Rheological Behavior of Powdered Baby Foods

Abstract

The rheological properties of four types of powdered baby foods have been investigated. Density and apparent viscosity have been measured at different temperatures. The values of the both properties decrease with the temperature. The experimental data belonging to apparent viscosity are represented by a power law. The relationship between the power law parameters and temperature has been analyzed.

Keywords:

• Powered baby foods
• Density
• Apparent viscosity
• Power law
• Hershel-Bulkey

Introduction

The knowledge of the rheological properties is important for design and evaluation of the process, process control,1–3 consumer acceptability of a product,[4] and the optimization of process variables. The physical properties of raw materials and transformed substrates have a great importance, as by intrinsic factors of quality as by the form in which such properties conditionate the manipulations and manufacturing or domestic transformations that the raw materials experience until becoming products ready for the consumption, without forgetting fashion, culture, and their organoleptic characteristics.[5] Food is the base or the physical vehicle of the essential nutritional principles in life, and at the same time it is a robust element of entailment for determined life forms.

A great amount of powdered foods are mixtures of basic ingredients, nutrients and agents which provide flavor and other compounds with specific functionality. The aim of powdered food preparation is to increase its life utility, making agile its handling and storage and facilitating the preparation for its consumption.[6] The analysis and the quantification of the rheological behavior of foods and the investigation of the chemical and structural causes determining them have a great interest in the food science and it is important to establish relationships between structure and flow, and to correlate physical parameters.[7] [8] In addition, the importance of rheological knowledge is also economic and commercial. The effects of transport and manipulation upon the physical integrity of fresh or cooked foods, their behavior during the processes of elaboration and texture quality of the product depend on their answer when external forces are applied,[9] but also on shear rate and time of shearing.[10] Almost all food is heat-treated before being eaten. During the heating process a lot of different physical, chemical, and structural changes take place,[11] for example diffusion of heat and water, gelatinization of starch, denaturalization of proteins, formation of aroma compounds, and so on.

Drying is currently used in the food industry to dry heavy pastes and thick liquids, as cooked starch, baby food, mashed potatoes, concentrated liquids, caseinates, malt-dextrins, yeast creams, fruit pulps, etc. The obtained dried product is porous, easy to rehydrate and ready to use.[12]

Baby foods are elaborated with some different products, such as flours with or without gluten, honey, fruits, etc. They are enriched with iron, calcium, and vitamins. The advantages of these foods are great nutritional quality, good digestibility, fast preparation without lumps, stable consistency, and excellent flavor.

Various studies have paid attention at the application of honey as a food ingredient, acting as a sweetener, flavor, humectant, and source of reducing sugars.[13] [14] These functional properties can be attributed to the high concentration of sugars present in honey; however, the minor constituents can also play a role, for example, as an antimicrobial,[15] antioxidant,[16] antibrowning,[17] or antistaling agent.[18] Another studies concluded that a gradual loss of gluten in the foods provokes a reduction of viscoelastic properties of flour.[19]

In this work, the rheological behavior of four types of powdered baby foods with and without gluten or honey was investigated. Besides the effect of temperature on the apparent viscosity, density was measured. The results are tested via different rheological models.

Materials and Methods

The employed baby foods were: cereals flour with honey, flour with cocoa, flour without gluten, and rice flour. These were obtained from a local supermarket. All of these were prepared by dissolving 25 g of the solid food in 150 mL of warm deionized water. The ingredients of cereals flours with honey were: flours 91 wt% (wheat, hydrolyzed wheat, maize, barley, rye, miller, rice, sorghum, and oats), sugar, honey, (7 wt%), calcium and iron mineral salts, vitamins and flavor. The ingredients of flour with cocoa were: dextrinated cereal flours 67 wt% (wheat, rice, oats, barley, rye), cocoa 19 wt% (sugar, powder cocoa, kolamalteated cereal cream: flour of wheat, extract of malt, natural flavor: extract of nut of tail, calcium phosphate, flavors, salt) sugar, fructoligosacarids 3 wt%, mineral salt and vitamins. The ingredients of flour without gluten were: flours of dextrinated cereals 81 wt% (rice, maize) dextrinose, sugar, fructoligosacarids 3 wt%, mineral salts (calcium phosphate, ferrous sulphate), vitamins and flavor. The ingredients of rice flour were: flours 95 wt% (of rice and hydrolyzed rice), sugar, mineral salts of calcium and iron, vitamins and flavor. It does not contain gluten. The amount of vitamins and minerals of all the foods are shown in Table 1.

