| | Introduction
Pre-cooling is the key component in the preservation of quality for perishable fresh produce in post-harvest systems. Pre-cooling is also very closely linked to the other operations such as handling and storage.
The practice of pre-cooling fruits and vegetables after harvest has existed for many years. In that time, several methods and techniques of pre-cooling were developed, primarily to meet requirements of the large producers and markets in the United States and Europe.
Pre-cooling
Interest in pre-cooling fresh fruits and vegetables is increasing in Alberta. These products need an improved shelf life and must be of high quality to enter and compete in retail markets. Many Alberta market gardeners, however, do not pre-cool their produce. Often, they do not pre-cool because of the initial costs involved, the problem in deciding which pre-cooling method to use and not knowing how to effectively set up a pre-cooling process.
This publication discusses the various cooling technologies and the basis for selecting and setting up systems that will meet the needs of a wide range of market gardeners.
What is pre-cooling?
Pre-cooling is the rapid removal of heat from freshly harvested produce. This process is typically done before the produce is shipped to market or put into cold storage.
Although produce may be pre-cooled in a cold storage facility, pre-cooling differs from cold storage. In cold storage, the temperature is simply maintained at a predetermined low temperature. If the cold storage facility is to double as a pre-cooling facility, higher refrigeration capacity is required as well as appropriate provisions for pre-cooling and handling of the produce.
Why pre-cool fresh produce?
Fresh produce starts to deteriorate immediately following harvest. Respiration due to enzymatic oxidation in the growing produce continues after harvest. This process results in the consumption of sugars, starches and moisture without replenishment by the plant.
Carbon dioxide and other gases along with heat are generated in the process. If the heat is not removed, the process is accelerated. Growth of molds and the loss of moisture from the produce are also accelerated by heat.
Bruising of the produce further accelerates these processes, resulting in the loss of texture, firmness, colour, flavour and appearance. In addition, some nutritional value may also be lost. When these losses occur, the produce is generally considered to have lost its freshness and quality.
Rapid lowering of produce temperature and then maintaining it at a constant low temperature minimizes the enzymatic and other processes that cause these losses. Pre-cooling as quickly as practical is therefore a very important requirement for maintaining optimum produce quality, especially for those types with naturally high respiration rates. Other benefits resulting from pre-cooling fruits and vegetables include the following:
- minimized production losses
Weather variability during the harvest season can cause produce to mature earlier or later than planned. If the produce is not harvested at the optimum time, losses will occur. Late harvesting may also occur if market opportunities are not available at the desired time. Pre-cooling and cold storage are valuable tools that better allow produce to be harvested on time and sold when markets become available.
- improved economics of harvest operations
The daily harvest may be increased with the assurance that produce quality will be preserved. This confidence permits harvesting to be done over a longer period, thus spreading out workloads. Daily harvest hours could also be extended because the effect of ambient temperatures on the produce would be lessened. This flexibility results in better use of equipment and personnel.
- minimized losses during marketing
Some types of produce spoil very quickly if not pre-cooled immediately after harvest. These types have to be sold within a day or two of harvesting to be of acceptable quality. Strawberries are a good example. Other examples are leafy vegetables that require trimming to maintain a fresh appearance. Pre-cooling extends the shelf life and thus the opportunity for sale before the produce is no longer marketable. See Table 1.
- improved utilization by consumer
Consumers can be supplied with top quality produce with a longer shelf life through pre-cooling. This extended shelf life lessens the urgency to consume or process produce quickly after it is purchased. Consumers are more likely to make larger purchases, enjoy lower handling costs and have more timely product utilization.
- expanded market opportunities
Retail marketers require produce of the highest quality that has longest practical shelf life. A highly perishable product that is not pre-cooled, kept cold and handled with care will lose quality and have difficulty competing with a quality imported product. It is also difficult for the lower quality product to meet grading standards. Pre-cooling is, therefore, the key for perishable produce to enter and compete in retail markets.
Factors contributing to spoilage
Different types of fruits and vegetables spoil naturally at different rates. Table I shows the length of time fresh produce (not rapidly pre-cooled) will last in a refrigerator at 4 degrees centigrade.
Rapid pre-cooling and storage at a stable temperature will extend this shelf life for most produce subject to rapid spoilage. For higher storage temperatures, it is a generally accepted rule of thumb that "deterioration of fresh produce doubles for every 10 degrees centigrade above the optimum storage temperature." Types of produce with high spoilage rates are often of higher value than those with low spoilage rates. This aspect makes control of spoilage an especially important factor for them.
Produce such as onions, potatoes and winter squash spoil rapidly if not cured as soon as possible after harvest. Curing at an elevated temperature heals cuts and bruises and forms a tight outer skin that resists further deterioration.
Atmospheres in storages also affect the spoilage rate. Excess carbon dioxide and insufficient oxygen result in rapid deterioration and off flavours. Ethylene produced by some fruits accelerates the ripening of many fruits. It is therefore important to consider the gases that will be associated with the cold storage.
