Freezing vegetables

Abstract

A process for the production of a frozen vegetable or part thereof, wherein said process comprises the steps: (i) subjecting a vegetable or part thereof to a firming treatment selected from: a) immersing the vegetable or part thereof in a solution of a calcium salt. b) heating the vegetable or part thereof to a temperature in the range 50 to 70° C., and c) a combination of a) and b); (ii) under-cooling to a core temperature of less than or equal to -5° C.; (iii) reducing the temperature to less than or equal to -18° C. The frozen vegetables, when thawed, possess a texture and appearance which closely resembles that of fresh vegetables.

Description

[0001] The invention relates to a process for freezing vegetables and the frozen vegetables provided thereby. More particularly the invention relates to a freezing process which provides frozen vegetables of excellent quality when defrosted for consumption.BACKGROUND TO THE INVENTION[0002] Various attempts have been in the art to improve the quality of vegetables which have been stored frozen by way of the freezing process applied.[0003] U.S. Pat. No. 3,736,154 describes a process of ultraslow freezing which discloses the maintenance of intact cell membranes in the product by way of a freezing regime with a cooling rate of about 0.1 to 0.3.degree. C. per hour. This process is disclosed as achieving dehydration of the inner cell as water within the cell moves outside the cell membrane where it freezes without the destruction of the cell membrane.[0004] Unfortunately this process if not a viable approach to the commercial preparation of frozen vegetables. Not only does this process no...

AI Technical Summary

Benefits of technology

[0031] The addition of calcium to vegetables is known to limit the softening that vegetables undergo on subsequent higher temperature heating, such as canning or cooking. However, the quality benefits of the calcium addition prior to conventional freezing is somewhat limited. Surprisingly the combination of calcium addition and the controlled freezing regime according to the invention provides a significant improvement.

[0032] Pectin is a key component of the cell walls of vegetables and fruits and exists in the form of a gel network. The pectin in the middle lamellae between the primary cell walls of adjacent cells essentially acts as an adhesive between the cells, whilst the pectin matrix within the primary cell wall itself is believed to control the porosity. Enzymes present within the tissue can act on the pectin causing it to change its structure and properties. For example, pectin methyl esterase can de-esterify pectin. The presence or addition of calcium can then lead to cross-links being formed between pectin molecules, which can strengthen the pectin network. This change in the structure and properties of the pectin gel can produce different tissue properties, for example it reduces the softening of the tissue on heating, by limiting the loss in cell adhesion.

[0041] In accordance with the present invention there is provided a process therein the rate of cooling is slowed sufficiently to achieve only a small temperature difference between core and surface and thereby induce under-cooling at the core of the vegetable material to a maximum temperature of less than or equal to -5.degree. C. Ice formation with further temperature reduction can then occur throughout the vegetable material at approximately the same time. This has been found to result in a higher proportion of ice crystal formation within the cell structures defined by the cell walls i.e. intracellular ice and more favourable vegetable properties when consumed.

[0043] Sensory analysis has confirmed that both the appearance and the texture of vegetables prepared according to the present invention show improvement over the conventional freezing methods known in the art and the results obtained closely resemble those for fresh unfrozen vegetables. In particular the firmness of vegetables according to the invention are significantly improved over frozen vegetables known in the art.

[0066] Vegetable or parts thereof according to the present invention can be readily used in a variety of commercial catering or domestic frozen food products. In particular vegetables of the present invention are ideally suited to frozen ready prepared meals where their superior texture considerably improves the product quality. Therefore a further aspect of the invention relates to the use of a vegetable of part thereof as described above in a frozen meal.

Problems solved by technology

Unfortunately this process if not a viable approach to the commercial preparation of frozen vegetables.

Not only does this process not tolerate a blanching step which is necessary as a microbiological and enzyme deactivation step in modern vegetable processing, but also this process takes several days to complete.

It is disclosed that free water moves from the intracellular fluid to the extra cellular fluid, resulting in the simultaneous dilution of the extra cellular fluid and concentration of the intracellular fluid, which makes it easier for the extra cellular fluid to freeze and, conversely, more difficult for the intracellular fluid to freeze.

This is a necessary precursor to subsequent cooling steps in the process as any variation in solute concentration will give rise to relative variation in the temperature at which freezing will occur and thereby reduce the ability to achieve effective under-cooling.

Preferably heat treatment is undertaken by blanching as this results in the destruction of cell membranes and the inactivation of some or all of the endogenous enzymes present.

The addition of calcium to vegetables is known to limit the softening that vegetables undergo on subsequent higher temperature heating, such as canning or cooking.

However, the quality benefits of the calcium addition prior to conventional freezing is somewhat limited.

The quality benefits of low temperature firming and conventional freezing are limited.

It has been shown that merely reducing the core to -1 or -2.degree. C. without further under-cooling is not sufficient for the rapid initiation of freezing needed for the desired reduction in extracellular ice and consequent product benefit.

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