Table 1 Nutritional information per 100 g of food

Cereals with honey	With cocoa	Without gluten	Rice
Proteins (g)	8.2	8.2	6.4	6.3
Hydrates (g)	84.0	80.5	83.9	85.5
Sugars	36.0	32.0	24.0	24.0
Fats (g)	1.2	2.0	1.4	0.6
Saturated	0.2	0.2	0.2	0.2
Fiber (g)	2.7	5.0	5.7	2.8
Na (g)	0.006	0.045	0.02	0.005
Ca (mg)	175	500	489	100
Fe (mg)	14.5	2.5	7.0	14.5
Cu (µg)	190	—	—	250
Vitamin A (µg)	210	450	450	210
Vitamin B1 (mg)	—	0.5	0.5	—
Vitamin B2 (mg)	—	0.6	0.6	—
Vitamin B6 (mg)	0.2	0.8	0.8	0.2
Vitamin B12 (µg)	—	2	2	—
Vitamin C (mg)	65	50	50	65
Vitamin D (µg)	6.3	7.5	7.5	6.3
Vitamin E (mg)	5.0	4.4	4.4	5.0
Vitamin K (µg)	—	40	40	—
Thiamin (mg)	0.5	—	—	0.5
Niacin (mg)	8	—	—	8
Folic acid (µg)	19	40	40	19
Biotin (µg)	37	15	15	37
Calcium pantothenate (mg)	—	2.8	2.8	—
Nicotinamid (µg)	—	40	40	—

Physical Properties Measurement

The density was determined using 25 cm³ pycnometer of the Gay-Lussac type from 25 to 40°C. The pycnometer containing the solution were thermostated to maintain a constant temperature with a precision of ± 0.05°C. They were weighed with the same balance. Each density value was the average of at least five measurements.

The rotational, concentric cylinder viscosimeter (Viscotester VT550, Germany) was used in this study. It is Searle type, in which the outer cylinder is fixed and the inner cylinder rotates. This apparatus measures the shear rate and the apparent viscosity of a fluid at a certain temperature. The apparent viscosity, η, is defined as the ratio of shear stress, τ, to that of shear rate, γ, (η = τ/γ), which was varied between 20 and 500 s⁻¹. The SV1 sensor system was used (bob length = 61.4 mm, bob diameter = 20.2 mm, gap width = 1.45 mm). This system is usually used for apparent viscosity measurements of high viscosity liquids and pastes such as greases, creams, ointments, plastisols, etc., working in the low medium shear rate range. The amount of sample to be used must be adjusted in order for the top surface or the inner cylinder to be just covered. The temperature of this system was kept constant by means of a thermal liquid circulator and it is varied from 25 to 40°C.

Results and Discussions

Density

In this work the density, ρ, of the different substances was measured at different temperatures. The food with higher value is the flour without gluten and the one with lower value is the cereal flour with honey (Fig. 1). Density decreases lineally with the temperature for all foods.

where ρ is the density in kg m⁻³ and T the temperature in °C. The fitting parameters, A ₀ and A ₁ are displayed in Table 2.

Table 2 Relationship between density and temperature

Flours	A 0 (kg m^−3)	A 1×10^+ 4 (kg m^−3 °C^−1)	R ^2
Cereals with honey	1073.7	−6.484	0.9992
With cocoa	1087.0	−7.344	0.9992
Without gluten	1101.1	−7.760	0.9991
Rice	1104.4	−10.140	0.9992

Figure 1 Experimental density data of the tested baby foods at different temperatures: cereals with honey (○), with cocoa (•), without gluten (▪), rice (□).

