A mechanical thumb for measuring the firmness of fruits

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Marketing Bulletin No. 25
U. S. DEPARTMENT. OF AGRICULTURE

Agricultural Marketing Service
Market Quality Research Division

A MECHANICAL THUMB FOR MEASURING
FIRMNESS OF FRUITS

H. A. Schomer, K. L. Olsen, and J. N. Yeatman
Horticultural Crops Branch
Agricultural Marketing Service

Scientists of the Agricultural Marketing Service have devised a ''mechan-
ical thumb" with which fruit inspectors can measure objectively the firmness
of the flesh of fruits, without cutting or discarding the samples. It is used as
an attachment on the Magness-Taylor pressure tester by substituting it for
the regular plunger at the end of the tester shaft (fig. 1).

The ''thumb'! consists of a plunger 1/2-inch in diameter, with a slightly
convex end, encased ina close-fitting but free-sliding sleeve (fig. 2, p. 4).
The plunger does not penetrate the fruit flesh, but, as it is pressed against
the fruit, the sleeve is forced back as the plunger indents the fruit to a pre-
set distance. At this point, an electrical contact is made which lights a small
attached flashlight, and the firmness is read on the Magness-Taylor tester.
These testers are available with scales reading either to 10 pounds or 30
pounds pressure, depending on the sensitivity desired and the product being
tested. A tester reading to 30 pounds is used for apples.

Material components:

Suggested material components shown in the scale drawing (fig. 2) are as
follows, or as otherwise indicated in the drawing:

—
e

Plunger and housing of stainless steel or aluminum.
Sleeve of brass or synthetic material.

Spring wire of .032-inch diameter, 5 revolutions, steel.

ete oes,

All components easily accessible for purposes of maintenance and
ease of calibration.

Specific requirements:

1. The plunger head is a segment of a l-inch sphere. The actual height
it extends above the sliding sleeve is not critical. It could actually be

Washington, D. C. 2 May 1963

BN-18679

Figure 1.--**Mechanical thumb’ fruit firmness tester.

down inside the sleeve or extend out to nearly the depth that the
plunger depresses the fruit tissue. It is critical that the head be uni-
form and that the instrument is set so that the plunger will depress
the fruit tissue to a uniform depth, in the case of apples 0.055 inch.}!

2. The calibration adjusting screws should allow for at least .045-inch
sleeve movement.

3. Exact specifications for the flashlight are not critical. All that is re-
quired is an indicator light which will go on when electrical contact
is made in the ''thumb'"!' accessory.

Directions for use:

To give accurate readings, the tester must be used carefully. It must be
held at a right angle to the surface of the product; otherwise erroneous read-
ings will be obtained. The product, such as an apple, may be held in one hand

1Schomer,H.A., and Olsen, K. L. A Mechanical Thumb for Determining Firmness of Apples. ASHS, Vol. 81,
pp. 61-66, 1962.

END OF PRESSURE TESTER SHAFT

aN
; ! 625 DIA
Sean a | a WIRE TO FLASH LIGHT
STRAIGHT Gees a FIBER WASHER
KNURL SSL

1.250 DIA.

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1290 1D.

Wa FESS WASHER
1.375 0.0.

CONTACT PLATE
(ELECT)

785 032 DIA
SPRING WIRE—>
5 REV. (STEEL)

LOCK NUT

4
Lil |

LONGITUDINAL SECTION 067
aj
PLUNGER oF 8
785
ie x x2 SLOT CLEARANCE FOR PIN
nile 180° FROM SCREW HOLES
fe 1.125->4
124 DIA —-»| NO. 2 SCREW
t I
i DIA. PIN 12 LONG
650 a
SLEEVE 1
| be—.500 1.D.
END VIEW MECHANICAL THUMB

% ALL MATERIALS ALUMINUM UNLESS INDICATED

3 3, 3 U
7 7g AE 3 Lr ig x |4 THD.
37g0.90. -Yeld

FEE AE EEETTETETEPRIETTTTTEFTTT

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A |
KNURLED bs LB NUT 40.D. INSULATOR
3g x16 THD.
(BRASS) \FIBER BATTERY

INSULATOR: 2 x20

LIGHT INDICATOR AND BATTERY CASE
FOR MECHANICAL THUMB

Figure 2.--Scale drawing of ‘‘mechanical thumb’® attachment to Magness-Taylor pressure tester.

and the pressure applied with the tester in the other hand. However, more
uniform results are obtained when the tester is clamped to a bench or table.
Another method is to hold the product rigidly against a stationary surface
with application of the ''mechanical thumb"! perpendicular to the product sur-
face, as shown in figure 1. Pressure should be applied steadily and slowly.

4

* U.S. GOVERNMENT PRINTING OFFICE : 1963 O - 687196