The Influence of Storage Time on Rheological Properties and Texture of Yoghurts with the Addition of Oat-Maltodextrin as the Fat Substitute

Abstract

Yoghurts from cow's milk containing 2 kg milk fat/100 kg or 2 kg of an oat-maltodextrin/100 kg (maltodextrin included 5 kg b-glucan/100 kg), were weighed on a laboratory scale and stored in refrigerator conditions for 21 days. Non-fat yoghurt without the addition of maltodextrin was used as a control product. The yoghurts were analysed after 1, 7, 14, and 21 days of storage. Sensory evaluation, instrumental texture profile analysis, and rheological investigations were carried out. They included the determination of the flow curves and the description by Ostwald de Waele and Casson models as well as an account of the apparent viscosity. Differences in the sensory quality of yoghurts containing milk fat or maltodextrin were not found, whereas these yoghurts were characterized by better sensory quality than the control product. The storage time had a significant influence on the sensory evaluation and the texture parameters. During the storage time, the apparent viscosity of yoghurts decreased. A decrease in consistency index value, in deviation from Newtonian flow, and yield stress was noticed, whereas in Casson's viscosity of the yoghurts, an increase was found.

Keywords:

• Maltodextrin
• Rheology
• Storage
• Texture
• Yoghurt

INTRODUCTION

One of the quality parameters of food products, which is important in the opinions of consumers, is the rheological properties of food. Knowledge of them is very important to the technologists concerned with the manufacture, the storage, designing processes, product development, and creation of quality,[1,2] Yoghurt is a dairy product of high nutritional and health value. Consumption of yoghurt can cause a reduction in cholesterol, provide anti-cancer effects, and improve antimicrobial activity and immunity in the human body. Beside the nutritional value, the rheological properties and texture influence, in a considerable way, the general sensory quality of this product. Yoghurt belongs to non-Newtonian, viscoelastic, and pseudoplastic fluids. It is an unstable rheological fluid, shear thinning, i.e., the viscosity decreases with a shear rate increase and depends on “shear history.” Yoghurt behaves as a thixotropy fluid, although, partial or non-reversed thixotropy, because the consistency of yoghurt cannot quite be rebuilt during the relaxation time, when shear forces are relented.[2 –6]

Consumers are more interested in nonfat or low-fat yoghurts; therefore, efforts are being made to find efficient possibilities to replace fat while producing the smallest change possible in other features of the product. Among the fat substitutes used for production of yoghurts are the following: modified starch, gelatine, whey proteins, pectin, or carrageen. A large group of fat substitutes include maltodextrin, products of partial depolimerization of starch of different origin including oat-starch.[7] Oat-maltodextrins, which are obtained by hydrolysis of flour or oat bran, can decrease the level of cholesterol and regulate the level of glucose in blood through the β-glucan included in them, which is the main component of soluble fraction of oats nutritive fiber. In the human alimentary tract, oat-maltodextrins stimulate secretion of bile, which absorbs cholesterol and decrease its assimilation.[8,9] Substitution of milk fat can influence the general quality of the fresh product and its behaviour during storage time. The quality includes features such as: flavor, rheological properties, and texture. The aim of this work was to examine changes of the rheological properties and texture, as well as the general sensory quality of yoghurts produced from milk containing 2 kg fat/100 kg or 2 kg oat-maltodextrin/100 kg (including 5 kg β-glucan/100 kg) during 21-days of refrigerated storage, in comparison to the properties and quality of non-fat yoghurt without maltodextrin.

MATERIAL AND METHODS

Cow's milk for yoghurt production was obtained from a farm and had met quality requirements of Polish Standard PN-86002:1999.[10] The investigations were carried out during the cow's feeding in the barn. Milk from the morning milking was cooled down and transported to a laboratory, where it was used for production of yoghurt after an initial analysis. It was centrifuged using the centrifuge machine type LWG-24E (Spomasz, Gniezno, Poland). Next, part of the milk was standardized to content of 11.5 kg of solids non-fat/100 kg milk using skim milk powder and to 2 kg of fat/100 kg milk. Oat-maltodextrin was added to standardized non-fat milk in the quantity of 2 kg/100 kg. Maltodextrin was obtained in the laboratory by enzymatic hydrolysis of ground oat grains. Then, the prepared milk was twice homogenized using homogenizer FT 9 (Armfield, UK) at a pressure of about 7 MPa, and pasteurized at 90°C for 10 minutes. After that the milk was cooled down to 44°C and inoculated with yoghurt culture YC-180, obtained from Christian. Hansen (Denmark) in a quantity of 2 kg/100 kg of batch starter culture. The carefully mixed milk was poured into containers and incubated at 44°C for 4-5 hours to a pH of 4.7. When the suitable pH was reached, the yoghurt was cooled down to 5°C and stored for 21 days.