Apparent Viscosity

The apparent viscosity, η, of the different kind of baby foods was tested at various shear rates at different temperatures. From this, plots of shear stress vs. shear rate can be obtained at the four different temperatures. Some of these plots are shown in Figs. 2 and 3 for the four foods at the same temperature. If these figures are compared, we can observe that there is a decrease in shear stress when the temperature increases. The suspensions exhibited non-Newtonian and pseudoplastic behavior and therefore η must decrease with the shear rate and with the temperature (Figs. 4 and 5). The temperature dependency can be represented by means of the following Arrhenius-type equation:

where η is the apparent viscosity in Pa s, η ₀ (Pa s), and ΔE (J/mol) are Arrhenius parameters, R is the ideal gas constant in J/mol K and T the temperature in K. The Arrhenius parameters can be obtained from the intercept and the slope of the linear plots of ln(η) vs. 1/T for each shear rate, respectively. The ΔE represents the activation energy of flow and the activation energy data are displayed in Table 3. It can be observed that the values of η ₀ decrease when the shear rate increases (Fig. 6). The values of η ₀ can be fitted to an equation similar to Power-law (Table 4).

where γ is the shear rate in s⁻¹, a and b are fitting parameters, a is expressed in Pa s^−b .

Table 3 Activation energy, ΔE in J/mol, for all tested foods

Cereals with honey		With cocoa		Without gluten		Rice
γ (s^−1)	ΔE	R ^2	ΔE	R ^2	ΔE	R ^2	ΔE	R ^2
20	353.88	0.9949	688.47	0.9761	351.84	0.9946	215.35	0.9950
40	320.53	0.9986	649.08	0.9796	325.36	0.9920	210.41	0.9990
60	308.90	0.9986	627.62	0.9784	307.80	0.9921	204.96	0.9994
80	297.68	0.9934	624.88	0.9787	290.77	0.9911	198.56	0.9991
100	301.67	0.9966	611.81	0.9806	282.48	0.9900	199.20	0.9985
120	302.40	0.9970	604.29	0.9810	274.48	0.9905	195.13	0.9984
140	295.59	0.9993	590.09	0.9804	263.83	0.9885	193.33	0.9964
160	294.48	0.9993	587.94	0.9803	256.33	0.9909	190.16	0.9971
180	293.48	0.9993	579.51	0.9793	249.99	0.9894	187.37	0.9959
200	284.85	0.9991	575.10	0.9807	253.78	0.9840	180.90	0.9926
220	276.97	0.9988	567.22	0.9820	246.64	0.9837	179.55	0.9964
240	275.67	0.9980	563.36	0.9818	245.84	0.9838	178.93	0.9980
260	270.41	0.9973	556.01	0.9814	240.59	0.9838	175.15	0.9953
280	259.51	0.9987	551.23	0.9819	236.52	0.9836	173.48	0.9952
300	256.43	0.9977	543.25	0.9823	236.43	0.9841	173.48	0.9952
320	252.92	0.9962	535.75	0.9826	229.38	0.9840	175.79	0.9983
340	245.97	0.9963	532.51	0.9829	223.93	0.9832	174.13	0.9987
360	238.35	0.9989	527.17	0.9828	221.73	0.9809	172.86	0.9964
380	236.20	0.9984	526.09	0.9830	220.27	0.9806	168.40	0.9965
400	230.73	0.9977	523.04	0.9825	217.18	0.9815	166.50	0.9963
420	232.72	0.9992	524.52	0.9826	216.51	0.9787	161.80	0.9959
440	231.35	0.9980	525.62	0.9827	216.60	0.9790	160.37	0.9945
460	229.02	0.9985	520.40	0.9834	216.46	0.9774	158.77	0.9965
480	225.24	0.9975	518.68	0.9834	217.05	0.9766	155.92	0.9966
500	223.56	0.9971	513.77	0.9840	219.56	0.9723	156.13	0.9960

Table 4 Relationship between η ₀ and shear rate

Flours	a (Pa s^−b)	b	R ^2
Cereals with honey	0.4812	0.6176	0.9871
With cocoa	0.1254	0.6999	0.9952
Without gluten	0.6397	0.6862	0.9958
Rice	1.9716	0.5544	0.9991

Figure 2 Shear stress vs. shear rate for the tested baby foods at 30°C: cereals with honey (•), with cocoa (○), without gluten (□), rice (▪).

