Rheological properties of gelatinized starch solutions as influenced by thermal processing in an agitating retort

JournalofFood Copyright Prkedin

Engineering 25 (1995) 441-43 0 1995 Elsevier Science Limited Great Britain. All rights reserved 0260-X774/95/$9.50

0260-8774(94)00009-3

Rheological Properties pf Gelatinized Starch Solutions as Influenced by Thermal Processing in an Agitating Retort H. S. Ramaswamy,” S. Basak, C. Abbatemarco Department

of Food Science,

Macdonald

Campus

& S. S. Sablani

of McGill University,

Ste. Anne de

Bellevue, PQ, Canada H9X 3VO (Accepted

3 March

1094)

ABSTRAC? Rheological changes associated with a canned model food (gelatinized starch) were evaluated in relation to factors (retort temperature, rotation speed, can headspace and starch concentration) which inf7uence heating rates of processed foods in an agitating retort. The rheology of gelatinized starch was well described by the power law model. Power law parameters (consistency coefficient and f7ow behaviour index) were generally in$!uenced by all factors tested with the exception of can headspace in the 8/32-16132 inch range. Even though the starch suspensions were pregelatinized, the rheological characteristics continued to change with processing under all conditions. Heating rates were modeled as a function of retort temperature (110-13O”C), rotation speed (IO-20 rpm) and product initial apparent viscosity (7~ at a nominal shear rate of 100 SK’, and the apparent viscosity of the processed product was related to 7, and the process cook value.

INTRODUCTION The majority of liquid foods are non-Newtonian in nature. Thermal process designs for liquids with or without particulates require data on the rheological properties of the fluid to arrive at processing conditions which ensure safety and improve quality factor retention. Starches are commonly added to popular foods such as soups and sauces to increase their consistency and improve their

*Author

to whom correspondence

should be addressed. 441

442

H. S. Ramaswamy

et al.

mouthfeel characteristics. The thickening activity of starches results from the swelling of the starch granules occurring at gelatinization temperatures (Self et al., 1990). Recently, modified cross-linked starches have been used to simulate carrier fluids in aseptic processing of liquid foods containing particulates (Chandarana et al., 1989a, b; Harrod, 1989a, b, c). It has been recognized that rheological properties of fluids depend on the concentration of the active ingredient (starch, gelatin, pectin, etc.), temperature and the shear rate. Some data on rheological properties of starch under aseptic processing conditions are available (Dail & Steffe, 1990a, b; Harrod, 1989u, b, c; Self et al., 1990). Sterilization in rotary autoclaves has been recognized to promote high product quality due to rapid heating of the contents through container agitation (Clifcorn et al., 1950). HTST heating techniques which are difficult to achieve in still retorts for processing of viscous foods have been made possible with rotary sterilizers. Several factors influence the heat penetration rates and product quality during rotational processing: product apparent viscosity, can size, type of product, rotation speed, retort temperature, headspace and ratio of liquid to particles in the product (Eisner, 1988). Among other sensory characteristics, textural attributes such as mouthfeel or consistency constitute an important quality parameter of the processed product. Rheological properties have been correlated with sensory properties and, being more fundamental in character, they provide objective measurements of food quality. Rheological properties not only influence the heating rates of the product during rotational processing, but are also influenced by the same process. The objective of this study was to evaluate the effect of retort temperature, rotation speed, can headspace, starch concentration and processing cycle on the rheological properties of a canned model food (gelatinized starch) processed in an agitating retort.

