RHEOLOGICAL CHARACTERIZATION OF COMMERCIAL BABY FRUIT PUREES

International Journal of Food Properties, 11: 321–329, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1094-2912 print / 1532-2386 online
DOI: 10.1080/10942910701359424

RHEOLOGICAL CHARACTERIZATION OF COMMERCIAL
BABY FRUIT PUREES
E. Álvarez, M.A. Cancela, N. Delgado-Bastidas,
and R. Maceiras
Chemical Engineering Department, ETSEI, University of Vigo, Vigo, Spain
The rheological behaviour of fruit purees was measured at different temperatures (20ºC–
40ºC) in a rotational viscometer. The rheograms were fitted with the Power Law or Ostwald
de Waele model, the Herschel-Bulkley model, the Casson model, and the Cross model. The
best adjustment was obtained with the Cross model, except for systems fruit purees 2 and 4,
which were fitted satisfactorily with Power Law model. Flow curves exhibited at all temperatures a pseudoplastic character after applying a shear stress higher than a critical value.
Keywords: Viscosity, Purees, Pseudoplastic, Cross model, Power law model.

INTRODUCTION
Commercial baby food is a good nutritional complement for the suckling baby and
they are very useful for parents. Milk as unique food from the six months does not provide
energy and nutrients that baby needs at this age, in addition, as their digestive functions
have matured, new foods must be included in his diet, following regulated norms.
The habitual form to introduce a complementary feeding must be replaced each milk
intaking that baby receives by different components from the complementary feeding
(cereals baby food, fruit puree, vegetable puree). These replacements must be done of one
by one, with a sufficient interval so that baby accepts new foods, and hereby to confirm
his tolerance before introducing a new food, this process is for giving time to the adaptation of his organism. It is very important in this period, to allow that the amount of food
can vary from a day to another one and from one week to another one, according to the
appetite of the baby.
In diverse circumstances, infants need to feed with homogenized infantile foods.
These foods can be made up of: vegetables, fruits, meats, fish, milky or mixes, whose

exclusive purpose is to establish an infantile nutritional regime. The nutritious composition and norms of quality, production and elaboration of these products are collected in the
Director 96/5/CEE, which demands that these products must be elaborated following strict
norms of control of quality and with a suitable nutritious value.[1]

Received 31 October 2006; accepted 23 March 2007.
Address correspondence to M. A. Cancela, Chemical Engineering Department, ETSEI, Rúa Maxwell, s/n,
University of Vigo, Vigo 36310, Spain. E-mail:
321


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Fruit purees are elaborated with varied fruits (peach, apple and banana, or others
like apricot, orange or pineapple), and enriched or not with vitamins. Water or juices
are added to fruits. Moreover these can include milk, cheese, biscuits, cereals and
sugar that they will indicate in the label. Their ingredients are controlled strictly; this
guarantees the quality and the nutritious contribution of these purees to obtain a correct grown of baby. They avoid the existence of preservatites or pesticides to offer a
first quality product.
The fruits purees produced commercially have increased in popularity during
the last years, due to nutritional benefits for the infants, as also by the comfort in the
handling and consumption of these products. For reasons described previously and
because it is very important for the market in Europe, it is necessary to assure the
quality of baby foods. On the other hand, for the food industry is essential to have
knowledge of some parameters as apparent viscosity among others, because the flow
behaviour plays an important role in the design and optimization of the processes,
control process, quality control[2–5] within the food industry in particular for the
derivatives of fruits.
A point of considerable interest is the effect of temperature upon the flow properties. Various studies made have looked and analyzed the effect of temperature on