As a control, yoghurt from non-fat milk without maltodextrin was used. After 1, 7, 14, and 21 days, the yoghurts were examined for sensory quality, for texture analysis, and for rheological investigations. The experiment was carried out in three independent replicates and results were described statistically. The two-factor ANOVA was done and differences between means were assessed with the Duncan test. The sensory evaluation of produced yoghurts was done on a 5-point scale. The following properties of quality were estimated: color, flavor, consistency, and syneresis. Each of these properties had the proper index of importance. This evaluation was done by a trained panel of 5 persons, whose sensory sensitivity had been checked.

Instrumental Texture Profile Analysis (TPA) was carried out using the 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 the penetration rate was 1 mm/s. Diagrams of the dependence of force to time obtained in the test were analyzed using the computer program Texture Expert for Windows v. 1.05 (Stable Micro Systems, UK) with algorithm Fracture TPA. The texture parameters that were assessed included hardness, adhesivness, cohesiveness, and gumminess. Hardness is defined as that force necessary for obtaining the precise deformation of the probe. Adhesivness is the work necessary for overcoming attractive energy between the area of foodstuff and other solids coming into contact with it. Cohesiveness is defined as a force of internal bonds, which keeps the product whole. Gumminess is the energy necessary for crumbling a semisolid consistency product into a state ready for swallowing.[11]

The rheological properties of yoghurts were estimated using rotary viscometer Rheotest RV2 (VEB MLW, Medingen, Germany) with controlled shear rate in, coaxial cylinder system s/s₂ in the measuring range Ia. The proportion of internal to external radius of cylinder was 0.94. The analysis included the calculating of flow curves for shear rate from 1 to 243 s⁻¹ and from 243 s⁻¹ to 1 s⁻¹. At the shear rate γ = 9 s⁻¹ at rising curve, apparent viscosity of yoghurts was calculated. The flow curves were described by Ostwald de Waele and Casson models, which include such parameters as consistency coefficient K, flow behaviour index n, yield stress τ_0, and Casson's viscosity η_c. The hysteresis loop areas were also calculated. That analysis was done using computer programme US 200 (Physica Messtechnik GmbH, Stuttgart, Germany).

RESULTS AND DISCUSSION

The mean results of sensory evaluation and texture analysis of yoghurts produced from milk containing 2 kg fat or 2 kg oat-maltodextrin/100 kg, in comparison to the control yoghurt during refrigerated storage, are presented in Table 1. Tables 2 and 3 show the results of statistical analysis of the influence of fat or maltodextrin in milk (that type of milk) and the storage time on sensory evaluation scores and texture properties of yoghurt. Generally, the yoghurts from milk containing 2 kg fat or 2 kg maltodextrin/100 kg had better sensory qualities than the control yoghurt. The highest scores in sensory evaluation were obtained with yoghurts after 7 days of storage. Longer storage time caused worse sensory quality evaluation. After 21 days, all yoghurts got similar scores in sensory evaluation. Statistical analysis shows significant influence of both used milk and storage time on the sensory evaluation. No differences were found in sensory quality between yoghurts containing milk fat and maltodextrin, which indicates a good replacement of fat by the substitute.

Table 1 Sensory evaluation scores (overall preference) and instrumental texture parameters of yoghurts produced with addition of milk fat or maltodextrin during 21 days of cooled storage (The mean values form 3 ± mean standard error).

