A method has been developed which predicts relative paste consistency of tomato breeding lines by measuring the viscosity of unconcentrated pulp. A gross viscosity measurement was decided advantageous only after failure of the following partial measures or their combinations: visual selection for "dryness", soluble and insoluble pectic substances, total dry matter, insoluble solids, and serum viscosity.
In preparing samples, any method of preservation is suitable which rapidly (in seconds) inactivates pectic enzymes of freshly broken fruit or which utilizes unbroken fruit. Where a few samples are required for immediate analysis, we slice a known weight of fruit into boiling distilled water, maintain the temperature of the mixture above 190 deg. F, and then bring back to the original weight. For large numbers of samples, 20 oz. of fruit are placed whole in #2.5 cans, put in an exhaust box at 212 deg. F. for 30 minutes, sealed, and cooked for 30 minutes. Large fruits are halved or quartered with a sharp knife and quickly put into the steam in the latter method. Water condenses and dilutes the samples, but this is usually a constant factor. Either method gives evidence of slight pectic degradation and a better preservation technique is being sought.
The enzyme-inactivated samples are juiced in an R.Y.P. "Healthmaster" food juicer. Here, tomatoes are forced down a short cylinder in the top of the machine onto the toothed bottom of a perforated basket turning at 7000 rpm. The basket is fabricated from a flat piece of 20-mesh stainless steel screen. Seeds and skin are thrown off at the top and pulp passes thru the perforations. Air whipped into the juice can be removed by mechinically stirring under vacuum for 5 to 15 minutes.
The gross viscosity of the deaerated juice is measured by timing the flow of a constant volume (19.5 ml), under constant pressure (20 cm of H\2\O), at 77 deg. F. through a 2 mm capillary tube 90 cm long. Time in seconds can be used as a satisfactory measurement for comparative purposes or the "apparent viscosity" in centipoise (cp) can be calculated from the following formula:
Tomato Juice (cp) = 60% sucrose (cp) X
sp. grav. tomato juice time for tomato juice
_____________________ X _____________________
sp. grav. 60% sucrose time for 60% sucrose
Different readings on tho same well-mixed juice vary only 0-5% from the
average reading; duplicates from the same samples show no significant
differences. Paste consistency, as measured by the Bostwick flow test, of 8
different "lines" showed a good relationship to unconcentrated juice viscosity
as measured by the capillary tube.