Effectiveness of the Pressure-Load Meterin Measuring Firmness of Tomato Fruit1

Francis E. Sobotka, Alley E. Watada2 and Robert G. Diener3

West Virginia University, Morgantown

Abstract. The Pressure-Load (P-L) Meter, asm ,11 portable instrument, was as sensitive asan Instion Machine in measuring firmness ofriptsning tomato fruit. Firmness, as measuredwitih the P-L Meter, was significantlycorrelated with changes In water andNaOH-soluble pectic substances of ripeningfruit.

In the last 20 years many types ofinstruments have been developed forme-asuring firmness of fruit. Someinstruments macerate the fruit at thetime of measurement whereas othersleave the fruit intact. The latter typesare desired when fruits are needed forcontinuing studies. One has beendescribed (4), that measures firmness onthe basis of single point compression,without sacrificing the fruit, and many(4>, measure firmness on the basis ofmmltipoint compression. These includedthe Firm-o-meter, (5), McCollumFirmness Meter, and the Asco FirmnessMeter . Perhaps a more accuratem e a s u r e m e n t is o b t a i n e d withi n s t r u m e n t s t h a t are controlledelectronically and have a potentiometricrecorder (8). However these instrumentsare extremely expensive, bulky and notportable. In seeking a portable andsens i t ive ins t rument to measurefirmness, the Pressure-Load (P-L) Meter,which was used for cherry fruits (10),was modified and evaluated for tomato.In this study we compare the P-LFirmness Meter to an Instron and alsothe relationship between firmness, asmeasured with the P-L Meter, andchanges in pectic substances.

The P-L Meter. The instrument (Fig.1) operates on a creep principle inwhich a known weight is applied andthe resulting displacement measured. Itconsists of a reverse reading dial gauge,lowering mechanism, stand and a seriesof assorted probe tips and load weights.The dial gauge, model D81T, wasmodified by the manufacturer (FederalProduc ts Corporation, Providence,Rhode Island) to operate on a creepprinciple by removing the return springfrom the shaft. The shaft is lowered byan adjustable air dashpot loweringmechanism (Ai rpo t Corporation,

1 Received for publication September 20,1971. Published with the approval of theDirector of the West Virginia AgriculturalExperiment Station as Scientific Paper No.1152.

^Present address : Horticulture CropsResearch Branch, MQD, ARS, U. S. Dept. ofAgriculture, Beltsville, Md.3 Divis ion of Resource Management,Agricultural Engineering.

Norwalk, Connecticut). The sampleholder consists of a 4 x 4 inch brickplate fashioned with a 3 inch diamconcave depression suitable for securinglarge or small fruits. The fruit waspositioned with the calyx to stylar planehorizontal to the base of the stand.

In the present studies a 200 g weightand a 1/4-inch radius convex probe tipwere used. The total load weight of theshaft (i.e. the total weight placed on thefruit) was 220 g. To measure firmnessthe probe was lowered and an initialreading ( D j ) of the dial gauge was madeat the moment the probe made contactwith the fruit. To avoid variation in therate of lowering, the probe was elevatedafter it made fruit contact and waslowered under the control of thedashpot lowering mechanism. Thelowering mechanism disengaged whenthe probe came in contact with thefruit. A second reading (D2) was taken15 sec after the lowering mechanismdisengaged.

Firmness was measured as a functionof force and deformation by thefollowing equation:

FK = D i - D 2

ADJUSTABLE AIR DASHPOTLOWERING MECHANISM

DASHPOT DISCONNECTWEIGHT

CAM LOWERING LEVER

FRUIT SEATINGSUPPORT

RUBBER FEET

Fig. 1. Physical description of Pressure-LoadMeter.

where F is the load weight (220 g), Djis the initial dial gauge reading, D2 is thefinal reading and K is the elasticstiffness reading in g/cm. The elasticstiffness reading (K) was used as anindex of firmness.

