EVALUATION OF THE VOLAND-STEVENS LFRA TEXTURE ANALYZER FOR MEASURING THE STRENGTH OF PECTIN SUGAR JELLIES

STEVAN A. ANGALET

General Foods Corporation Technical Center

Cranbury, New Jersey 08512

Manuscript received July 18, 1984 Accepted for publication December 12, 1985

ABSTRACT

The Voland-Stevens LFRA texture anal.yzer was compared to the Tarr-Baker gelometer for its applicability to measuring jelly strength. Both instruments showed a linear relationship between jelly break strength and pectin content. The test results from the two instruments were highly correlated (r = 0.94). The coefficient of variation relative to the overall average of measurement was 5.0% for the Voland-Stevens LFRA and 8.4% for the Tarr-Baker gel- ometer. The Voland-Stevens LFRA instrument may be equal to or better than the Tarr-Baker gelometer for measuring jelly break strength. It offers the advantages of reduced errors in sample preparation and ease of instrument calibration.

INTRODUCTION

The annual world production of pectin is now approximately 32 million pounds. The pectin industry serves mainly the needs of the jam and jelly manufacturers, who in the United States pro- duced 720 million pounds of products in 1977 (Anon 1984).

Journal of Texture Studies 17 (19861 87-96. Al l Rights Reserved. (GCop-yright 1986 buy Food & Nutrition Press, Inc., Westport, Connecticut. 87

88 S. A. ANGALET

Jellying power is the most important property of pectin. It is measured by testing gels prepared under standard conditions. The results are dependent on the jellying strength of the pectin and on the manner of preparation of the jellies (Doesburg 1965). The strength and quality of pectin-sugar jellies depend on six factors which may be adjusted within limits: pectin content, solids content, pH value, amount and kind of metal ions present, jelly process used, and size of the container (Bender 1949).

Instrumental methods for testing pectin gels can be divided into two groups (Christensen 1954; Szczesniak 1975; and Mitchell 1976). The first group consists of methods which measure sag without exceeding the elastic limits of the gels. The best known member of this group is the Ridgelimeter described by Cox and Higby (1944). The second group consists of methods in which the elasticity of the gels are exceeded. The Tarr-Baker gelometer and the Voland-Stevens LFRA are examples of this group.

The Tarr-Baker gelometer, also named- the Delaware jelly strength tester, was developed by Tarr and Baker (Baker 1926 and Tarr 1926). It measures the force necessary to drive a plunger into the jelly. The apparatus was improved by Baker (1938) and modified by Bender (1949). Bender noted that slices as thin as 5 mm or as thick as 8 mm produced results different from the stan- dard 6.5 mm thickness results.

The Tarr-Baker gelometer was commercially available from Arthur H. Thomas, Philadelphia, until it was discontinued in 1965 (Anon 1982). Since this instrument is no longer available, an in- vestigation was undertaken to replace the Tam-Baker apparatus with the Voland-Stevens LFRA Texture analyzer (Voland Corp., Hawthorne, New York), which was developed by the Leatherhead Food Research Association in England at the request of the British gels & thickeners panel (Marrs et al. 1980).

The Voland-Stevens LFRA texture analyzer appears a good al- ternative to the Tarr-Baker gelometer for the quality control of pectin and jelly manufacture. Its appealing features are compact- ness, operational similarity to other texture measuring instru- ments, portability, ease of use and calibration.

The two instruments are shown in Fig. 1.

MATERIALS AND METHODS

The experimental design called for one inswument operator, four concentrations of pectin, with five replications for each concen- tration. Each replication was evaluated by both instruments.

VOLAND-STEVENS LFRA TEXTURE ANALYZER 89

FIG

. 1. P

HO

TO

GR

APH

S O

F T

HE

IN

STR

UM

EN

TS

Left: T

arr-

Bake

r G

elom

eter

; Rig

ht: V

olan

d-St

even

s L

FRA

90 S. A. ANGALET

The rapid set dry pectin used in all experiments was a com- merical product having the following analytical values:

IFT Sag Grade

Degree of methylation 72.4

229 “as is” 246 “dry basis”

Different levels of pectin were combined with dextrose and fumaric acid in the proportions shown in Table 1 to obtain the fruit pectin dry mixes. Each sample, sufficient for a single jelly recipe, was individually weighed and mixed manually.

TABLE 1. Composition of fruit pectin dry mixes (gram)

Sample Number

I n g r e d i e n t s 1 2 3 4

Dextrose 42 .62 4 2 . 2 1 4 1 . 9 2 41 .22

P e c t i n 3 .99 4 .34 4 . 6 9 5 .39

Pumaric Acid 3.00 3.00 3.00 3 .00

The jellies were prepared by diluting 207 g of synthetic strawberry juice (Table 2) with 520 g of distilled water, adding 49.6 g of fruit pectin mix and stirring until dissolved. The mix- ture was brought to a boil and 899.8 g of sucrose was added in two steps while stirring constantly. The mixture was allowed to come to a boil again and cooked, with stirring, until the final weight of 1542.85 f 2.75 g was reached. The foam was skimmed off the top and the liquid was poured into jelly glasses.