Parameter	Day of storage	Type of yoghurt
		Control 0 kg fat/100 kg yoghurt	2 kg fat/100 kg yoghurt	2 kg maltodextrin/ 100 kg yoghurt
Sensory evaluation	1	4.39 ± 0.06	4.70 ± 0.09	4.62 ± 0.07
[scores]	7	4.51 ± 0.06	4.82 ± 0.05	4.77 ± 0.04
	14	4.29 ± 0.05	4.51 ± 0.06	4.48 ± 0.06
	21	4.12 ± 0.07	4.15 ± 0.08	4.12 ± 0.07
Hardness TPA[N]	1	0.82 ± 0.03	1.10 ± 0.00	0.87 ± 0.03
	7	1.13 ± 0.02	1.22 ± 0.00	1.03 ± 0.08
	14	1.21 ± 0.12	1.20 ± 0.06	1.09 ± 0.12
	21	1.16 ± 0.08	1.03 ± 0.11	1.03 ± 0.05
Adhesivness TPA	1	3.74 ± 0.46	5.18 ± 0.05	4.25 ± 0.26
[|N·m|] × 10^−3	7	8.09 ± 0.69	5.18 ± 0.58	4.66 ± 0.43
	14	3.89 ± 0.23	5.44 ± 0.61	5.12 ± 0.49
	21	4.63 ± 0.41	3.70 ± 0.34	5.67 ± 0.57
Cohesiveness TPA[−]	1	0.48 ± 0.02	0.47 ± 0.01	0.50 ± 0.03
	7	0.42 ± 0.02	0.45 ± 0.01	0.47 ± 0.02
	14	0.42 ± 0.01	0.40 ± 0.01	0.45 ± 0.01
	21	0.44 ± 0.02	0.44 ± 0.00	0.38 ± 0.01
Gumminess TPA[N]	1	0.40 ± 0.05	0.52 ± 0.01	0.44 ± 0.05
	7	0.47 ± 0.03	0.55 ± 0.01	0.48 ± 0.05
	14	0.50 ± 0.05	0.48 ± 0.03	0.49 ± 0.06
	21	0.50 ± 0.03	0.45 ± 0.05	0.39 ± 0.02

The type of processed milk did not have a significant influence on the texture properties of investigated yoghurts. Hardness, adhesivness, and cohesiveness of the yoghurts had changed significantly during the 21 days of storage. The hardness of yoghurts had been increasing till the 14^th day of storage and then decreased. Statistical analysis also showed the interaction of both investigated factors, type of milk and storage time, regarding the changes of adhesivness and cohesiveness. Yoghurts demonstrated the highest mean adhesivness after 7 days of storage, but longer storage time resulted in a decrease. In the group of yoghurts containing milk fat, yoghurt showed the highest adhesivness after 14 days of storage; and in yoghurts with maltodextrin, after 21 days of storage. The mean cohesiveness of investigated yoghurts decreased during storage, and changes of gummines were not statistically significant.

Table 2 The mean squares of deviation from variance analysis concerning the influence of fat or maltodextrin content in milk and storage time on sensory evaluation scores (overall preference) and instrumental texture parameters of yoghurts.

Source of variance	Degrees of freedom	Sensory evaluation	Hardness	Adhesivness	Cohesiveness	Gumminess
1. Type of milk	2	0.1569**	0.0517	0.1479	0.0004	0.0080
2. Time of storage	3	0.5390**	0.0969**	4.3923**	0.0089**	0.0065
Interaction 1 x 2	6	0.0158	0.0241	5.5400**	0.0025**	0.0063
Error	24	0.0122	0.0157	0.6421	0.0007	0.0045
** - highly significant differences between means at p £ 0.01.

Table 3 The mean of the smallest squares from variance analysis concerning the influence of fat or maltodextrin content in milk and storage time on sensory evaluation scores (overall preference) and instrumental texture parameters of yoghurts.

Parameters	Type of milk			Storage time [days]
	Control 0 kg fat/100kg yoghurt	2 kg fat/100kg yoghurt	2kgmd*/100kg yoghurt	1	7	14	21
Sensory evaluation [scores]	4.33^A,B	4.54^A	4.50^B	4.57^A,a,b	4.70^a,B,C	4.43^b,B	4.13^A,C,D
Hardness TPA [N]	1.08	1.14	1.01	0.93^A,B,a	1.13^A	1.17^B	1.07^a
Adhesivness TPA[│N·m│] x 10^−3	5.09	4.87	4.92	4.39^A	5.98^A,B,C	4.81^B	4.67^C
Cohesiveness TPA [-]	0.44	0.44	0.45	0.49^A,B,C	0.45^A,a	0.42^B	0.42^C,a
Gumminess TPA [N]	0.47	0.50	0.45	0.45	0.50	0.49	0.45
^A-C - highly significant differences between means marked with the same letters in a row (p £ 0.01), every factor separately.
^a-b - significant differences between means marked with the same letters in a row (p £ 0.05), every factor separately.
*md - maltodekstrin.

Figure 1 shows change of apparent viscosity (for shear rate γ = 9 s⁻¹) of the control yoghurt and of yoghurts with an additional 2 kg fat/100 kg or 2% maltodextrin/100 kg during 21 days cooled-storage. The apparent viscosity of all types of yoghurts decreased during storage. In relation to the control yoghurt (without fat and maltodextrin), the addition of 2 kg milk fat/100 kg caused a decrease in apparent viscosity, but the addition of 2 kg maltodextrin/100 kg caused its increase A quick decrease in viscosity during storage was demonstrated in the yoghurt with maltodextrin. After 7 days, the apparent viscosity of yoghurt with maltodextrin was lower than for the control yoghurt.