MATERIALS

AND METHODS

Therm-F10 Starch (Lot #EA 4767, National Starch and Chemical Corporation, Bridgewater, NJ), a cross-linked waxy maize starch, was used to prepare samples of gelatinized 3% and 4% starch solutions. Appropriate amounts of starch and water were mixed thoroughly and brought to boiling in a steamjacketed kettle. The mixture was allowed to boil for 45 min followed by simmering at approximately 85-90°C for 2 h. Water was added to the mark to compensate for evaporative losses. The pregelatinized starch solutions were hot filled (80°C) into 307 X 409 cans allowing for two levels of headspace: 8/32 inch (6.4 mm) and 16/32 inch ( 12.8 mm). CNS copper-constantan thermocouples (Ecklund-Harrison Technologies, Cape Coral, FL) were positioned at the geometric center of each can to obtain heat penetration data. The cans were then placed in a steamer for thermal exhausting of air to ensure adequate formation of vacuum and sealed in a seaming machine (Double Seamer, Type W-200, Continental Can Co. of Canada Ltd.). Thermal processing was carried out in a Lagarde Rotary Simulator using a steam/air medium consisting of 75% steam. Temperature readings were computer-recorded at 15 s intervals via a data logger (Dash-g, Metra-Byte

Rheology of gelatinizedstarch

443

Corp., Tauton, MA) as cans were subjected to end-over-end rotation. A 3 X 3 factorial experimental design was employed in this study with three different retort temperatures ( 110, 120 and 130°C) and three rotation speeds ( lo,15 and 20 rpm). All cans were subjected to a 10 min come-up period, a 30 min process time and a 25 min cooling time period. For each experimental run at a constant temperature and rotation speed, several cans containing thermocouples were used to obtain heat penetration data during three consecutive runs. Heat penetration data were analyzed to obtain the heating rate index fh according to procedures detailed in Stumbo (1973). Rheological properties of the processed starch solutions were evaluated both before and after processing using a Haake rotational viscometer Model RV20 (Haake Hess-Technik GmbH u. Co., Karlsruhe, Germany) equipped with an M5 OSC measuring head employed in the rotational mode and computer controlled via a Rheocontroller RC20 module. After equilibration at 25°C test samples were sheared at a programmed rate increasing from 0 to 200 s- ’ in 4 min (upward flow curve) and subsequently decreasing from 200 to 0 s- ’ in the next 4 min (downward flow). The flow curves (rheograms) were evaluated by using the following rheological models: o= mfll 1. Power law model: (1) where o is the shear stress (Pa), 9 is the shear rate (s- I), m is the consistency coefficient (Pa s”), and n is the flow behavior index (dimensionless). 2. Herschel-Bulkley

o= a,, + mj”

model:

(2)

where 0,) is yield stress. 3. Casson model:

o”” = ( mo)o.s+ m,( p)0”

(3,

where m,, and m, are Casson yield stress and Casson viscosity, respectively. Apparent viscosity has no real meaning when considering non-Newtonian fluids. However, several simple instruments such as Brookfield viscometer which are routinely used in the food industry for quality control applications rely on estimated shear stress values at some nominal shear rate. They yield a consistency or an apparent viscosity value meaningful enough for routine quality control testing. As an estimate of one such parameter, an apparent viscosity value was compared at a nominally chosen shear rate of 100 ss ’ as a ratio of GlY. In order to quantify the degree of cooking, a ‘cook value’ (Cv) is generally defined as equivalent minutes of heating at 100°C (Teixeira et al., 1969; Eisner, 1988). In thermal processing operations, temperature of a canned product follows a characteristic profile gradually approaching the operating conditions which determines the degree of cooking. The cook value has been assumed to follow a logarithmic change with respect to temperature, with every 33°C (z value) change in temperature resulting in a lo-fold change in the cook value. For example, a cook time of 1 min at 100°C is equivalent to 10 min cooking at 67°C and 0.1 min at 133°C. The accumulated cook value of the process can be obtained by numerical integration: I

(--v

=

I

1O’T-

0

‘Olli/~ dt

(,4)

444

H. S. Ramaswamy

et

al.