A comparison of P-L Meter andInstron machine. The Instron machine iscontrolled electronically (9). The probe,which is regulated by a hydraulic unit,moves at a constant speed and measuresresistance by an electronic load cell inseries with the probe. Resistance wasrecorded continually during penetrationof the probe by a potentiometricrecorder. Firmness of the fruit wasdetermined from the force deformationcurve of the sample. A 1/2-inch radiusspherical probe tip was used to measurefirmness.

The P-L Meter was compared withthe Instron Machine by measuring thelargest diam of 500 'Manhattan' tomatofruit with both instruments. Fruitmaturity ranged, according to thecriteria of Workman et al. (12), fromthe mature-green to table-ripe stage.Each category contained 100 fruit.

The P-L Meter and the Instronp e r f o r m e d similarly in measuringrelative changes in compression (Fig. 2).The compression, measured by the P-LMeter, of a light pink and table-ripefruit was 40 and 23%, respectively, ofthat of a mature-green fruit and 41 and25%, respectively, with the Instron. Thecoefficient of correlation between the 2instruments was 0.77 which wassignificant at the 1% level. Thecompression values at different stages ofmaturity differed significantly whenmeasured by either instrument.

The diam of the fruit tested rangedfrom 4.78 to 8.64 cm and averagedapprox 7.24 cm. The coefficient ofcorrelation between the compressionvalues as measured by the P-L Meter andfruit diam was 0.10. Therefore, withinthe diam range studied, P-L Metermeasurements were not affected byfruit size.

The P-L Firmness Meter is aseffective as the Instron Machine formeasuring changes in firmness ofripening tomato fruit. Although theactual firmness reading obtained withboth instruments are not comparable,the relative differences in firmness ofripening fruit measured by bothinstruments were similar. The P-L Meterhas a definite advantage over an InstronMachine or any other hydraulic and/orelectronically controlled machine incost, size and speed of measurement. Aswi th o t h e r i n s t r u m e n t s (3),measurements with the P-L Meter arenot affected by fruit size.

Firmness in relation to changes inpectic substances. The relationship offirmness, as measured with the P-LMeter, to the quantitative changes inpectic substances was studied with

34 HORTSCIENCE, VOL. 7(1). F E B R U A R Y 1972

50

ct 4O

30 -

VJ -s

I"

20

10

P-L METER

INSTRON

MG LPMATURITY

DP

Fig. 2. Firmness of ripening 'Manhattan'tomato fruit as measured with P-L Meterand Instron. MG (mature green), B(breaker), LP (light pink), DP (dark pink),TR (table ripe).

at the mature-green stage to 2543 g/cmat 6 days following the breaker stage,which was a 58% loss in firmness. Theremaining lines lost about 72% of theircompression within the same perio.d oftime.

The water-soluble pectic substancesincreased significantly during ripesningof fruit of all tomato lines (Table fl). Acontinual increase occurred withripening in 'T-3640' and 'West Viriginia63'; whereas a drop was observed afterthe 2nd to 4th day following thebreaker stage in the remaining lines.. Theincrease observed in fruit showingincipient color was significant only with'T-3585'. In other lines, the increasebecame significant after the fruitshowed incipient color.

The pectic substances soluble inammonium oxalate increased withripening up to 2 days following thebreaker stage (data not shown).. Theincrease noted in fruit at the breakerstage was significant in all lines e;xcept' M a n h a t t a n ' . The increase in'Manhatten' became significant aft-er thefruit passed the breaker stage.

The pectic substance soluble in

sodium hydroxide decreased withripening in all lines (Table 1). As notedwith the water-soluble pectic substance,only 'T-3585' had a significantreduction in this fraction as fruitshowed incipient color. In all other linesthe decrease was significant after thefruit passed the breaker stage. Analysisof the pectic substances was continuedat 2-day intervals up to 12 daysfollowing the breaker stage. All linesexcept 'T-3640' showed a continualdecrease in the NaOH-soluble pecticsubstance, although the decrease in theadvanced stages of ripeness (4 and 6days) was not statistically significant forthe 'Manhattan', T-3640' and 'W. Va.63' cultivars.