Figure 1 Changes in apparent viscosity (for (= 9 s⁻¹) of yoghurts produced with addition of 2 kg fat or maltodextrin/100 kg/yoghurt.

Figure 2 shows flow curves of examined yoghurts during 21 days of storage. All curves have the shape of a hysteresis loop. The hysteresis loop area can be interpreted as a measure of yoghurt structure breakdown during shear, and the slope of the flow curve indicates the resistance of yoghurt gel on the action of shear forces.[4,6,12] The hysteresis loop areas are presented in Table 4. In comparison to the hysteresis loop area for the control product, in fresh yoghurts containing 2 kg fat/100 kg, a decrease of the hysteresis loop area was found, while in those products with 2 kg of maltodextrin/100 kg, an increase was found. The histeresis loop area for all stored yoghurts was greater than for fresh yoghurts, with the exception of yoghurt with 2 kg of maltodextrin/100 kg after 21 days of storage. In comparison to the control yoghurt, a decrease of shear stress at maximum shear rate for yoghurts with the addition of fat and maltodextrin was observed after 21 days of storage.

Figure 2 Flow curves of yoghurts with addition of 2 kg of milk fat or maltodextrin/100 kg yoghurt after 1, 7, 14, and 21 days of storage.

Table 4 shows rheological parameters of the models applied for the description of flow curves. A general decrease of the values of the consistency coefficient K, relevant to the apparent viscosity of the product during storage of yoghurts, was observed. A higher decrease of K was determined for the control yoghurt and for the yoghurt containing maltodextrin than for yoghurt containing 2 kg fat/100 kg fat. The tendency for these changes was similar to changes of apparent viscosity of yoghurts. The value of exponent n, called the flow index, is a measure of deviation from Newtonian flow, for shear thinning fluids n < 1 and for Newtonian fluids n = 1.[6] During storage, the increase of exponent n value was stated, which testifies to the decrease in deviation from Newtonian flow of stored yoghurts. In analyzed yoghurts, the yield stress was stated, i. e., such values of shear stress below which yoghurt behaves as a solid state.[6] The values of yield stress in Casson model during the storage of all yoghurts were decreased. An increase in Casson's viscosity during storage of control yoghurts was also found. In yoghurts with the addition of 2 kg fat/100 kg or 2 kg maltodextrins/100 kg, Casson's viscosity increased after 14 days of storage, but after 21 days of storage, it decreased and was similar to the viscosity of fresh yoghurts. This experimental data shows a good fit to Ostwald de Waele model (R² in range 0.9275 – 0.9954) and a worse fit to Casson model (R² in range 0.7640 – 0.9291).

Table 4 Rheological parameters of yoghurts with addition of 2 kg milk fat or oat-maltodextrin/100 kg yoghurt.

Type of yoghurt	Time of storage [days]	Ostwald de Waele model			Casson model			Hysteresis loop area[W/m^3] x 10^3
		K[Pa.s^n]	n[-]	R^2	τ0[Pa]	ηc [mPa.s]	R^2
Control 0 kg fat/100 kg yoghurt	1	24.41	0.18	0.9892	26.25	48.01	0.9116	4.437
	7	20.37	0.22	0.9691	21.88	69.68	0.8496	5.398
	14	14.90	0.30	0.9350	15.41	117.70	0.7787	5.117
	21	14.32	0.31	0.9275	14.70	124.84	0.7640	5.262
2 kg fat/100 kg yoghurt	1	15.53	0.26	0.9911	16.50	93.62	0.9187	3.301
	7	15.60	0.26	0.9914	16.58	91.60	0.9291	5.717
	14	11.71	0.32	0.9832	11.96	130.53	0.8808	5.063
	21	12.15	0.28	0.9954	12.62	92.70	0.9261	5.131
2 kg malto-detrin/100 kg yoghurt	1	21.29	0.22	0.9562	22.90	69.22	0.8230	5.750
	7	18.22	0.26	0.9414	19.46	92.29	0.7909	6.074
	14	14.72	0.30	0.9321	15.39	121.41	0.7798	6.085
	21	14.77	0.25	0.9929	15.70	74.61	0.9164	4.538

The composition of processed milk, especially content of dry matter and protein, is one of the basic factors influencing sensory features, texture, and the rheological properties of yoghurt. The content of fat or its substitutes, stabilizers, and other carbohydrate additives are also very significant.[13 –16] According to Fernandez-Garcia et al.,[13] the addition of oat fiber causes an increase of sensory quality and apparent viscosity of yoghurts. Similar results were ascertained, by Zuraw[15] for different modified starches.

