SOP: Determination of Gel Strength with the Tarr–Baker Gelometer
GIFSQ — Global Institute for Food Safety & Quality
Principle
The breaking strength of the set jelly is measured as the water pressure required to force a weighted piston through the gel. The pressure at which the jelly breaks, read on a water manometer, is used to determine the grade of the pectin by reference to a nomogram relating breaking strength to the quantity of pectin used.
Apparatus required
- Tarr–Baker Gelometer, consisting of:
- A — 25-litre levelling bottle
- B — 2 stop cocks
- C — 2-litre bottle
- D — 3-way stop cock
- E — piston (a 10-ml Luer syringe with the piston plain cut and round edged; weight of the piston 12 g; diameter of the pressure plate approximately 2.1 cm; the proportion between the area of the piston and that of the pressure plate must be 0.485)
- F — revolving table
- G — manometer (a 1300-mm glass tube with a 100-ml reservoir, half-filled with coloured water)
- H — graduated scale (fixed with a constant zero-point at the middle of the reservoir)
- I — drain
- Capillary tube (for regulating the water stream before stop cock B)
- Copper jelly rings, 35 mm internal diameter and 27 mm height (for production control)
- Glass plate
Reagents
a. Paraffin oil — for lubricating the piston and the inside of the jelly rings.
b. Vaseline — for seating the jelly rings tightly on the glass plate.
c. Alcohol — for moistening the woollen thread used to open jars.
Sample preparation
Allow the sample to set for at least 24 hr before testing.
If the jam or jelly is packed in tins, cut open the bottom of the tin in the usual way and use the tin for measuring the strength. If the product is packed in jars, cut a slot 1 cm from the bottom of the jar with a glazier’s diamond; alternatively, turn the jar bottom up, place a thick woollen thread moistened with alcohol approximately 1 cm from the bottom, set the thread on fire and the bottom will usually separate after the first kindling — the bottom may have to be dipped in cold water after the jar is heated, and if it does not separate, repeat the process.
Procedure — Adjusting the gelometer
Place a firm body under the piston; open the 3-way stop cock to equalize any variation of pressure and close it again
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Open the closure tap B; the pressure begins to rise
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Measure the time required for the manometer fluid to rise to the 300 mm mark
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If more than 60 seconds are required, raise the level of the fluid in bottle C and measure again; repeat until the pressure rises to 300 mm in 60 sec
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The gelometer is now adjusted and may be used for determining the strengths; generally it is not necessary to adjust more than once a day
Procedure — Measuring the gel strength
Place the jam or jelly contained in the can or jar, turned upside down, under the piston E of the gelometer on table F
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Open the 3-way stop cock to equalize any difference in pressure, and close it again
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Open the closure tap B; the pressure begins to rise and the manometer fluid rises slowly until the jelly is broken by the piston
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Note the manometer reading at the breaking point and remove the jelly
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Using the drain I, regulate the fluid level in C to the starting point, until the manometer shows pressure 0
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Test the remaining jellies in the same way
Procedure — Production control with jelly rings
Apply vaseline on the edges of the copper rings so they sit tightly on the glass plate; apply paraffin oil on the inside of the rings to facilitate withdrawing the rings easily
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Fill the rings with jelly and allow to set at 20 ± 2°C for 3 hr
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Place the rings on the platform F and measure the jelly strength as in the main procedure
Calculation
The breaking strength is read directly as mm of water pressure (W.P.) on the manometer. The grade of the pectin is determined from a nomogram relating breaking strength to the quantity of pectin used.
Worked example as given in the source: if the average breaking strength of 5 or 6 readings is 530 mm of water pressure and the quantity of pectin used is 0.5 g, a line drawn connecting 530 mm W.P. with 0.5 g of pectin on the nomogram intersects the scale at point 252, which is the grade of the pectin.
(The source’s nomogram is a manufacturer’s chart and is not reproduced here; its use is as described above.)
Note: the book’s pectin chapter gives other gel-strength methods needing specialized equipment and setting in specified containers; the Tarr–Baker gelometer “could easily be fabricated” and is the factory method described above.
Precautions
- The piston must always be well lubricated with paraffin oil and must never be allowed to adhere; if any jelly sticks to the piston, remove the piston, clean it thoroughly, dry it, lubricate with paraffin oil and insert it again.
- Keep the proportion between the area of the piston and that of the pressure plate at 0.485.
- Regulate the stream of water passing from A to C with the capillary tube before stop cock B for a steady pressure rise.
- After each breaking, bring the manometer back to zero pressure before the next test.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
Genu Pectin, Handbook for Marmalade, Jam and Jelly, manufacturers Københavns Pectinfabrik, Copenhagen, Denmark — the gelometer design and nomogram reference, as cited in the book.