Vol. 65, No. 4, 2000—JOURNAL OF FOOD SCIENCE
641
? 2000 Institute of Food Technologists
F o o d E n g i n e e r i n g a n d P h y s i c a l P r o p e r t i e s
Effect of Cloud Particle Characteristics on the Viscosity of Cloudy Apple Juice
D.B. G ENOVESE AND J.
E. L OZANO
ABSTRACT: Particle size, shape, and volume fraction as well as soluble pectin content and electro-viscous effects on cloudy apple juice viscosity were investigated. Particle characteristics were micrographically determined by elec-tron microscopy. More than 2300 particles were digitalized and statistically analyzed. Cloudy apple juice resulted in a suspension of irregular shape particles ranging from 0.25 to 5 ?m in size. A 10 °Brix juice had a particle volume fraction of ?0= 3.93 ∞ 10-3. Relative viscosity values were fitted to the generalized Sudduth equation. Intrinsic viscosity resulted 5.8 times greater than the theoretical value predicted by Einstein (? = 2.5). This was attributed to the first electro-viscous effect. Results also indicate that the effect of particle non-sphericity on cloudy apple juice viscosity can be neglected. Soluble pectin was found to increase juice viscosity significantly. Combined effects result in a strong linear increase of relative viscosity.
Key Words: cloudy apple juice, viscosity, particle size, particle shape, electro-viscous effects
Introduction
M
OST LIQUID FOODS ARE POLY -DISPERSE SYSTEMS . C LOUDY
apple juice has solids (particles) of various dimensions dis-tributed in a liquid constituted by a colloidal solution of pectins in a true solution of sugars, organic acids, and salts (serum or clarified juice) (Moyls 1966; Beveridge and Tait 1993; Rao 1999).Particles have electrical charge and are constituted by carbohy-drates and proteins surrounded by insoluble pectins (proto-pec-tins). One of the main problems with cloudy apple juice produc-tion is cloud stability (Chobot and Horulaba 1983; Beveridge and Harrison 1986; Gierschner and Baumann 1988; Genovese and others 1997). Even after prolonged storage none or only a very small part of the cloud particles should precipitate.
Rheological behavior of a dispersion is governed both by: (a)liquid (continuous) phase characteristics (viscosity, chemical composition, polarity, and electrolyte concentration) and (b) sol-id (dispersed) phase characteristics (particle size, shape, size dis-tribution, and volume fraction as well as electro-viscous effects).So it is useful to study the flow behavior of the cloudy juice and its liquid phase (solution of pectins in serum) and link their rheo-logical behaviors with juice structure and composition, since these properties modify juice viscosity, compromising the stabili-ty of the colloidal system.
Characterizing the cloudy apple juice microstructure is diffi-cult, but most of the variables involved are reflected in 1 param-eter: the volume fraction of particles (?). Diluted dispersions can be modeled in terms of the intrinsic viscosity [?] (Sherman 1970), which depends on particle shape, size distribution, and also on the 1st electro-viscous effect (Krieger 1972; Russell 1980):when diluted dispersions are sheared, the electrical double layer (shear layer) is distorted, which leads to an increased viscosity.The behavior at high concentrations is governed by the fact that beyond a volume fraction ?MAX , called the packing fraction, the dispersed particles lock into a rigid structure, and flow ceases (Krieger 1972). Another factor that increases viscosity in concen-trated dispersions is the 2nd electro-viscous effect due to particle repulsion, which was claimed (Sherman 1970) to be proportional to ?2 and inversely proportional to pH.
The objectives of this study were: (1) to examine the influence of size, size distribution, shape, and charge of cloud particles on the viscosity and (2) to quantify the effect of soluble pectin con-tent on the flow behavior of cloudy apple juice.