A z value of 33°C has been generally assumed (Eisner, 1988) to represent the temperature sensitivity of typical food quality factors (color, texture, nutrients). The cook value therefore is a measure of the severity of the process and the destruction of product quality. Hence, in order to optimize product quality, it is desired to keep the cook value as low as possible. RESULTS AND DISCUSSION Rheological characterization of starch Typical upward and downward flow curves for 3% and 4% starch suspensions are shown in Fig. 1. The 4% starch had a higher consistency as indicated by the higher shear stress values under a given shear rate. A comparison of different rheological models as shown in Table 1 for describing the flow behavior of starch suspensions under typical conditions indicated better R 2 values with the power law model. The power law model generally gave a higher R 2 and showed a better fit for data points under most circumstances. Hence, this model was used for all subsequent analysis in this study because of its inherent compatibility as far as engineering calculations are concerned. Test samples showed some textural breakdown, since the upward and the downward flow curves did not overlap (Fig. 1). The magnitude of structure breakdown is generally evaluated from the area under the hysterises loop

20

15

IO

5

0 0

20

40

60

80

100

120

140

160

180

200

Shear rate (s-’ )

Fig. 1.

Typical upward and downward flow curves for starch suspensions processing).

(prior to

Rheology

Verification

Starch concentration

of gelatinized

445

starch

TABLE 1 of Different Models for the Rheology of Canned Gelatinized Processed in a Rotational Retort at 12O”C, 15 rpm for 30 min Down cl4rve

Up curve

Model

Starch

(%I)

3

Power law” H. Bulkley ’ Casson’ Power law H. Bulkley Casson

4

intercept

Slope

R2

0.128 0.046 0.143 2.447 0.176 3.933

0.668 0.876 0.016 0.452 0.960 0.056

0.99 0.98 0.99 1.00 0.77 0.96

Intercept

Slope

R’

0.053 0.04 1 0.050 1.820 0.352 3.346

0.84 1 0.890 0.018 0.505 0.8 1s 0.3 16

1W 1+10 0.99 1.00 0.92 0.99

“Power law: regression of log (u) vs p. hH. Bulkley: regression of log (u- a,,) vs p. ‘Casson: regression of a”” vs pll ‘.

enclosing the upward and downward flow curves. In an earlier study (Ramaswamy & Basak, 1991), it was shown that the structural breakdown of a non-Newtonian fluid (stirred yogurt) correlated well with the ratio of apparent viscosities, at an intermediate shear rate, from the upward and downward flow curves. Figure 2 is a combined plot of apparent viscosities (for both 3% and 4% starch) calculated at 100 SC’ from downward curves vs those from upward curves (prior to processing) indicating a good correlation. Similar results were observed with processed samples. The following were the regression equations relating the apparent viscosities: before processing:

l;l,doWnward, = 0.966

after processing:

r,+dounward) = 0.899 x v(“,,,+~~~) + 0.0117;

X l;lfupwardl +

04057;

R ? = 0.97

(51

R 2 = 090

16)

The results indicate that apparent viscosities of test samples in the downward cycle are 5-10% lower than in the preceding upward curves indicating some structure loss. Processing

factors influencing

rheological

properties of starch

Typical upward flow curves of starch suspensions subjected to selected rotational processing conditions are compared in Fig. 3. All process parameters, especially temperature and processing cycle, influenced the rheological properties. Higher temperatures and higher rotation speeds with larger can headspaces generally contribute to higher viscosity build-up in the product. It was initially anticipated that the rheological changes associated with the processing would be small because the starch used in the study was pregelatinized for a long period prior to processing. However, as seen from the results, the viscosity build-up, especially due to additional processing cycles, was far more pronounced than from other parameters under the study. Hence, the number of processing cycles was used as one of the additional parameters, providing samples with a wide range of apparent viscosities (0.0 l-O.5 Pa s).

H. S. Ramaswamyet al.

446

0

I

I

I

0.1

0.2

0.3

I

0.4

0.5

Upcurve apparent viscosity (Pas)

Fig. 2.

Apparent

viscosities from flow curves calculated at a shear rate of 100 SC’: downward vs upward (prior to processing).