The jellies were cooled at ambient temperature (21 “c) for 5 min before being covered and allowed to set at 20 “c for 18-24 h. The soluble solids of all jellies were 63.5% and the pH was 3.00.

Tam-Baker Gelometer Four slices of jelly were evaluated from each glass container.

The jelly was gently slid out of the container onto a sheet of 20 lb substance plain duplicator paper without disturbing the struc- ture. The bottom 5 mm of jelly was sliced off with a metal spatula

VOLAND-STEVENS LFRA TEXTURE ANALYZER 91

and discarded. Four successive 6.5 .5 mm thick jelly slices were cut with the spatula and transferred to a 12.7 x 12.7 cm square

TABLE 2. Composition of synthetic strawberry juice ~

I n g r e d i e n t s Grams

Sodium S a l i c y l a t e 0.96

S a l i c y l i c Acid 0-63

___

Tr ipo ta s s ium Phosphate , Anhydrous

Tr ipo ta s s ium C i t r a t e , nonohydrate

11.50

47.10

Trisodium Ci t ra te , Dihydrate 6.10

C i t r i c A c i d , Anhydrous

i4agnesium Ace ta t e , Te t r ahydra t e

Tr i ca l c ium Ci t ra te , Te t r ahydra t e

D i s t i l l e d Water

186 .90

15.20

1 1 . 4 0

2720 .21

TOTAL 3000.00

pH = 3.10 ;t 0.02

Solids = 8.6 f 0.02% ( b y r e f r a c t i v e index) ~-

of paper. The remaining jelly was used for pH and solids analysis. The average jelly break strength for a cook was determined by evaluating slices from two glasses. The jelly break strength of each pectin concentration was averaged from five replication cooks.

Voland-Stevens LFRA Texture Analyzer The tape was removed from each glass container of jelly and

the jelly surface leveled flush with the rim using a wire cheese cutter or a sharp nonstick knife. The glass was placed on the in- strument’s sample support platform and the maximum force re- quired to break the jelly surface with a 2.54 cm diam AOAC probe was recorded.

The instrument conditions are shown in Table 3. The jelly break strength was determined as the peak height on the recorder trace. The average jelly break strength for a cook was determined by evaluating four glasses. The jelly break strength for each pectin concentration was the average from the five replication cooks.

92 S. A. ANGALET

TABLE 3. Comparison of test conditions for the two instruments

Tarr-Baker Voland-Stevens LPRA

Glass, Inside Dimensions

Height 7.5 cm 8.0 cm

Bottom Diameter 3.3 cm 4.1 cm

Top Diameter 5.3 cm 6.2 cm

Probe, Type

Slice, 6.5 5 .5 F u l l Glass Thick

Flat Plunger, Flat Plunger 1.90 cm Dia 2.56 cm Dia

Compression Speed 0.082 mm/s 1.0 mm/s

Depth of Penetration 4 m m 15 nun

Chart Speed N/A 20 cm min

RESULTS

Table 4 lists the mean values and standard deviations for each pectin level as measured by each instrument. The results from the two devices for each of the 20 jelly preparations are shown in Fig. 2. The correlation coefficient between each measurement and pectin concentration was r = 0.92 for the Tarr-Baker gel- ometer and r = 0.96 for the Voland-Stevens LFRA.

DISCUSSION

The two instruments measure the jelly strength differently: one uses a 6.5 mm thick slice, the other 80 mm of jelly in a glass con- tainer. However, they have similar levels of precision as measured by the coefficient of variation.

When the data are converted to units of stress (Pascals), the Tarr- Baker gelometer readings were numerically approximately twice those of the Voland-Stevens. This may be explained as follows. The Tarr-Baker gelometer measures the force to rupture the in- ternal jelly structure; the jelly slice can only deform about 4 mm before the internal structure ruptures. The Voland-Stevens LFRA measures the force to break the jelly surface. The jelly surface in the glass deforms approximately 8 mm before the surface

VOLAND-STEVENS LFRA TEXTURE ANALYZER 93

TABLE 4. Actual readings and calculated values

Tarr-Baker 3eloneter Voland-Stevens L E

Pectin Level [,lean (Grams) ( l l=5 )

CII of llater

3 .99 65.2 4.34 74 .5

5 .39 1 0 2 . 6 4.G9 83.8

3.99 4315

4 .69 5374 5.39 6453

4.34 4845

Coefficient Mean of Variation __ (14=5) - I - - - ~

Newtons Force

8.0% 1 . 0 3 2

G . 8 % 1 . 3 7 1 4.7% 1.694

13.7% 1.258

- 8 . 4 a ( l )

Pascal

6. 8% 2040 11.9% 2442

6 .08 2 7 0 1 4 .3% 3344

7 . 4 % ( 1 )

_1_

Coefficient of Variation

2 . 4 8 3.5% 6.7% 4.9%

5 . 0 % ( 1 )

-

2.4% 6.0% 6.9% 5.0%

5 .6%( 1)

~

(1) Coefficients of variation are based upon the average sample variance and not the sim ple average of the sample coefficient of variation.

