Cooling and Storing Lettuce Post-Harvest

Lettuce is one of the world’s most popular leafy green vegetables. It is often consumed on hamburgers and sandwiches or in wraps or soup. Additionally it may be an ingredient in various recipes and dishes. Regardless of its use, quality and freshness is crucially important and in order to achieve maximum freshness the lettuce must be carefully cooled after harvest and handled and stored properly. What follows is a discussion of several pieces of key information about lettuce as well as factors that affect cooling and storage and proper cooling and storage methods.

General Facts About Lettuce

Despite the dramatic difference in appearance as far as what someone is likely to visualize, lettuce is actually a member of the sunflower family. It is harvested before it has a chance to flower, however it does flower it will produce a small yellow blossom on a stalk about 3 feet high. At this point the lettuce is no longer edible. Additionally though it is today one of the most popular agricultural plants it began as a weed that was cultivated and selectively breed to produce better taste and larger leaves.

There are several different cultivars of lettuce in existence. The most popular three are the looseleaf lettuce, the iceberg or crisphead lettuce, and the Romaine or cos lettuce. In addition to its popular use as a leafy green, lettuce has historically also be grown for its seeds and oil. In 2010 alone about 23,620,000 metric tons of lettuce was grown, over half of which came from China. Nutritionally it is a good source of potassium and vitamin A.

General Information About Cooling and Storing Lettuce

Proper cooling, storage, and handling of lettuce will depend on the particular cultivar. For example Romaine lettuce that has been cut or broken will discolor more rapidly than iceberg lettuce. Likewise even among the same cultivars, the density of the heads can also affect cooling, storage, and shelf life. However, despite all this there are some major similarities between cultivars and heads and some general factors to bear in mind when cooling and storing lettuce.

Temperature – Temperature has an extremely significant impact on product quality and shelf life. It is estimated that by reducing temperatures from 50°F to 41°F a doubling of shelf life occurs. Lowering temperatures from 41°F to 36°F will result in another double of shelf life. However, optimal temperature is about 32°F. Temperatures lower than this risk chill damage.

Relative Humidity – Lettuce has a high water content and thus it is important to maintain a high relative humidity in order to prevent the leaves from drying out and losing mass and quality. A relative humidity of about 98% to 100% is generally optimal.

Ethylene Sensitivity – Lettuce itself releases very low levels of ethylene and is unlikely to significantly affect other ethylene-sensitive produce. However, lettuce itself is very sensitive to ethylene and will rapidly brown and decay in the presence of high levels of ethylene. For this reason lettuce should be kept isolated from higher ethylene producing produce.

Shelf Life – Under optimal conditions it has a shelf life of about 2-3 weeks.

Methods of Cooling Lettuce

Vacuum Cooling – The most common method of cooling associated with lettuce is vacuum cooling. This method involves using a vacuum seal around the lettuce and using pressure to vaporize the water present. The water vaporization in turn results in a very fast cooling. However, research indicates that vacuum cooling of lettuce is most effective when the vacuum film used is vented.

Forced-Air Cooling – Forced-air cooling involves placing the lettuce in a cooled room such as a cooler and pulling air through the lettuce. This forced air movement results in much faster and more thorough cooling than room cooling alone, which is not optimal for cooling lettuce.

Hydrocooling – Hydrocooling involves submerging the lettuce in rapidly moving, near freezing water. This results in a very fast lowering of temperature, especially at higher initial temperature levels. However, it is not ideal for lowering lettuce temperatures from mid or low level ranges to optimal temperature because the cooling load is much higher at lower temperatures.

At SEMCO/SEMCOLD LLC we are committed to helping our clients get the most value and quality from their lettuce. We provide high-quality, fully customizable cooling systems which can be optimized to fit the particular needs of the customer. These systems are ideal for lettuce and other produce.