Figure 3 Shear stress vs. shear rate for the tested baby foods at 35°C: cereals with honey (•), with cocoa (○), without gluten (□), rice (▪).

Figure 4 Viscosity vs. shear rate for all foods at 30°C: cereals with honey (•), with cocoa (○), without gluten (□), rice (▪).

Figure 5 Viscosity vs. shear rate for all foods at 35°C: cereals with honey (•), with cocoa (○), without gluten (□), rice (▪).

In this study, two models were used to represent the rheological data. These are:

where η is the apparent viscosity in Pa s, τ ₀ is the yield stress expressed in Pa, γ is the shear rate in s⁻¹, n is the behavior index and K is the consistence index in Pa s⁻ⁿ . These models were selected because of their simplicity. These are commonly used to describe the rheology of non-Newtonian systems. Although both models were able to represent the experimental data reasonably well with the R ² value, which is a measure of the goodness of fit, higher than 0.999; the parameter τ ₀ estimated for the Hershel-Bulkey model was physically incorrect. Therefore, it was decided to use the power law model, due to the fact that its two parameters—consistence and behavior index—showed excellent representation of the data for all range of shear rate used in this work. The model parameters are displayed in Table 5.

Table 5 Power law parameters

T (°C)
Flours	Parameters	25	30	35	40
Cereals with honey	n	0.4260	0.4539	0.4775	0.5049
k (Pa/s^n)	4.8466	3.4351	2.5539	1.9589
R ^2	0.9991	0.9998	0.9998	0.9999
With cocoa	n	0.4412	0.4658	0.5048	0.5416
k (Pa/s^n)	7.2280	4.5409	2.5626	1.5051
R ^2	0.9994	0.9993	0.9998	0.9996
Without gluten	n	0.4757	0.5048	0.5317	0.5583
k (Pa/s^n)	6.6547	4.4087	3.4089	2.7116
R ^2	0.9996	0.9995	0.9993	0.9994
Rice	n	0.4626	0.4758	0.4901	0.5067
k (Pa/s^n)	7.5501	6.2540	5.1937	4.3435
R ^2	0.9992	0.9996	0.9997	0.9998

The Power-law parameters changed with temperature. The behavior index, n, increases lineally with temperature, while the consistence index, K, decreases lineally when the temperature increases.

where B _i and C _i are fitting parameters expressed in (°C)⁻ⁱ and Pa s⁻ⁿ  (°C)⁻ⁱ , respectively, and T represents the temperature in °C. The values of the fitting parameters are displayed in Table 6.

Table 6 Relationship between model parameters and temperature

n			K
Flours	B 0	B 1 (°C^−1)	R ^2	C 0 (Pa s^−n)	C 1 (Pa s^−n °C^−1)	R ^2
Cereals with honey	0.2964	0.0052	0.9990	9.4024	−0.1909	0.9641
With cocoa	0.2672	0.0068	0.9913	16.4050	−0.3829	0.9649
Without gluten	0.3391	0.0055	0.9995	12.6350	−0.2566	0.9274
Rice	0.3885	0.0029	0.9973	12.7770	−0.2136	0.9914

Conclusions

Four analyzed baby foods have a non-Newtonian behavior and the temperature influences the values of apparent viscosity and density. The power law was used to fit the apparent viscosity data; this indicates that the examined materials are pseudoplastic fluids. The four baby foods have rice as one of its ingredients and the results lead us to conclude that the density and apparent viscosity increase with the amount of rice. The tested powered baby foods without gluten, flour without gluten, and rice flour, have a higher viscosity and density than the flours with honey or cocoa. When we establish comparisons between foods with gluten, we observed that the flour with cocoa has higher values of both physical properties.