G1603

Hydrocooling Peaches Food Processing for Entrepreneurs Series

Principles of commercial hydrocooling can be used by small commercial growers or farmers in areas where large-scale refrigerated storage and shipping is not available.

Durward Smith, Extension Food Processing Specialist; Carol Ringenberg, Extension Educator; and Erik Olson, Manager, Arbor Day Farm

Hydrocooling has for several decades been the method of choice to rapidly remove the field heat from produce before it is placed in a cold room or shipped. Hydrocooling is a more rapid method of achieving refrigerated temperatures than air-chilling thus produce quality is enhanced especially when bulk handling or pallet handling systems are used. With hydrocooling riper produce may be picked and shipped to supply the consumer with better quality produce. The following information describes research done on an international assignment to aid peach growers in a less-developed region, and which may be of value to small commercial growers. Various arrangements of water immersion or drench can be used to accomplish hydrocooling.

Advantages

Pre-cooling with low temperature water is rapid, can be continuous and is compatible with packing shed line operations. To obtain the full benefits of hydrocooling the fruit must be sufficiently cooled. Cooling peaches before shipping or storage results in lower fruit temperatures during transit. This slows the growth of rot-causing organisms. Cooling also retards respiration and ripening of the fruit. To further protect against rot, a very small amount of chlorine may be added to the cooling water (Use caution as excess chlorine will harm the fruit.)

Direct benefits of hydrocooling of peaches include: 1) greatly reduced losses from decay, 2) less bruising, 3) decreased shrinkage from evaporation and 4) lowered refrigeration loads for refrigerated rooms or trailers.

Hydrocooling permits picking somewhat riper fruit; in turn ripening time and decay at destination are reduced, resulting in improved color and flavor. Hydrocooling thus results in lower shipping and marketing losses and a better quality product for the consumer. All of these benefits should be reflected in more favorable prices and increased returns to the grower.

Research on Hydrocooling

For the process to yield the advantages, the peaches must be adequately cooled. A temperature of 57°F. or lower at pit depth is desired. This corresponds to a fruit temperature of about 50°F as measured with a thermometer. Usually peaches are not cooled to this extent by conventional hydrocooling operations. The time in the cooler is generally too short for this amount of cooling due to low heat exchange between fruit and the cooling water. During rush season and/or very hot weather, the fruit may be insuffciently cooled.

Research on hydrocooling was done on methods of increasing the cooling capacities of small and/or low-technology peach hydrocoolers by improving the efficiency of heat exchange between fruit surface and cooling water. Excellent results were obtained from the use of a wetting agent either in the hydrocooler water or as a pre-dip before cooling. The effect of the wetting agent was 1) to wet the entire surface of the peach through the fuzz and 2) to keep the flowing water in direct contact with the fruit surface throughout the cooling period. Under normal conditions air bubbles trapped in the peach fuzz inhibit contact of the cooling water with the peach surface and insulate the peach. Without the wetting agent, the peach had a tendency to shed water much like an umbrella. Also, fruit in the middle of a basket cooled more slowly than did fruit at the top or bottom of the pack. Use of the wetting agent eliminated this irregular cooling and resulted in much faster cooling throughout the basket. Results of some of the tests are given in Table I.

Some detergent wetting agents used in the hydrocooler water caused considerable foaming while other non-foaming wetting agents left a noticeable white spot residue on the fruit. These problems were solved by using the wetting agent as a pre-dip before hydrocooling. In tests conducted with a small, commercial, cascading water peach hydrocooler, the wetting agent pre-dip increased cooling capacity of the hydrocooler by from 40 to 60 percent depending on the load and its initial temperature. Further increases would be attainable by lowering the temperature of the cooling water. These increases in capacities could be used to great advantage in obtaining adequate cooling of the present volume of fruit and/or handling increased volumes from expanded acreages.

In experimental hydrocooling, thermocouples were temperatures in several different positions within a peach. used for recording temperatures at different depths within Temperatures, accurate to one-tenth degree F were read at the peach and at different positions in the basket. Thermo-two-minute intervals at the exact position of the small thercouples, because of their small size, can accurately measure mocouple junctions.

Table I. Effect of wetting agent in pre-dip and of different hydrocooler rates on temperature decrease of Elberta peaches.
Water g.p.m./ sq. ft. Beginning Temperature Wetting Agent1 Temperature decrease Pit depth2 Minutes Cooled Increased cooling From pre-dip3 Minutes Cooled
Deg F
10 Deg F
15 Deg F
20 Deg F
10 Pct.
15 Pct.
20 Pct.
5
84.6
None
4.4
8.0
13.2
—
—
—
Pre-Dip
11.4
19.4
26.4
160
143
100
10
83.9
None
5.8
12.1
18.8
—
—
—
Pre-Dip
9.4
18.2
26.4
62
51
41
15
81.4
None
6.8
14.3
21.4
—
—
—
Pre-Dip
10.9
19.2
26.6
58
34
24
20
83.0
None
7.1
14.1
20.7
—
—
—
Pre-Dip
10.0
18.7
26.6
41
33
29
30
79.7
None
10.8
17.9
24.5
—
—
—
Pre-Dip
14.6
22.9
29.7
35
28
21
1Submerged 30 seconds, 5000 p.p.m. of wetting agent.
2Firm-ripe fruit, 2 5/8 in. average. Pit depth temperatures at top, middle and bottom.
3Percentage of increase in amount of cooling as a result of pre-dip.


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Index: Food & Nutrition
Safety
Issued January 2006

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