Results and Discussion
A
TYPICAL FREQUENCY HISTOGRAM OBTAINED THROUGH THE statistical analysis of apple juice cloud particles is shown in Fig. 1. Cloudy apple juice resulted in a suspension of irregular shape particles ranging from 0.25 to 5 ?m in size, with a mean di-ameter D = 0.84 ?m. Calculated maximum and minimum mean diameters resulted in L a = 1.01m and B a = 0.74 ?m, respectively.A 10 °Brix juice had a particle volume fraction of ? o = 3.93 ∞10-3. This value is in accordance with published data. It was found (Ruck and Kitson 1965; St?hle-Hamatschek 1989) that rel-ative cloud material in apple juice was < 0.5%. Using this ?o value at 10 °Brix, the volume fraction of particles in the assayed apple juice at other soluble solids
resulted:
(3)
Combining Eq. 1 and 3, soluble pectin content can be related
to volume fraction of particles:
(4)
Figure 2 shows a typical log shear stress (?) in contrast to log
shear rate (?
и) curve for different soluble solids concentrations of JFS: Food Engineering and Physical Properties
642JOURNAL OF FOOD SCIENCE —Vol. 65, No. 4, 2000
F o o d E n g i n e e r i n g a n d P h y s i c a l P r o p e r t i e s
Cloudy Apple Juice Viscosity . . .
cloudy and microfiltered apple juice at 25 °C. It was confirmed
that ? increased linearly with ?
и and all curves go to the origin (r 2 =0.9982). Moreover, viscosity was also independent of time. Then cloudy apple juice (up to 50 °Brix) was confirmed to be Newto-nian (Saravacos 1970), as well as microfiltered juice. Figure 3compares viscosity with soluble solids for clarified, microfiltered,and cloudy apple juice in the range 10 to 50 °Brix, at 25 °C.
In order to quantify the effect of microstructure on cloudy ap-ple juice viscosity, relative (cloudy/microfiltered) viscosity values in contrast to ? were fitted with Sudduth’s generalized Eq. 11 and plotted (Fig. 4). Fitting parameters were [?] = 14.5; ?MAX
= 0.0611;
Fig. 1—Particle size distribution histogram: particle relative number
(% N) in contrast to particle diameter (D)
Fig. 2—Log-Log plot of shear rate (??
) and shear stress (?) at 25 °C for cloudy and microfiltered apple juice at various soluble solids
s = 3.29 and r 2 = 0.996. According to Eq. 9, in the dilute region (?< 0.01) the dispersion showed linear behavior, although [?] was 5.8 times greater than the theoretical value (? = 2.5) predicted by Einstein. This elevated value indicates that the 1st electro-vis-cous effect cannot be neglected in cloudy apple juice up to 20°Brix (? Х 0.008). The low value of maximum packing fraction suggests that at higher concentrations the 2nd electro-viscous ef-fect is significant too. Finally, with the estimated L a and B a parti-cle axial values, p a parameter resulted equal to 1.365. It can easily be calculated that the term accompanying the a coefficient in Eq.13 is practically irrelevant (less than 0.6% the value of [?]),
and
Fig. 3—Viscosity of cloudy, microfiltered, and clarified apple juice as a function of soluble solids at 25 °
C
Fig. 4—Relative viscosity of cloudy apple juice (?r ) as a function of the volume fraction of particles (?). Full line represents Eq. 11with slope (dotted line) equal to the intrinsic viscosity [?]. Dashed line represents Eq. 6.
Vol. 65, No. 4, 2000—JOURNAL OF FOOD SCIENCE
643
F o o d E n g i n e e r i n g a n d P h y s i c a l P r o p e r t i e s
the effect produced on the viscosity due to nonsphericity of par-ticles can be neglected. When size of particles is considered, be-low about 0.5 μm dia, a higher relative viscosity is always to be expected.
When relative (cloudy/clarified) viscosity values were plotted (Fig. 4), a strong linear behavior (r 2 = 0.996) with a high slope (96.4) was obtained. However, this slope also includes the effect of soluble pectin, absent in clarified juice. To quantify this effect,a linear equation is proposed which contains the intrinsic viscosi-ty previously obtained and includes a term considering the solu-ble pectin concentration:
Table 1—Cloudy apple juice* characteristics Soluble solids (°Brix)10pH 3.4
Color
Yellowish white
(L= 47.7; a= -0.18; b= 8.5)
Pulp content % (V/V)< 1.2(before centrifugation)Foreign matters Free Iron (ppm) 1.36Calcium (ppm)35.6Zinc (ppm)0.47Potassium (ppm)
615.1
*Obtained from a 65 °
Brix concentrate.