Table 1. Shelf-life of Fruits and Vegetables Refrigerated at 4°C for Best Flavour and Nutrition
| # | Product | Shelf-life | # | Product | Shelf-life |
| 1 | Asparagus | 2 days | 19 | Peas | less than 1 day |
| 2 | Beans (green,wax) | 5 days | 20 | Peppers | 7 days |
| 3 | Beets | 21 - 28 days | 21 | Potatoes (mature) | 9 months (7 - 10°C) |
| 4 | Broccoli | 3 days | 22 | Potatoes (new) | 7 days |
| 5 | Brussel sprouts | 5 days | 23 | Pumpkins | -no data - |
| 6 | Cabbage | 14 days | 24 | Radishes | - no data - |
| 7 | Carrots (mature) | several weeks | 25 | Raspberries | 2 days |
| 8 | Carrots (young) | 14 days | 26 | Rhubarb | 3 days |
| 9 | Cauliflower | 10 days | 27 | Rutabagas | several weeks |
| 10 | Celery | 14 days | 28 | Saskatoons | - no data - |
| 11 | Chinese Vegetables | - no data - | 29 | Spinach | 4 days |
| 12 | Cucumbers | 10 days | 30 | Squash (summer) | 7 days |
| 13 | Kohlrabi | - no data - | 31 | Squash (winter) | several months |
| 14 | Leafy Greens | - no data - | 32 | Strawberries | 2 days |
| 15 | Leeks | - no data - | 33 | Sweet Corn | less than 1 day |
| 16 | Lettuce | 7 days | 34 | Swiss Chard | - no data - |
| 17 | Onions | 28 days | 35 | Tomatoes | 7 days |
| 18 | Parsnips | 28 days | 36 | Zucchini | - no data - |
(Adapted from Publication 1695, Agriculture Canada).
Significant moisture loss affects the appearance and firmness of produce. Table 2 shows the maximum amount of moisture the produce may lose before losing its appearance and firmness. Room cooling, forced air cooling and vacuum cooling all remove some moisture from produce during pre-cooling. The amount of moisture lost will depend on design and operation of the system. More moisture will be lost if the produce is held for extended periods before cooling.
Water used in direct contact with the produce can affect produce quality as well. Rapid deterioration and off flavours can be triggered by water containing iron, high levels of minerals, bacteria and other organic material.
Contaminated water used on fresh produce can also mean that people who consume it become ill. The cleanliness, quality and sanitation of water used in produce facilities is therefore very important.
Table 2. Percentage of Weight (Moisture) Loss from Fruits and Vegetable that Affects Produce Quality after Harvest
| # | Product | Weight Loss (%) | # | Product | Weight Loss
(%) |
| 1 | Asparagus | 8.0 | 19 | Peas | 5 |
| 2 | Beans (broad, runner, snap) | 6.0, 5.0, 41.0 | 20 | Peppers | 7.0 |
| 3 | Beets | 7.0 | 21 | Potatoes (mature) | 7.0 |
| 4 | Broccoli | 4.0 | 22 | Potatoes (new) | 7.0 |
| 5 | Brussel sprouts | 8.0 | 23 | Pumpkins | - no data - |
| 6 | Cabbage | 8.0 | 24 | Radishes | - no data - |
| 7 | Carrots (mature) | 8.0 | 25 | Raspberries | 6.0 |
| 8 | Carrots (young) | 4.0 | 26 | Rhubarb | 5 |
| 9 | Cauliflower | 7.0 | 27 | Rutabagas | - no data - |
| 10 | Celery | 10.0 | 28 | Saskatoons | - no data - |
| 11 | Chinese Vegetables | - no data - | 29 | Spinach | 3.0 |
| 12 | Cucumbers | 5 | 30 | Squash (summer) | 23.9 |
| 13 | Kohlrabi | - no data - | 31 | Squash (winter) | - no data - |
| 14 | Leafy Greens | - no data - | 32 | Strawberries | - no data - |
| 15 | Leeks | 7.0 | 33 | Sweet Corn | 7.0 |
| 16 | Lettuce | 3.7 | 34 | Swiss Chard | - no data - |
| 17 | Onions | 10.0 | 35 | Tomatoes | 7.0 |
| 18 | Parsnips | 7.0 | 36 | Zucchini | - no data - |
(Adapted from Postharvest Physiology of Perishable Plant Products by Kays, S.J., 1991).
In preparing fresh produce for long term storage, remove free surface moisture on the product. This moisture can support the growth of spoilage bacteria. This situation is particularly true in bruised and cut areas of produce such as root crops. If mechanical damage is minimized and the tops are removed with a clean cut, the removal of free moisture is easier.
The spoilage rate of produce and its contributing factors are the most important considerations when selecting the type of pre-cooling and handling system to be used.
Source: Agdex 736-14. 2002. |
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