A greater hardness of yoghurt from milk containing milk fat in comparison to non-fat yoghurt obtained in this work can be explained by the influence of fat homogenization and the incorporation of homogenized fat globules covered with new membranes, composed of whey-proteins and casein submiceles into the structure of the protein matrix of yoghurt gel.[14,17] The investigations of sensory quality changes, rheological properties, texture of natural yoghurts, and yoghurts with different carbohydrate and protein additives as fat substitutes during storage have been carried out by Barrantes et al.,[18] Dankow et al.,[19] Grega et al.,[20] and Zuraw.[15] Protein fat substitutes in yoghurt milk used by Barrantes et al.[18] caused a decrease in yoghurt hardness in comparison to the product containing anhydrous milk fat. This was caused by the substitute molecules having a bigger diameter than the fat globules in milk. The authors did not ascertain the significant increase of yoghurt viscosity with substitutes after 14 days of storage. Dankow et al.[19] examined the influence of storage time of natural yoghurts from goat's milk containing 3.6 kg fat/100 kg for their sensory quality and viscosity. Yoghurts in the 7th day of storage obtained the highest scores in sensory evaluation. Similarly as in this work yoghurts, viscosity decreased after 15 days of storage.

Grega et al.[20] investigated the sensory quality and texture of natural yoghurts from cow's milk and yoghurts with the addition of complete and ground amaranthus seeds during 14 days of cooled storage. The sensory quality of yoghurts decreased after 14 days of storage, while the highest scores were obtained, as in this work, by yoghurts after 7 days storage. The authors determined the increase in hardness during storage, whereas the values of other texture parameters were submitted to only small fluctuations. The values of such texture parameters as hardness, adhesivness, and gumminess were lower than those obtained in this work. While examining the influence of different modified starches on rheological properties and sensory features of yoghurt, Zuraw[15] determined a considerable decrease in sensory quality both for the control yoghurt and for modified yoghurts after 21 days of storage. Yoghurts after 3 days of storage got the highest score. Viscosity of the control yoghurt and most yoghurts containing modified starch decreased after 21 days of storage.

According to Benezech and Maingonat,[1] the Ostwald de Waele, Casson, and Herschel-Bulkley models are most often used to describe the flow curves of yoghurts. These models were used in reological investigations of yoghurt and concentrated yoghurt (labneh) by Rohm,[6] Fortuna et al.,[21] Jumah et al.,[22] Aby-Jdayil et al.[23] and Dello Staffolo et al.[24] Rohm[6] and Jaros et al.,[12] in rheological investigations of yoghurts from cow's milk, have obtained flow curves similar in shape to curves obtained in this work. Fortuna et al.[21] used the Casson and Ostwald de Waele models for the description of flow curves and to assign values of rheological parameters for commercial yoghurts. The values of the consistency coefficient K obtained by them using the Ostwald model were significantly lower than those obtained in this work, but comparable values were obtained in the case of exponent n and Casson's model i.e., the yield stress and Casson's viscosity. Abu-Jdayil et al.[23] have found the Ostwald model to be the most appropriate for the rheological investigations of labneh. Dello Staffolo et al.,[24] in rheological and sensory investigations of yoghurt with the addition of different origin alimentary fiber, have found that both storage time and type of fiber have a significant influence on the viscosity and values of rheological parameters of yoghurts. The apparent viscosity of all yoghurts decreased after 21 days of storage, which is consistent with the results of this work.

cONCLUSIONS

The yoghurts containing milk fat or maltodextrin have shown to have better sensory qualities than the control yoghurt. No differences in sensory quality were found between yoghurts with fat or maltodextrin. The storage time of yoghurt significantly influences the results of sensory quality and the values of most texture yoghurt parameters. A decrease in the apparent viscosity of yoghurts during storage was ascertained. A higher decrease of viscosity was found in yoghurts containing maltodextrin than in those with milk fat. During storage of yoghurts, the value of the consistency coefficient and yield stress decreased, as did the deviation from Newtonian flow, while Casson's viscosity increased.

Notes

10. Polish Standard PN-86002:1999. Mleko surowe do skupu. (Raw milk for collection). (In Polish).

14. Tamime, A.Y; Robinson, R.K. Yoghurt - Science and Technology. Woodhead Publishing Limited: Cambridge, England, 1999.