110- 0

100 - h

I Z

Lz W

5 0 c 100 I20 I40 I60 I80

VOLAND- STEVENS LFRA ( G )

FIG. 2. LINEAR REGRESSION ANALYSIS FOR THE TARR-BAKER AND THE VOLAND-STEVENS LFRA INSTRUMENTS

94 S. A. ANGALET

breaks. This requires less force than to rupture the jelly structure. The 2:l ratio is consistent with literature values. Baker and

Goodwin (1941) measured the surface jelly strength of a standard jelly and reported a value of 50 cm water. Bender (1949) measured the jelly slice strength of a standard jelly and reported a value of 106 cm of water.

The Tarr-Baker gelometer has a potential for operator errors which may include: (1) Instrument calibration - since the plunger is driven by air pressure the compression speed may vary for jellies of different break strengths. (2) Slicing of jelly - correct results are achieved when the cut surface is parallel to the piston sur- face. Otherwise, the force to break the gel is decreased. (3) Thick- ness of the slice - if too thick, the extra gel cushions the piston thus delaying the surface rupture and resulting in a lower reading. If the slice is too thin, the break reflects internal rupture rather than surface break.

The Voland-Stevens LFRA has the following benefits when used to measure the break strength of pectin jellies: (1) Instrument calibration -the instrument is provided with a 1000 gram weight for use in calibrating the load cell and range scale. (2) The Voland- Stevens LFRA has a constant rate of compression. (3) Sample preparation - the operator needs to make only one step in prepar- ing the jelly for evaluation, i.e., a smooth cut to remove the excess jelly above the rim of the glass container. (4) The time to evaluate one Tarr-Baker container is 10 min compared to 1 min for the Voland-Stevens container. The Voland-Stevens LFRA graphs the increasing force as the plunger is penetrating the jelly. The slope of the increasing force is a measure of elasticity (Szczesniak et al. 1963 and Bourne 1976). This value may be compared with the sag values obtained using the ridgelimeter.

CONCLUSION

The data obtained in this study show that the Voland-Stevens LFRA texture analyzer is well suited for measuring jelly break strength.

Further studies employing a greater range of samples are war- ranted to verify the results.

VOLAND-STEVENS LFRA TEXTURE ANALYZER 95

ACKNOWLEDGMENTS

The author wishes to acknowledge the work of Messrs F. L. Wat- son and R. Carlson in sample preparation and data collection, Mr. A. Merlino for photography, Mr. P. Manni for his analysis and statistical interpretation of the data, and Dr. Alina S. Szczesniak for her advice and encouragement in the preparation of this article.

REFERENCES

ANON. 1984. The Almanac. Edward E. Judge & Sons, Inc., Westminster, Maryland.

ANON. 1982. Personal Communication. Arthur H. Thomas Com- pany, Philadelphia.

BAKER, G. L. 1926. Jelly strength of pectin jells. End. & Eng. Chem. 18, 89-93.

BAKER, G. L. 1938. Improved Delaware jelly strength tester. Fruit Prod. J. 17(11), 129.

BAKER, G. L. and GOODWIN, M. W. 1941. Fruit jellies - XI: The role of pectin - 7. Demethylation of pectin and its effects upon jelly properties, Univ. of Delaware Agric. Exp. Sta. Bulletin No. 234.

BENDER, W. A. 1949. Grading pectin in sugar jellies. Anal. Chem. 21, 408-411.

BOURNE, M. C. 1976. Interpretation of force curves from in- strumental texture measurements. In RheoZogy and Texture in Food Quality, (J. M. DeMan, P. W. Voisey, V. F. Rasper and D. W. Stanley, eds.).

COX, R. E. and HIGBY, R. H. 1944. A better way t o determine the jellying power of pectins. Food Inds. 16, 441-442.

CHRISTENSEN, P.E. 1954. Methods of grading pectin in rela- tion to the molecular weight (intrinsic viscosity) of pectin. J. Food Sci. 19, 163-172.

DOESBURG, J. J. 1965. Pectin substances in fresh and preserved fruits and vegetables. Inst. Res. on Storage and Proc. of Hort. Produce, Wageningen, The Netherlands. I.B.V.T. - Com- munication Nr. 25.

MARRS, W. M., PINFOLD, M. J., WEIR, G. S. D. and WHITE, J. A. 1980. The Stevens LFRA texture analyzer. A new instru- ment for gel testing and for evaluation of texture. The Brit. Food Manufact. Ind. Res. Assoc. Technical Circular No. 706.

96 S. A. ANGALET

MITCHELL, J. R. 1976. Rheology of gels. J. Texture Studies 7,

SZCZESNIAK, A. S., BRANDT, M. A. and FRIEDMAN, H. H. 1963. Development of standard rating scales for mechanical parameters of texture and correlation between the objective and sensory methods of texture evaluation. J. Food Sci. 28,397403.

SZCZESNIAK, A. S. 1975. Textural characterization of tempera- ture sensitive foods. J. Texture Studies 6, 139-156.

TARR, L. W. 1926. Univ. of Delaware Agri. Exp. Sta., Bulletin No. 142.

313-339.