In order to quantify the effect of various factors on the rheological properties, the two power law parameters (m and II values) and the apparent viscosity value (at the mid-shear rate of 100 s- i j before and after processing were compared (Fig. 4). The points on the diagonal line in these figures represent parameters not influenced by processing, while points above the line represent those with a positive change and below, a negative change. The majority of points for the m value data are above the diagonal line indicating a consistency build-up due to processing. A similar effect is also observed for the apparent viscosity data. The n value data show the contrary indicating that the resulting product is becoming more pseudoplastic following processing. The influence of specific factors on the rheological changes was assessed using the parameters m, n and q, as their respective ratios (for example, m value ratio is m value after processing divided by m value before processing). An analysis of variance of these ratios is shown in Table 2 and various effect curves are shown in Figs 5-7 for both 3% and 4% starch suspensions. Headspace, as a main effect or interaction, was not significant (p> 0.05; Table 2) under any of the test conditions. Starch concentration, temperature, rotation speed and repeated processing were all significant (p < 0.05) with reference to influencing both m value and the apparent viscosity ratios, with the number of processing cycles being the most dominant factor. Temperature-processing

447

Rheology of gelatinized starch 20Processing

Headspace

cycle #3

16132

15. P % I

-

g lo$

Q is

5:f

d

200

0

40

Shear rate (s-l )

40

80

120

Shear rate (s-l )

120

180

200

160

200

Shear rate (d’ )

60

0

80

160

200

Rotation speed 20 rpm

0

40

80

120

Shear rate (s-l )

Fig. 3. Typical effects of processing conditions on flow curves of starch suspensions (base levels of test factors except when used as variable: starch concentration, 4%; headspace, 16/32 inch; retort temperature, 120°C; rotation speed, 10 rpm; processing cycle, 1).

cycle interactions were significant with both m and q ratios. In addition, interactions of rotation speed with temperature and processing cycles were significant with m value ratio while interactions of starch concentration with processing cycle were significant with r] value ratio. With respect to n value ratio, only temperature and repeated processing, and their interaction were significant (p< 0.05).

H. S. Ramaswamy et al.

448

2

4

Initial m value (Pa8 ) 0.9

o

l

0.8-

Initial n value

E ‘6 0.4 8 5

0.3 -

5 zB 0.2 -

a p O.lE 3 1 0, II: 0 Fig. 4.

Rheological

0.2 0.3 Initial Apparent Viscosity (Pas)

0.1 parameters

of starch

0.4

0.5

suspensions as influenced by processing

(pooled data).

Figures 5 and 6 show the changes in m and 7 value ratios as influenced by various factors. They both remained higher than unity indicating structure build-up, but decreased with repeated processing. The associated changes in the ratios were more pronounced in the first processing cycle and with the 3% starch. Temperature and rotation speed contributed significantly to structure

Rheology of gelatinized starch

TABLE 2 Analysis of Variance of Factors Influencing Rheological Processing Source of variation (degrees of freedom)

m Value ratio

449

Parameters

n Value ratio

During Rotational Viscosity ratio

F ratio

Sig. level

F ratio

Sig. level

F ratio

Main effects (8)

17.01

0~000

3.71

0.00 1

24.00

Headspace ( 1) Starch cont. (1) Temperature (2) Processing cycle (2) Rotation (2)

0.59 5.73 28.45 30.57 5.89

0.455 0.019 0.000 0~000 0.004

0.21 0.85 4.10 8.92 1.29

0.650 0.370 0.020 0.000 0.282

0.15 18.00 40.87 44.86 1.21

0.703 0~000 0~000 0.000 0.306

Interactions

11.07

0.000

1.62

0.057

6.42

0~000

0.83 0.50 1.49 0.43 1.55 1.17 1.81 22.78 3.44 4.58

0.376 0.6 10 0.233 0.647 0.220 0.352 0.172 0.000 0.012 0.002

0.30 0.00 0.46 0.00 0.66 0.93 2.43 5.07 1.39 1.37

0.590 0.997 0.634 0.998 0.522 0.400 0.09 5 0.00 1 0.247 0.253

0.16 0.06 1.74 0.06 4.14 IO.18 0.08 30.82 0.79 0.32

0696 0.942 0.182 094 1 0.020 0.000 0.924 0.000 0.538 0.863

(25)

