the control yoghurt and for modified yoghurts than obtained in this article. Yoghurts displayed a significant increase in hardness, adhesiveness and gumminess when ground seeds of amaranthus were added.

Significantly higher values of texture parameters in comparison to those obtained in this work, have been obtained by Antunes et al.[22] in investigations of yoghurt from full-fat and non-fat milk with different content of whey proteins. The hardness of yoghurt from full-fat milk investigated by them was higher than non-fat yoghurt and together with the increase of whey proteins content in milk used for yoghurt, the hardness of yoghurt increased too. The yoghurt from non-fat milk with normal whey proteins content was characterized by the highest sensory quality. Sandoval-Castilla et al.,[23] in investigations on texture yoghurts, produced with different fat substitutes have stated that yoghurt with decreased fat content was characterized by less hardness and adhesiveness, but higher cohesiveness than yoghurt from full-fat milk. Usage of protein fat substitutes generally caused a production of non-fat yoghurt texture similar to the texture of yoghurt from full-fat milk, whereas carbohydrate fat substitutes caused an increase in yoghurt hardness and had no influence on cohesiveness and adhesiveness.

According to Benezech and Maingonnat,[24] the Ostwald de Waele and Casson models, along with the Herschel-Bulkley model, are the models most often used to describe the flow curves of yoghurts. These models were used in rheological investigations of yoghurt and concentrated yoghurt (labneh) among others by Rohm,[13] Fortuna et al.,[25] Jumah et al.,[26] Abu-Jdayil et al.[27] and Dello Staffolo et al.[19] Rohm[13] and Jaros et al.[12] researching rheological properties of yoghurts from cow’s milk, have obtained flow curves in a shape similar to curves obtained in this work. Fortuna et al.[25] in rheological investigations of commercial yoghurts have used among others, Casson and Ostwald de Waele models. The values of consistency coefficient K in Ostwald model, obtained by them were significantly lower than those obtained in this work, whereas those in case of exponent n and parameters of Casson’s model i.e. yield stress and Casson’s viscosity were comparable. Among eight commercial yoghurts evaluated by Fortuna et al.[25] half did not get 4 points in a 5-point scale. Abu-Jdayil et al.[27] have found the Ostwald model to bee the most appropriate for the rheological investigations of labneh. Keogh and O’Kennedy[16] have stated in their investigations that the addition of hydrocoloids into milk used for yoghurt, and also adding of milk fat and homogenization of the milk, cause an increase in the consistency of the coefficient K value and decrease of flow behavior index n. In this article, similar changes have been observed, only with an addition of 1 kg fat/100 kg yoghurt. Dello Staffolo et al.[19] examined the sensory quality and rheological properties of yoghurt with the addition of different origin alimentary fibers. In rheological investigations, they demonstrated an increase in the consistency of coefficient K and a decrease in the flow behavior index n in comparison to control yoghurt only for fiber from apples. In the case of other fibers, opposite changes have been noticed. In the sensory evaluation, significant differences in the quality of control yoghurt and yoghurts with additions of fibers have not been shown. The apparent viscosity of all investigated yoghurts was higher than the viscosity of yoghurts obtained in this article.

CONCLUSIONS

Oat-maltodextrin replaces fat in yoghurts effectively, not causing essential differences in sensory quality, rheological and most texture parameters. In the texture of yoghurts containing 1 kg and 3 kg fat or maltodextrin/100 kg, yoghurt significant changes have not been observed. Only with the addition of 2 kg/100 kg the yoghurts differed in hardness. In comparison to the control yoghurt, only the addition of 1 kg fat or 2 kg maltodextrin/100 kg yoghurt caused an increase in apparent viscosity. Oat-maltodextrin, similar to fat, caused a decrease in the value of the consistency coefficient, in deviation from the Newtonian flow and in yield stress, and an increase in Casson’s viscosity in most investigated yoghurts. No significant relationships between the results of the sensory evaluation and the instrumental texture and rheological parameters, especially in yoghurts containing milk fat, were found.

Notes

7. Polish Standard PN–86002:1999. Mleko surowe do skupu. [Raw milk for collection.] [In Polish.]

8. Polish Standard PN–86061:1983. Napoje mleczne fermentowane. [Fermented milks.] [In Polish.]

11. Schramm, G. Reologia. Podstawy i zastosowania. [Rheology. Fundamentals and Practice.] Osrodek Wydawnictw Naukowych PAN: Poznań, Poland, 1998, 17–76. [In Polish.]