Freezing of fruit

Eikiin, K., Tsvetkov, O., Laptev, Y, Fikiin, A., and Kolodyaznaya, V. 2003. Thermophysical and engineering issues of the immersion freezing of fruits in ice slurries based on sugar-ethanol solution. Ecolibrium 2(7) 10-15. 

The recommended uses for low-ester pectins have been numerous, and previous publications (I, 2 P, If, 12 15y 17) have mentioned their adaptations, primarily in low-solids desserts, salads, spreads, and metallic pectinates, in the freezing of fruits, as film coatings of foods, etc. While the uses of low-methoxyl pectinates have been associated largely with the idea that they will gel under certain conditions without the addition of sugar or with low-sugar content, sugar is usually added in small quantities to improve the flavor of the gel. 

Fruit pulps and purges as well as juices and concentrates may be preserved with sulfur dioxide. This practice is quite widespread abroad, as was mentioned before, although it is not used in the United States, since preservation freezing of fruits and fruit pulps is preferable to sulfiting for subsequent use by jam and preserve industries. The preservation of fruit pulps with sulfur dioxide is described by Atkinson (1941), Atkinson and Strachan (1941), and Charley (1934). Both cold fruit pulps and hot fruit pulps may be barrelled with SO2 the latter usually require a lower concentration of SO2 for preservation. 

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... 

Figure 2.3 illustrates a process in which the outer layer of a product is quickly frozen as a congealed crust. These CRUSToFREEZE plants have a capacity between 1500 and 5000 kg/h and require 0.5 to 0.8 kg LN2 pro kg of product, which has to be frozen totally on a conveyor belt. Figure 2.4 shows the product exit of the plant in Fig. 2.3. The freeze drying of coffee and tea extracts, fruit pulps or small pieces of meat require a multi- stage pretreatment. The granulated end product from coffee and tea extracts should have a defined grain size, a desired color, and a predetermined density. Fruit pulps should become granulated, with the appearance of fruit pieces, while meat pieces should not stick together like a small meat ball, but be recognized as single pieces when presented in a meal.

Fig. 4.4. Influence of fruit and vegetable type on color, taste and consistency of freeze dried products.

Sauvageot, F., Simatose, D. Some experimental data on the behavior of fruit juice of volatile components during freeze-drying. International Institute of Refrigeration (Comm. X, Paris, 1969)... 

FIG. 15 Texture modification (percentage of raw fruit texture) and protopectin content (mg/100 g wet weight) before (BF) and after (AF) freezing of strawberry slices not pretreated (NT) or air dried at 80 °C up to 60% weight reduction without (AD) or following 60-min osmotic dehydration (OAD) in 60% (w/w) sucrose solution at 25 °C at atmospheric pressure (Brimar, 2002). 

Crystallisation by freezing, or freeze crystallisation, is a process in which heat is removed from a solution to form crystals of the solvent rather than of the solute. This is followed by separation of crystals from the concentrated solution, washing the crystals with near-pure solvent, and finally melting the crystals to produce virtually pure solvent. The product of freeze crystallisation can be either the melted crystals, as in water desalination, or the concentrated solution, as in the concentration of fruit juice or coffee extracts. Freeze crystallisation is applicable in principle to a variety of solvents and solutions although, because it is most commonly applied to aqueous systems, the following comments refer exclusively to the freezing of water. 

Arthey, D., Freezing of vegetables and fruits, in Mallett, C.P., (ed.), Frozen food technology, Blackie, Glasgow, 1993. 

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. 

Although a combination of product quality and cost considerations will dictate the methods used for bulk processing of fruit juices, there are instances where the flavour components present in the juice at e vulnerable to any form of heating din ing concentration. Strawberry juice is perhaps the best example of this, being one of the most heat sensitive of fruits, and it works well with alternative processes for concentration such as freeze-concentration and hyperfiltration. 

Large amounts of centrifuge pulp in a sample is an indication of high extractor and finisher pressures applied to the fruit during manufacture. It may also indicate the softness or freeze damaged fruit. 

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