Loss of firmness is one of theattributes in determining the holdingquality of fruit. Tomato lines in thisstudy differed in firmness and in therate of softening. A larger percentage oforiginal firmness was maintained by'T-3640' which was reported to have avery long shelf life (7). The remaininglines softened rapidly and were reportedto have a shorter shelf life. Perhaps theholding quality of a line can be

Table 1. Firmness and water and sodiuim hydroxide-soluble pectic substances in ripening fruitof 5 tomato lines.

ripening fruit of 5 tomato lines thatdiffered in marketable shelf life (7).Measurements were made on fruitharvested at the mature-green andbreaker stage, and of fruit held 2, 4 and6 days at 22°C following harvest at thebreaker stage. The tomato lines usedwere 'Manhattan', 'West Virginia 63'and 3 T-lines (7) homozygous recesivefor either high pigment (hp), crimson(o£c), or both.

The pectic substances soluble inwater, ammonium oxalate, and sodiumhydroxide were extracted by theprocedures outlines by Rouse andAtkins (11). Water-soluble pecticsubstances have sufficiently highmethoxyl content to be termed pectins(6). Ammonium oxalate-soluble pecticsubstances are the insoluble salts ofpectic acid and the low-methoxylpectinic acids and are called pectatesand pectinates, respectively. The sodiumhydroxide-soluble pectic substancerepresents the protopectin fraction. Thedifferent pectic fractions were analyzedfor galacturonic acid content by thecarbazole-hexuronic acid-sulfuric acidmethod of Dische (2) as modified byAtkins and Rouse (1).

The rate of softening, as measuredwith the P-L Meter, differed amongtomato lines (Table 1). The compressionvalue of 'West Virginia 63' was relativelyhigh (7026 g/cm) at the mature-greenstage but was the lowest (1162 g/cm)among the lines at 6 days following thebreaker stage. This was approx an 82%loss in iirmness. The compression value°f 'T-3640' decreased from 6063 g/cm

MaturityNo. of days after breaker stage

Tomatoline

MatureGreen

0 2 4 6(--light pink-) (dark pink)

FirmnessManhattanT-3640 (Tip ogC)W. Va. 63T-3624 |7ipJT-3585 (ogC)

4888 az

6063 a7026 a7229 a6198 a

Pectic

4550 b5909 a4664 b4489 b4694 b

subistances (^M

2751 c4181 b2503 c2828 c2828 c

d-glacturonic

1461 d2693 c178S d2596 c2241 d

acid/lOOg fresh wt)

1386 d2543 c1162 e1995 d1735 e

Water solubleManhattanT-3640 (hp of)W. Va. 63T-3624 (hp)T-3585 (og^

78 a65 a81 a47 a54 a

515 a154 a139 a139 a242 b

324 b378 b349 b368 b407 d

387 b399 b357 b498 c338 cd

377 b469 c438 b410 b270 be

Soluble in sodium hydroxide

ManhattanT-3540 (ftp ogCjW. Va 63T-3624 (hp)

T-35&S(ogC)

439 a488 a366 a526 a486 a

420 a405 a371 a528 a270 b

180 b188 b1 12 b249 b149 c

132 b138b65 b

2OO c89 c

89 b118 b72 b78 d74 c

zMeans within each row followed b^y the same letter are not significantly different at the 5%level (Duncan's New Multiple Range test).

Table 2. Coeffiecient of correlatioms between firmness and pectic substances in fruit of 5tomato lines.