apparent viscosity for different fruits juice and fruit purees.[6–11] In addition, some
further investigations have characterized the flow behaviour of samples of commercial banana and peach baby foods. [12] In these studies were observed the behaviour of
flow of the different samples, in which it was found that the flow parameters
were changing significantly according to the manufacturer and the type of sample.
All the made studies confirm non-Newtonian behaviour of many of fruit purees
analyzed.[7,13–16]
The present paper was one focused in the study of the effect of temperature upon the
rheological behaviour of different kinds of commercial fruit purees. In this study some
rheological models were used to determine the relationship between shear stress as function of shear rate to different temperatures. Rheograms were fitted according to the
following models: Power Law or Ostwald de Waele model, Herschel-Bulkley model,
Casson model, and Cross model.
MATERIAL AND METHODS
Infant Foods (commercial fruit purees)
The ingredients of the puree fruits are showed following and the physicochemical
characteristics are refers in Table 1.
Puree 1. Apple (76%), peach (18%), honey (4%), apple juice concentrated and
vitamin C, without gluten.
Puree 2. Banana (35%), water, orange (17%), sugar, biscuits without gluten (7%),
lemon, rise starch, and vitamin C.
Puree 3. Peach pulp (27%), water, milk, sugar, creamed fresh cheese (6%), apricot,
rise starches and maize, lemon juice, apple concentrated, rice flour, cream, and vitamin C.
Puree 4. Apple (67%), pineapple (19%), pear (4%), sugar, and cereals (3%) (rice,
maize, tapioca).
Puree 5. milk (40%), apple, pineapple, banana, and sugar.


RHEOLOGICAL CHARACTERIZATION OF FRUIT PUREES

323

Table 1 Nutritional information per 100 g and pH.

Proteins (g)
Hydrates (g)
Sugars (g)
Fats (g)
Saturated (g)
Pectine (g)
Na (mg)
Ca (mg)
Vitamin C (mg)
Water content (g)
pH

Puree 1

Puree 2

Puree 3

Puree 4

Puree 5

0.3
—
15.0
0.4
—
—

2.0
—
11
77.3
3.57

0.8
24.7
19.0
1.4
0.4
1.4
10
—
35
75.1
3.62

1.4
21.8
16.4
0.9
0.6
0.6
14
—
35
78.1
3.65

0.5
8.5
—
0.1
—
1.2
6.2
—
25
78.7
3.41

2.9
20.5
—
3.2
—
—
37
130
—
79.8
3.68

Flow Properties Measurements
The rheological properties of fruit purees were carried out using rotational viscometer (Haake VT550/MV3, Searle type system). This viscometer is equipped with two coaxial cylinders, thereby that it has an inner cylinder rotating in a fixed outer cylinder. The
gap width between two cylinders was 1.45 mm. Radius and length of the rotating cylinder
were 10.1 mm and 61.4 mm, respectively. Thermostatic bath was used to control the
working temperature with a precision of ± 0.1°C.
Shear stress was analyzed as function of shear rate from 18 to 445 s−1 with an uncertainly of ± 0.001, while the temperature was kept fixed. On the other hand, also there was

analyzed the effect of the temperature on the rheological behaviour to different shear rate.
Measurements were carried out within temperature range from 20–40°C in 5 measures. In
order to investigate the reproducibility of the results, two replicates were made for most of
the experiments and the reproducibility was ± 5% on average
RESULTS AND DISCUSSION
Rheograms
Shear flow curves of commercial fruit puree samples in a temperatures range of
20–40°C are shown in Figure 1. In this Figure, it can be observed that there is a decrease
in shear stress when the temperature increases. Therefore, the suspensions exhibited nonNewtonian and pseudoplastic behaviour; thereby that η must decrease with the shear rate
and with the temperature as it is observed in the Figure 2. Likewise, in the Figure 3 can be
seen rheological behaviour of all systems of commercial fruit purees at 20 and 40°C. All
samples exhibited shear-thinning behaviour and therefore the shear stress decreases with
the temperature.
Rheological Parameters
The Cross model described well the flow behaviour of systems fruit purees for each
temperature, as compared with other models. However, in the case of systems fruit Purees 2
and 4 the model that better fitted the experimental data was the Power Law.


324

ÁLVAREZ ET AL.

90
80

τ (Pa)

70
60

50
40
30
20

0

100

200

300

400

500

γ (s )
–1

Figure 1 Shear Stress vs. shear rate for system fruit puree 5 at all temperatures: (ᮀ) 20°C; (᭹) 25°C; (Δ) 30°C;
(᭜) 35°C; (∇) 40°C.

