Fruit freezing temperature

The warm-weather climates necessary for citrus orchards generally minimize the risk of frost conditions. Nevertheless, occasionally measures are needed to protect the trees and their fruit from freezing temperatures. Citrus crops become threatened when temperatures fall below 28°F for four hours or moreJ l Heating... 

In the case of meat and most vegetables, phase changes induced by freezing temperatures are easily reversed with melting or thawing. The liquids and solids in meat freeze and melt without much noticeable affect on taste. Likewise, the liquids and solids in most vegetables can freeze and melt without trouble. But fruit is another story. 

Lyophilization is mainly used for the sample treatment of plants (and biological materials) having undefined quantity of water (fruits, vegetables) as well as when volatile elements are to be determined. Water is removed under low temperature and pressure. The freezing temperatures do not influence the mass of the final product. The enrichment factor depends on the quantity of water and is in the range 4-20 for fruits and vegetables. 

Extension of the season in the fall is accentuated by covers because of the enormous amount of heat stored in the soil as compared to springtime soil heat accumulation. A single layer SB cover will protect mature pepper and tomato fruits from freezing temperatures down to about —4°C, which is more than twice the protection afforded by covers in the spring. 

As a general rule, foods which are not to be frozen are handled and stored at a temperature just above their freezing point, providing this does no damage (exceptions are fruits such as bananas and lemons). Produce which is to be frozen must be taken down below the freezing point of the constituents. Since foodstuffs contain salts and sugars, the freezing process will continue down to -18°C and lower. 

In order to preserve, as much as possible, the phenolic content in fruit and vegetable samples, the literature proposed the application of cold temperatures, even reaching to freezing, when lyophilization is the objective. These procedures also could inactivate the enzymes. The freeze-drying is largely the main preservation technique used in the studies related to the identification and quantification of the phenolic compounds of fruit... 

Tomato Tomato is also popular for the expression of biophamaceuti-cals due to the relative ease of processing of the fruit, and the fact that the fruit can be consumed fresh, thus providing stability of the recombinant protein at room temperatures. Tomato fruit has a short shelf life but, alternatively, can be freeze-dried, an inexpensive and well-established technology. Freeze-drying results in concentration of the protein and maintenance of batch consistency. 

Lasztity, R., Sebok, A., Major, J. (1992). Textural properties of fruits and vegetables and their changes during freezing and storage at low temperature. Per. Pol. Chem. Eng., 36, 25-238. 

Continuous hoi air driers are used to prepare most of the high quality, dried, piece-furm fruits and vegetables produced in the United States. Liquids and pastes arc commonly dried in spray, drum, or feeze dryers. Particulate foods can be dried in batch or continuous air-fluidized beds or freeze dryers. Many agricultural commodities are sun-dried when weather conditions at harvest provide low humidity, warm temperatures, and good air circulation. 

Substances that do not freeze into a rigid solid at low temperatures, such as fruit juices, may be subjected to a devitrification treatment to avoid the soil-glass structure detrimental to optimum drying... 

Frozen materials should be stored at —20°C, fruits and salad vegetables at around 4°C, and canned foods at room temperature. Powdered and freeze-dried materials should be stored in the dark in their original containers. Storage of fresh materials should preferably not exceed 3 days. After the initial preparation (see below), fresh or cooked materials can be conveniently stored at -20°C for a short time prior to extraction. 

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