Coefficient of correlation

W^terPectic substance soluble in:

(NH4)2C204 NaOH

ManhattanT-3640 (hp og°)W. Va. 63T-3624 (hp)

T-3585 (agC)

.9 .19*

.8-46

.9 16*

.8*41*

.9 18*

.768*

.684

.662

.654

.775*

-.921*-.833*

-.836*-.750*

-.962*

I

*Significant at the 5%

HORTSCIENCE. VOL. 7(1), FEBRUARY 1972 35

estimated by measu r ing the ra te ofsoftenin* wi th the P-L Meter .

P o e t i c substances may not becompletely responsible for f i rmness :never theless , so f t en ing of the f r u i t hasbeen associated with changes in pecticsubstances. The relationship of f i rmness ,as measured w i th the P-L Me te r , wi thchanges in various pectic f rac t ions werecompared and found similar in all lines.Firmness was negatively correlated wi thc h a n g e s in water soluble pecticsubstances (Table 2). The coefficients ofcorrelation between firmness and thea m m o n i u m oxalate-soluble f r ac t ionwere significant only with 'Manhat tan 'and 'T-3585'. Changes in f irmness werecorrelated significantly with pecticsubstances soluble in NaOH. Thecoefficient values ranged from 0.75 forT-3624' to 0.96 for 'T-3585'. Thus,using changes in pectic substances as acriteria for sof tening, the P-L Meter waseffective in measuring firmness.

Li tera ture Cited

1 . A t k i n s , C. U. and A. H. Rouse. 1953 . Theeffect of d i f f e r e n t m e t h o d s of juicee x t r a c t i o n on the pect ic con ten t ofValencia orange juice. Proc. Flu. StateHort. Soc. 66:289-292.

2. Dische , 'L. 1947. A new specific colorreact ion of hexuron ic acids. J. Biol.Client. 167:189-198.

3. Gar re t t . A. W., N. W. Desrosier. G. D.K u h n and M. L. Fields. 1960. Eva lua t ionof ins t ruments to measure f i rmness oftomatoes. Food Tech. 14:562-564.

4. Hamson, A. R. 1952. Measu r ing firmnessof tomatoes in a breeding program. Proc.Amer. Soc. Hort. Sci. 60:425-433.

5. Ra t t an . A. A. 1 957. Changes in color andfirmness during r ipening of detachedtomatoes, and the use of a newins t rument for measuring firmness. Proc.Amer. Soc. Hort. Sci. 70:374-384.

6. Kertesz, Z. I., G. L. Baker , G. H. Joseph,H. H. Mattern and Aksel G. Olsen. 1944.Report of the Committee tor the revisionof nomenclature of pectic substances.Chem. Eng. News 22 :1 05-1 06.

7. Lampe, C., and AJley E. Vv'atada. 1971.Post harvest q u a l i t y of h igh pigment andcrimson tomato f r u i t . Proc. Ainer. Soc.Hort. Sci. 96:534-535.

8. Mohsen in , N. N. and H. Goel ich. 1962.T e c h n i q u e f o r d e t e r m i n a t i o n o fmechan ica l properties of f ru i t s andvegetables as related to design andprocessing m a c h i n e r y . J. Agr. Eng. Res.7:300-315.

9. Ourecky , D. K. and M. C. Bourne . 1968.Measurement of strawberry tex ture withan Ins t ron M a c h i n e . Proc. Amer. Soc.Hort. Sci. 93:317-325.

10. Parker , R. E., J. H. Levin and R. T.Whit tenberger . 1966. An i n s t r u m e n t formeasuring firmness of red tart cherries..Midi. Agr. Expt. Sta. Quart. Bui.48:471-482.

11. Rouse, A. H. and C. D. Atkins . 195S.Pectinesterase and pectin in commercialc i t rus juices as determined by methodsused at the c i t rus exper iment s tat ion. Fla.Agr. Expt. Sta. Bui. 570.

12. Workman. M., H. K. Pratt and L. L.Morris. 1957. Studies on the physiologyof tomato f ru i t . I. Respiration andripening behavior at 20 degrees C. asrelated to date of harvest. Proc. Amer.Soc. Hort. Sci. 69:352-365.