Materials and Methods
C
LOUDY APPLE JUICE OF VARIOUS SOLUBLE SOLID CON -
tents, in the range 10 to 50 °Brix, was made by reconstitut-ing with distilled water a 65 °Brix concentrate obtained from Coop Julia & Echarren (Buenos Aires, Argentina). This concen-trate was obtained by applying at industrial scale the method proposed by Genovese and others (1997). Soluble solids at 20°C were determined according to AOAC (1980) number 932.12method. The main characteristics of this juice are summarized in Table 1. Color was determined as Hunter L , a , and b param-eters with a Hunter Ultrascan XE Spectrophotometer (Hunter-lab Inc., Reston, Va., U.S.A.). The solution of pectins in serum was obtained removing cloudy juice particles by microfiltra-tion (0.45 μm). Particle size analysis of microfiltered juice re-sulted in particle diameter < 0.25 μm. Moreover, residual par-ticle volume was less than 2% of the initial particle volume in the cloudy juice. Viscosity (shear rate range from 10 to 1000 s -1)of cloudy and microfiltered juices was determined with a Bohlin CVO Rheometer (Bohlin, U.K.). Clarified apple juice viscosity values were obtained from literature (Constenla and others 1989).
Particle characteristics determination
Particle volume fraction, size, size distribution, and shape of cloudy apple juice particles were micrographically deter-mined with a JEOL Model 35CF SEM (JEOL LTD, Tokyo, Japan)at 5kV. Cloudy juice (10 °Brix) was centrifuged (9.000 ∞ g ∞ 1min ) in a Beckman Microfuge centrifuge (Beckman Instru-ments Inc., Calif., U.S.A.). To avoid agglomeration the 10 °Brix juice was previously diafiltered with a lab HFUF unit with a single Polysulfone hollow fiber (Amicon, Lexington, Mass.,U.S.A.; cut off 50,000 MW) operated in a batch mode until sol-uble solids reduction of 1.5 °Brix. An aliquot of this diafiltered juice was diluted 1:100 with distilled water. 10 μl of the diluted juice were put on a glass slide, vacuum dried at 40 °C and gold covered in a Pelco Model 3 Sputter Coater 91000 metal evapo-rator. Total volume of particles was calculated after digitaliza-tion of more than 2300 particles. Digital images of cloud parti-cles were statistically analyzed with the AnalySIS 2.1 (Soft-im-aging Software GmbH) version.
Besides the number of particles and mean particle diame-ter, this program also gave information about the length of the major axis (L a ) and the minor axis (B a ), valid for estimation of particle sphericity. Each particle diameter was calculated as a projected area. To evaluate maximum and minimum axis the program generates an axis, called evaluation axis, at a defined angle with respect to the particle. Then 2 lines perpendicular to the evaluation axis are generated completely, including the particle. L a and B a maximum diameters can be calculated by
moving the evaluation axis angle.
To calculate particle volume fraction (?) at other soluble solid concentrations, the following equation was deduced:
F o o d E n g
i n
e e r i n g a n d P h y s
i c
a l P r o p e r
t i e
s Cloudy Apple Juice
Viscosity . . .
(11)
where s is a particle interaction coefficient.
As previously mentioned, intrinsic viscosity is affected by
particle size, size distribution, and shape (sphericity). Cloudy
apple juice particles could be considered ellipsoidal rather
than spherical. (Fig. 5). In this case, the axial ratio of the parti-
cle (p a) should be considered:
(12)
where L a and B a are the major and minor axis, respectively.
Rutgers (1962) adapted Mooney’s (1951) equation to concen-
trated dispersions of ellipsoidal particles:
(13)
where a + 0.399(p a - 1)1.48 = [?]. Depending on the axial ratio of
particles, Eq. 13 gives information about the influence of
shape on the viscosity of cloudy juice.
