Peaches Treated By Immersion

Press, Volume CIV, Issue 30936, 17 December 1965, Page 9

Peaches Treated By Immersion

A method of controlling brown rot in peaches after harvesting, developed by the Plant Diseases Division of the Department of Scientific and Industrial Research, may improve the efficiency and output of the peachcanning industry. Experiments by the division have shown that if freshly picked peaches are pasteurised by immersion in water at a temperature of 120 deg F. for 10 minutes, spores of the brown rot fungus are killed, both on the skin of the fruit and in the sub-surface tissues. Golden Queen peaches thus treated have been kept in cool storage for more than three weeks with practically no loss from brown rot, whereas untreated peaches from the same lines had almost all rotted within seven days. TESTS ENCOURAGING

This pasteurisation technique was tried out on a small scale by some canneries last season. The results were encouraging, and more extensive trials will be conducted in the coming season to test the suitability of the technique for large-scale operation. Hitherto, brown rot has been a major problem in the peach canning industry. The danger of losses has been such that the fruit has had to be processed very promptly after picking. This, and the short harvesting season, have put heavy pressure on cannery facilities.

If some of the crop could be cool-stored and the flow of fruit to the canneries spread over a longer period, the efficiency of processing could be improved, and the reduced wastage through brown rot would result in a larger output of canned peaches.

Commercial peach growers achieve control of brown rot in the pre-harvest stages by a spraying programme. During the blossoming period, trees are sprayed as often as twice a week with a fungicide based on dichlone. Then, during the last month before harvesting, several sprayings with a fungicide such as captan are applied to protect the ripening fruit from windblown spores of the fungus. SPORES IN AIR This system gives good control of the fungus while the fruit is still on the trees. Nevertheless, the control is seldom complete, and there are usually some spores drifting about, or some invisible infection on the fruit when it is picked. The result often is that brown rot ruins the crop after harvest if it is not consumed quickly. It is not uncommon for the whole of a picking to be spoiled by brown rot within a week after harvest. The Plant Diseases Division has been concerned about this post-harvest wastage for several years. Tests with a variety of fungicides applied before harvest failed to reveal any which significantly reduced post-harvest infection below present levels. It was evident that a different approach to the problem was required. One new approach, suggested by Australian research work, was studied in New Zealand in the 1961-62 season. This involved harvesting peaches before they had reached a stage of ripeness suitable for growth of the brown rot fungus, and then holding them for ripening at a carefully controlled temperature of 94 to 95deg F. at which the fungus was inactivated. OFF FLAVOUR The method was effective, but there were difficulties both In selecting fruit at the right stage for picking and in

maintaining the critical temperature. Moreover, the fruit tended to develop an off flavour when canned. It was then decided that the possibilities of pasteurisation should be explored. The main problem was to find a combination of time and temperature that would be lethal to the fungus but harmless to the fruit. At first, brief treatments at relatively high temperatures were tried, with the object of achieving rapid pasteurisation of the fruit surface. It soon became evident, however, that infection was not all on the surface. After many tests, it was found that the most effective treatment was a 10-minute immersion in water at 120 deg F. This allowed the flesh temperature half an inch from the surface to rise to about 115 deg F.—sufficient to kill the fungus but not to affect the quality of the fruit.

DIFFICULTIES Mr K. M. Harrow, the scientist in charge of this work, says it is possible that the pasteurisation technique could be applied to peaches supplied to the fresh fruit market, but before this could be done, some difficulties would have to be ironed out. The treatment has no adverse effect on the quality of the fruit, but if captan has been used in pre-harvest spraying, there is a tendency for the water treatment to produce scald marks on the skin. These are only skin-deep and are removed by skinning so they make no difference if the fruit is used for canning. But they would certainly be a drawback in the fresh fruit market. Some alternative preharvest fungicides do not cause this skin scald when the fruit is pasteurised, but further research will be required to determine which would be best to use instead of captan.