Headspace-cont. (1) Headspace-temp. (2) Headspace-pr. cycle (2) Headspace-rotation (2) Concentration-temp. (2) Concentration-pr. cycle (2) Concentration-rotation (2) Temperature-pr. cycle (4) Temperature-rotation (4) Rotation-pr. cycle (4)

Sig. level

Residual (74) Total (107)

build-up with higher temperatures and higher rotation speeds resulting in higher ratios. Again, the changes were more pronounced with 3% starch. Changes in n value ratios showed somewhat mixed trends (Fig. 7). The ratio remained generally below 1.0 indicating that the n values of test samples after processing were generally lower than the n values of samples prior to processing. Headspace, starch concentration and rotation speed had no significant effect (p> 0.05) on n value changes. Since the y1 value ratio increased (even though it remained below 1.0) with the processing cycle, the first processing cycle had a greater effect than the third. Modeling of rheological changes The apparent although test developed for rotation speed (vi, in Pa s):

viscosity values were found to increase following each run samples were thoroughly pregelatinized. A relationship was the correlation of the heating rate index &) as a function of (rpm), processing temperature ( T ) and initial apparent viscosity

log(f,)=O.5835+2.429X

IO-“T-

1.95 X lo-‘rpm+

1.637qi(R2=0.72)

(7)

H. S. Ramaswamy et al.

450

00 6

16

6

110 120 130 110 120 130

16

Temperature eC)

Headspace (l/32”) ,.

. . I I..

. I..

. , . . . ,I

4

1

1

0

0 123123 No. of Process Cycles

Fig. 5.

10 15 20 10 15 20 Rotation Speed (rpm)

Effect curves of m value ratios as influenced by rotational (combined data from the factorial design).

process parameters

factors plus the cook value (Cv) which is a measure of the of cooking were used as factors in a stepwise multiple regression to the resulting apparent viscosity ( qr). A plot of the predicted vs experiapparent viscosity is shown in Fig. 8 with the following equation describregression equation (both vi and vr in Pa s):

All the above

degree predict mental ing the

qr= - 1.84 x 1O-4 + 2.8 x 1O-6 Cv+ 1=186qi

(R2=0.94)

(8)

Rheology of gelatinized starch

451

.g 2.1 !! .z? 8 1.8 5: ‘5 E $? 1,5 x z 1.2

t., , , , , , ,, , ,,, ,,,,.,

1.1t,....

16

8

8

,j

0.91, , , , , . , . ,

16

110 120 130 110 120 130

Headspace (l/32”)

1

2

3

1

2

Temperature (OC)

3

10 15 20 10 15 20 Rotation Speed (rpm)

No. of Process Cycles

Fig. 6.

. . , . , , ,j

Effect curves of apparent viscosity at the shear rate of 100 s- ’ as influenced rotational process parameters (combined data from the factorial design).

by

CONCLUSIONS changes associated with a canned model food (gelatinized starch) processed in an agitating retort were found to be influenced by all factors studied (retort temperature, rotation speed, can headspace, starch concentration and apparent viscosity), except headspace in the 8/32-16/32 inch range. Rheological

H. S. Ramaswamy et al.

452 102F’

KGF”

3% starch

1

i

1

3% starch

T

QQ-

fz : &xi0 ‘3

.

E !?j 93-

T c

’ 90-

1 871,. ...

, .. 8

. , ., . , 16

Headspace

106

4% starch ...,

8

.., 18

.,-j

82t,

. . . , . : . ,

.

, . . . ,-j

110 120 130 110 120 130 Temperature

(1132”)

(%)

I

4% starch

123123 No. of Process Cycles

Fig. 7.

10

15 20

IO

15

20

Rotation Speed (rpm)

Effect curves of n value ratios as influenced by rotational process parameters (combined data from the factorial design).