Fig. 5—Electron micrographs of particles from cloudy apple juice
(85 kV ∞ 4000)
References
AOAC. 1980. Official Methods of Analysis. 13th. Ed. Washington, D.C.: Association of Official
Analytical Chemists. Washington, DC.
Ball RC, Richmond P. 1980. Dynamic of colloidal dispersions. Phys. Chem. Liq. 9:99-116.
Beveridge T, Tait V. 1993. Structure and composition of apple juice haze. Food Structure.
12:195-198.
Beveridge T, Harrison JE. 1986. Clarified natural apple juice: Production and storage stability
of juice and concentrate. J. Food Sci. 51:411-414.
Chobot R, Horulaba A. 1983. Stabilization of naturally cloudy apple juices by mechanical
and heat treatment of must. Przemysl Spozywczy. 37:409- 411.
Constenla DT, Crapiste GH, Lozano JE. 1989. Thermophysical properties of clarified apple
juice as a function of concentration and temperature. J. Food Sci. 54:663-669
Genovese DB, Elustondo MP, Lozano JE. 1997. Color and cloud stabilization in cloudy apple
juice by steam heating during crushing. J. Food Sci. 62:1171-1175
Gierschner K, Baumann G. 1988. New method of producing stable cloudy fruit juices by the
action of pectolytic enzymes. Industr. Obst-Gemuesev. 54:217-218.
Kintner PK, Van Buren JP. 1982. Carbohydrate Interference and its Correction in Pectin
Analysis Using the m-Hydroxydiphenyl Method. J. Food Sci. 47:756-764.
644JOURNAL OF FOOD SCIENCE—Vol. 65, No. 4, 2000
Vol. 65, No. 4, 2000—JOURNAL OF FOOD SCIENCE
645
F o o d E n g i n e e r i n g a n d P h y s i c a l P r o p e r t i e s
Krieger IM. 1972. Rheology of monodisperse latices. Adv. Colloid Interface Sci. 3:111-136Metzner AB. 1985. Rheology of suspensions in polymeric liquids. J. Rheol. 29:739-747
Mooney M. 1951. The viscosity of concentrated solutions of spherical particles. J. Colloid Sci. 6:162-167.
Moyls AW. 1966. Opalescent apple juice concentrate. Food Technol. 20(5):121-123
Rao MA. 1999. Rheological behavior of processed fluid and semisolid foods. In: Rheology of Fluid and Semisolid Foods. Gaithersburg, Md.: Aspen Publishers Inc. p 221-223.
Ruck JA, Kitson J. 1965. Seasonal variation in the soluble solids and total acid content of opalescent apple juice. Wissen. Techn. Kommission Intarn. Fruchtsaft-Union. p 433-438Russel WB. 1980. Review of the role of colloidal forces in the rheology of suspensions. J.Rheology. 24:287-317.
Rutgers, IR. 1962 Relative viscosity and concentration. Rheologica Acta, Brand 2, 4:305-349.Saravacos GD. 1970. Effect of temperature on viscosity of fruit juice and purees. J. Food Sci.
35:122-127
Sherman P. 1970. Rheology of dispersed systems. In Industrial Rheology. London: Aca-demic Press Inc. p 97-183.
St?hle-Hamatschek S. 1989. Cloud composition and its influence on cloud stability in naturally cloudy apple juice. Fluss. Obst. 56:543-544.
Sudduth RD. 1993. A Generalized Model to Predict the Viscosity of Solutions with Sus-pended Particles. I. Journal of Applied Polymer Science. 48:25-36.MS 19990439 received 4/20/99; revised 1/4/00; accepted 3/30/00.
Authors are with the Plapiqui (Uns - Conicet) Camino La Carrindanga Km.7 (8000) Bahia Blanca, Argentina. Direct inquiries to J.E. Lozano (E-mail: jlozano@https://www.doczj.com/doc/dc10608869.html,.ar).