η (Pa.s)

2

1

0

0

100

200

300

400

500

γ (s–1)
Figure 2 Viscosity vs. shear rate for system puree 5 at all temperatures: (ᮀ) 20°C; (᭹) 25°C; (Δ) 30°C; (᭜)
35°C; (∇) 40°C.

Systems purees 1, 3, and 5. Systems purees 1, 3, and 5 were fitted with Cross
model, and the adjusted determination coefficient in all cases was higher than 0.998. The
Cross model was represented as:

ηa =η∞

⎛
⎞
η0 −η∞ ⎟
⎜
+⎜
.m⎟
⎜⎝ 1 + α γ ⎟⎠

c

(1)

Where γ is the shear rate (s−1), ηa the apparent viscosity (Pa s), h0 the zero-shear rate viscosity (Pa s), h∞ the infinite-shear rate viscosity (Pa s), ac (sm) is time constant, and m is
dimensionless constant. The constant αc is expressed by the ratio k1/k0, where k0+k1 γn is
the rate constant for the rupture of linkages; the parameter ac is related to the relaxation
time of the structural species responsible for shear thinning and the onset of shear-thinning


RHEOLOGICAL CHARACTERIZATION OF FRUIT PUREES

325

4
8

20°C

40°C
3
η (Pa.s)

η (Pa.s)

6
4

2
1

2

0

0
0

100

200

300

400

500

0

100

200

300

400

500

γ (s–1)

γ (s–1)

Figure 3 Viscosity vs. shear rate for all systems analyzed at: 20 and 40°C. (᭢) Puree 1; (᭹) Puree 2; (᭜) Puree
3; (ᮀ) Puree 4; (᭛) Puree 5.

behaviour.[17,18] A high value of ac implies a relatively large shear dependent contribution
to structural breakdown.[19] The exponent m is related to the power law exponent “n”
(flow behaviour index).[20] Systems in which h0 >> h∞ the Cross equation reduces to the
power law model or may be approximated to the Bingham model.[21,22] Newtonian fluids
have m = 0, while fluids that exhibit shear thinning have small< 1, positive exponents.
Moreover, such as ha >> h∞ the Cross model was used with only three adjustable parameters (assuming h∞ ≈ 0).[17,23] As a zero-shear viscosity was not obtained on these samples
the h0 was estimated to lower shear rate, whereby was used the relation Log η vs Log γ.
This rheological model adjusts the experimental values reasonably well for the different purees (Figure 4a). Furthermore, the time constants (αc) are larger for the system
puree 3 with values between 1.1333 and 0.7265 (sm), among 20 and 40°C respectively. On
the other hand, systems puree 1 and 5 presented a stable behaviour with respect to relaxation time and temperature, presented values between 0.6907–0.7280 and 0.7726–0.7597
(sm), respectively. Moreover, the exponent “m” in all cases was a positive exponent lower
than 1, therefore there is deduced that these systems exhibit a shear thinning behaviour.

4

3

40°C

40°C
3

η (Pa.s)

η (Pa.s)

2
2
1

1

0

0
0

100

200

γ (s–1)
(a)

300

400

500

0

100

200

300

400

500

γ (s–1)
(b)

Figure 4 Viscosity vs. shear rate for different systems fitted with rheological models: (᭢) Puree 1; (᭹) Puree 2;
(᭜) Puree 3; (ᮀ) Puree 4; (᭛) Puree 5.


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ÁLVAREZ ET AL.
Table 2 Parameters of the Cross model for systems puree 1, 3, and 5.
Systems

T (ºC)

αc (sm)

m

χ2

R2

Puree 1

20
25
30
35
40
20
25
30
35
40
20
25
30
35
40

0.691
0.702
0.710
0.727
0.728
1.133
0.831
0.772
0.751
0.727

0.773
0.746
0.743
0.747
0.760

0.641
0.655
0.660
0.672
0.686
0.690
0.696
0.690
0.679
0.659
0.756
0.751
0.748
0.747
0.739