Rheology of the gelatinized starch model both before and after processing ias well described by the power law model. Even though the starch suspensions were pregelatinized, the apparent viscosity continued to increase following the processing under all conditions. Heating rates were modeled as a function of retort temperature ( 110-l 3O”C), rotation speed (lo-20 rpm) and product initial apparent viscosity (up to 0.5 Pa s), while the resulting product apparent viscosity was related to the cook value of the process.

453

Rheology of gelatinized starch 0.6 -

1

““I’

I

“l”“l”“l1

“1

I

““I

0.5 -

0.1 -

O-

I I 81 I 0

0.1

I

0.2

I

I

I

I

I,

I,

0.3

,I

I,,

0.4

I

I 0.5

,,

,

,

I

0.6

Predicted apparent viscosity (Pas) Fig. 8.

Plot of observed vs predicted apparent viscosity of the product calculated at the shear rate of 100 s- ’ (pooled data).

Starch is generally added to formulations in the non-gelatinized form and gelation takes place as a result of the processing. The rheological changes as discussed in this study with the pregelatinized starch may therefore appear to be more of academic interest. However, it should be noted that the rheological changes were predictable and had good correlation with the cook value, a process parameter that has been implicated in several studies to be closely associated with quality of thermally processed foods. Hence, the results suggest that processing conditions for optimal quality retention could be modeled based on associated changes in rheological properties of foods especially those containing starch. ACKNOWLEDGEMENTS This research was supported by the Partnership Grant Program from Agriculture Canada, Natural Science and Engineering Research Council of Canada and Industry (Cordon Bleu Int. Ltd.). The authors wish to thank Cordon Bleu Int. Ltd for their cooperation in carrying out the heat penetration studies.

454

H. S. Ramaswamy et al.

REFERENCES Chandarana, D. I., Gavin, A. III & Wheaton, F. W. (1989a). Particle/fluid interface heat transfer under UHT conditions at low particle/fluid relative velocities. J. Food Process Engng, 13, 191-206.

Chandarana, D. I., Gavin, A. III & Wheaton, F. W. (19896). Simulation of parameters for modeling aseptic processing of foods containing particulates. Food Technof., 43 (3), 136.

Clifcorn, L. E., Peterson, G. T., Boyd, J. M. & O’Neil, J. H. (1950). A new principle for agitating in processing of canned foods. Food Technol., 4,450. Dail, R. V. & Steffe, J. F. (1990a). Rheological characterization of cross-linked waxy maize starch solutions under low acid aseptic processing conditions using tube viscometry techniques. 1. Food Sci., 55, 1660-5. Dail, R. V. & Steffe, J. F. (19906). Dilatancy in starch solutions under low acid aseptic processing conditions. J. Food Sci., 55, 1764-5. Eisner, M. (1988). Introduction into the Technique of Rotary Sterilization. Private Author’s Edition. Milwaukee, WI. Harrod, M. (1989a). Modelling of flow properties of starch pastes prepared by different procedures. J. Food Process Engng, 11,257-75. Harrod, M. (19896). Apparent concentration: a method to predict the flow properties of viscous foods for process applications. J. Food Process Engng, 11,277-96. Harrod, M. (1989~). Time-dependent flow behavior of starch pastes, with food process applications. J. Food Process Engng, 11,297-309. Ramaswamy, H. S. & Basak, S. (1991). Rheology of stirred yoghurt. J. Texture Studies, 22,231-41.

Self, K. P., Wilkin, T. J., Morley, M. J. & Bailey, C. (1990). Rheological and heat transfer characteristics of starch-water suspensions during cooking. J. Food Engng, 11, 291-316. Stumbo, C. R. (1973). Thermobacteriology in Food Processing. Second Edition, Academic Press Inc., NY. Teixeira, A. A., Dixon, J. R., Zahradnik, J. W. & Zinsmeister, G. E. (1969). Computer simulation of nutrient retention in thermal processing of conduction heating foods. Food Technol., 20 (3), 34 1.