CA1275196C - Dehydrofreezing of peeled apple pieces - Google Patents

Abstract

ABSTRACT
The invention relates to a method of freezing peeled apple pieces and the like. The apple pieces are blanched and are then dried, the dried apple pieces subsequently being frozen. Blanching is carried out in the range of 85 to 95°C and the drying is conducted at 60°C by means of hot air to reduce the weight by substantially one half. The freezing is conducted by placing in a temperature within the range of minus 30 to minus 40°C.

Description

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DEHYDROFREEZING OF PEELED APPLE PIECES
The present invention relates to a method of freezin~ peeled apple pieces and the like wherein the apple pieces are blanched, dried and frozen.
It is known to preserve apple pieces by deep-freezing, wherein the apple pieces are initislly dipped into a solution of ascorbic acid and/or sodium chloride or a syrup followed by freezing. In this procedure, the energy consumption during freezing is high, because a lot of water is frozen together with the apple pieces. For this reason the procedure is not satisfactory, but in addition the additives applied by dipping may cause a defective taste, and the product when thawed will be wet and mushy and collapse rather than behaving as fresh apple pieces.
According to the Swedish patent specification 125,580, vegetables, fruit, etc. are parboiled at about 100 C., followed by cooling under vacuum. The evaporating water cools and dries the goods which are then frozen. Since the product is parboiled at the high temperature of 100 C, the cell structure breaks down. It is reasonable to assume that too much water is left in the product and causes bursting of the cells during freezing, the product aPter freezing and unfreezing is thus soft and will have little resemblance to the fresh original, lacking the vitality and firmness characteristic of the fresh product.
U.S. patent specification 2,425,714 relates to quick-freezing of foods wherein the goods initially are blanched at a temperature over 74 C, and up to 110 C. The patent has not realized the importance of the cells not being broken down, because it is not mentioned to what extent the drying of the products is to be carried out, and the patent is silent on the f~ct that the temperature at blanching must not be too low, because the browning enzyme polyphenolic o~idase then will not be inhibited, to avoid discolouration of the apple pieces. As in Swedish patent specification 125,580 typical~y "boiled" products are concerned, which after thawing have little resemblance to the fresh product for vitality and firmness.
The British patent specification 2,005,983 relates to a method of preserving fruit, e.g. apples, or vegetables, which initially are boiled for 5 to 45 minutes at a temperature which is below about 82 C. Then, the goods are heated at a temperature in the range from 66 to 93 C for 5 to 20 minutes to reduce the moisture content by about 1 to 15~ before freezing takes place.

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Since the heating is carried to Q temperature of more than 60 C, a skin forms on the surface during drying which prevents liquid from being expelled from the goods. The products obtained nccording to the British patent specific~tion obviously have to be boiled or fried before use.
A further preservation method has been applied to apple rings including initial treatment of spple rings with sulphur to avoid discolouration ~browning), and then drying. The qddition of sulphur is not desirable, because it influences the smell and the taste.
Summarizing, it can be noted that until the present the importance of limiting the blanching to a specific temperature to prevent all breakdown in the product and reducing the liquid content to a low value and under such conditions that no miscolouring is caused and no skin is formed, which prevents expulsion of liquid from the product.
There is thus no s~tisfactory procedure for preserving apple pieces, so that they will resemble the fresh product. In food handling the use of additives should be avoided.
It is an object of the invention to provide a method of treating apple pieces and the like, to obtain a deep frozen product, which on thawing has, the same firmness and vitality as and is at least as rich in aroma as fresh apple, and which sdditionally contains no browning preventing additives, and which can be used in the same manner as fresh apple.
Hore particularly in accordance with the invention there is provided, a method of freezing peeled fruit pieces wherein the pieces are blanched, dried and frozen, characterized in that the blanching is carried out in water without additives at a temperature in the range from 85 to 9S C and that the drying is perfor~ed by means of hot air at a maximum tempesature of 60 C
until the weight of the piecas has been reduced to substant`ially half the weight, the dried pieces then being frozen. The blanching msy be carried out for one to three minutes. The drying may be performed in a convection dryer.
~referably the fruit is apple.
Embodiments of the invention will now be described for purposes of illustrating the invention.
Apples which have been peeled and the core removed, are sliced or made into cubes of suitnble size. The apple pieces then are blanched under strictly controlled conditions to incctivate the browning enzyme polyphenolic 75~g~

o~idase. This blanching takes place in pure water, i.e. water containing no additives, at a temperature in the range from ~5 to 95C for a period of one to three minutes. At lower temperature the blanching has to take place for a longer period.
The blanched apple pieces then are clried also under strictly controlled conditions. Preferably the drying is performed by mesns of hot air in a convection dryer at a temperature in the ranBe from 50 to 60 C. ~he dryine is carried to a specified water content~ preferably so that the weight of the apple pieces is reduced by about half. This may take about one hour depending on the size of the apple pieces and the construction of the convection dryer.
When the drying of the apple pieces has been completed, they are immediately ~rozen at a temperature in the range from minus 30 to minus 40 C
(deep-freezing).
The product obtained is superior to the products obtained by the preservation methods heretofore applied in that;
- the product is richer in aroma than fresh spple, because the volatile substances have been concentrated by drying;
- no discolouration occurs even through no additives are used;
- the product has a firm consistency and does not collapse during thawing;
the product provides a noticeable resistance when chewed;
- contrary to fresh apples which will be pulpy when used e.g. in an apple cake, the apple pieces processed as described maintain their apple structure and firmness after thawing;
- the thawed apple pieces can be used in salads and pastries without any pretreatment.
The novel method can be termed "dehydrofreezing" and has been successfully applied to apple pieces in accordance with the illustrative embodiment described above. According to indications it can also be applied to other similar ~ruits such as pears, peaches, and apricots.

Claims (4)

1. Method of freezing peeled fruit pieces wherein the pieces are blanched, dried and frozen, characterized in that the blanching is carried out in water without additives at a temperature in the range from 85 to 95°C and that the drying is performed by means of hot air at a maximum temperature of 60°C until the weight of the pieces has been reduced to substantially half the weight, the dried pieces then being frozen.

2. Method as defined in claim 1, characterized in that the blanching is carried out for one to three minutes.

3. Method as defined in claim 1, characterized in that the drying is performed in a convection dryer.

4. Method as defined in claim 1, 2 or 3 the fruit being apple.