2.0E-05

0.99929

3.27E-03

0.99872

6.0E-05

0.99938

Puree 3

Puree 5

The flow behaviour was largely affected by temperature. In the Table 2 can be observed
parameters of the Cross model that were obtained from the adjustment of the experimental
data of the systems mentioned previously. Equation 1 was used to fit systems by means of
the Cross model.
Systems fruit purees 2 and 4. As can be seen in Figure 4b, the Power Law
model described well the flow behaviour of systems puree 2 and puree 4, for each temperature, the adjusted determination coefficient being in all cases higher than 0.999. The
Power Law model was represented as:

η = K •γ n −1 ,

(2)

where K is the consistency coefficient, and n is the flow behaviour index. The values of the
flow behaviour index and consistency coefficient, n and K, are reported in Table 3. It can
be observed that n has a value less than 1, indicating pseudoplasticity. For flow curves of
system puree 2 the flow behaviour index varies from 0.374 to 0.414. Whereas, flow curves

Table 3 Parameters of the Power Law model for systems
puree 2 and 4.
System

T (ºC)

n

K

Puree 2

20
25
30
35
40
20
25
30
35
40

0.374
0.380
0.390
0.405
0.414
0.286
0.282
0.268
0.254
0.247

16.706

14.882
12.556
10.299
9.007
26.109
24.675
24.487
24.543
24.125

Puree 4


RHEOLOGICAL CHARACTERIZATION OF FRUIT PUREES

327

of system puree 4 the flow behaviour index varies from 0.286 to 0.280. Figure 5 shows the
influence of the temperature on consistency coefficient and flow behaviour index.
From a practical point of view, we have decided to describe effects of temperature
and total solids content on consistency and flow behaviour index by one combined model.

0.3
0.3

0.29

0.29

n

0.28

0.28

0.27

0.27

0.26

n

(a)

0.26

0.25

0.25

0.24

0.24

25

T(
°C)

30

21.

3

35
40 21.4

Solid

%

(b) 26.5

26.5

26

26

25.5

K

K

25.5

25

25

24.5

24.5

24

24
25
30
T (°
C)

21.

3

35
40 21.4

Solid

%

Figure 5 Response surfaces for the effect of temperature and total solids on rheological parameters of puree 4.


328

ÁLVAREZ ET AL.

Table 4 Regression coefficients of the models for rheological parameters of puree 2 and 4.
Parameter
N
K

a0

a1

a2

b1

b2

b3

0.3911
47.3254

−12
768

96
−10240

2.8545

−3.4707

−1.0371 10–3
0.1097

1.1333·10–5
−1.1467 10–3

In the literature, some authors[24,25] used the combined effect of temperature and total solids content on thses parameters to describe the flow behaviour of fruit purees. In this
work, a response surface methodology was selected to investigate this effect on rheological parameters of purees 2 and 4, and a quadratic polynomial regression model was
assumed for predicting the individual variables. The model proposed for rheological
parameters is:

y = a0 + ∑

ai
i

S

+ ∑ bi T ,
i

(3)

where a0 is a constant; and, ai and b i are regression coefficients of the model.
The regression coefficients are shown in Table 4. The predictive models developed
for n and K were considered adequate because they had satisfactory levels of R2
(R 2 > 0.9999).
The model indicated that temperature has significant effect on behaviour index. This

observation is also verified from canonical analysis of response surface. Figure 5 shows
the response surfaces for the effect of temperature and total solids on rheological parameters for puree 4.
CONCLUSIONS
From this study, it can be determined that temperature significantly affected the
flow characteristics of all cases analyzed. Systems purees 1, 3, and 5 could be accurately
described with 3-parameters of the Cross model, which were sensitive to change in temperature. On the other hand, systems 2 and 4 were fitted well by the Power Law. In the
same way, such as in the case of systems fitted by Cross model, Power Law parameters
(behaviour index and consistency coefficient) were affected by change in temperature.
This effect was analyzed with the Response Surface Methodolgy and three-dimensional
figures were presented to identify the effects of temperature and total solids content.
Good fit models were developed for consistency and flow behaviour index. The results
of this work have direct application to fruit puree processes involving fluid flow and
heat transfer.
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