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Patent Searching and Data


Title:
FROZEN PLANT TISSUE PRODUCT AND PROCESS FOR THE PREPARATION THEREOF
Document Type and Number:
WIPO Patent Application WO/1998/047383
Kind Code:
A1
Abstract:
A plant tissue product which is frozen to and stored at a temperature lower than or equal to its glass transition temperature and, prior to freezing, is not subjected to a process which inactivates lipoxygenase.

Inventors:
HOLT CHRISTOPHER BRUNSDEN (GB)
KIRK ELLIOTT (GB)
SHARP DAVID GEORGE (GB)
STEPHENSON PETER RICHARD (GB)
Application Number:
PCT/EP1998/001898
Publication Date:
October 29, 1998
Filing Date:
March 23, 1998
Export Citation:
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Assignee:
UNILEVER PLC (GB)
UNILEVER NV (NL)
HOLT CHRISTOPHER BRUNSDEN (GB)
KIRK ELLIOTT (GB)
SHARP DAVID GEORGE (GB)
STEPHENSON PETER RICHARD (GB)
International Classes:
A23B7/04; (IPC1-7): A23B7/04
Domestic Patent References:
WO1998020756A11998-05-22
Foreign References:
US5206048A1993-04-27
EP0052574A11982-05-26
US4220672A1980-09-02
Other References:
CLAUDIA DZIUK O'DONNELL: "The Formulation Challenge - Formulating for the big freeze", PREPARED FOODS, vol. 162, no. 6, 1993, pages 55 - 56, XP002039423
H.D. GOFF: "Low-temperature stability and the glassy state in frozen foods", FOOD RESEARCH INTERNATIONAL, vol. 25, no. 4, 1992, CANADA, pages 317 - 325, XP002039424
Attorney, Agent or Firm:
Kirsch, Susan Edith (Patent Division Colworth Hous, Sharnbrook Bedford MK44 1LQ, GB)
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Claims:
Claims
1. A plant tissue product which is frozen and stored at a temperature lower than or equal to its glass transition temperature, characterised in that, prior to freezing, the product is not subjected to a process which inactivates lipoxygenase.
2. A product as claimed in claim 1, wherein the product is stored for at least one week.
3. A product as claimed in claim 2, wherein the product is stored for at least one month.
4. A product as claimed in claim 3, wherein the product is stored for at least three months.
5. A product as claimed in any preceding claim, wherein the product is pasteurised prior to freezing.
6. A product as claimed in any preceding claim, wherein the plant tissue is selected from the group consisting of vegetables, fruit, leaf tea and herbs.
7. A product as claimed in claim 6, wherein the plant tissue is a vegetable selected from the group consisting of peas, green beans, spinach, broccoli, sweetcorn and potato.
8. A process for the preparation of a frozen plant tissue product, the process comprising the steps of: a) freezing a plant tissue product to a temperature lower than or equal to its glass transition temperature; and b) storing the product at a temperature lower than or equal to its glass transition temperature; characterised in that, prior to freezing, the product is not subjected to a process which inactivates lipoxygenase.
9. A process as claimed in claim 8, further comprising pasteurising the product prior to step a.
Description:
Frozen Plant Tissue Product and Process for the Prenaration Thereof Field of the Invention The invention relates to a frozen plant tissue product, such as vegetables, fruit, tea and herbs, and a process for the preparation thereof, which process does not include a step which inactivates lipoxygenase.

Background Art Traditionally, plant tissue is blanched to inactivate enzymes, including lipoxygenase and peroxidase, prior to freezing. This prevents undesirable and detrimental enzyme- mediated changes to the plant tissue during freezing and frozen storage. However, blanching has a number of consequences other than enzyme inactivation. For example, blanching results in a reduction of the nutritional and sensory quality; it reduces the vitamin content, in particular vitamin C; it also affects the cell membranes and cell walls, resulting in texture and colour alteration (eg softening).

It is therefore highly desirable to be able to remove the blanching requirement. However, to date, no process for

freezing plant tissue has been suggested that has no integral step to inactivate enzymes such as lipoxygenase.

FR 2634977 discloses freezing raw cep mushrooms to -120"C and storing them at -240C.

In CH 179756, fresh fruit or vegetables are frozen without any added preservative to a temperature of -15 to -25°C.

Once they are frozen, they are stored at -5°C, for example, with a constant circulation of cold air.

US 3600199 teaches freezing fresh root vegetables such as potatoes to a temperature in the range of -20 to -320C, keeping them at this temperature for two days and storing them at higher temperatures of, for example, -15 or -10°C.

Summary of the Invention According to the present invention, there is provided a plant tissue product which is frozen and stored at a temperature lower than or equal to its glass transition temperature, characterised in that, prior to freezing, the product is not subjected to a process which inactivates lipoxygenase.

Lipoxygenase is an enzyme which is known to catalyse reactions that generate off-flavours in plant tissue products. Traditionally, it is inactivated prior to freezing to prevent deleterious changes to a plant tissue product during frozen storage.

The inventors were suprised to find that plant tissue products, having active lipoxygenase, had a natural 'fresh' flavour and colour after prolonged frozen storage at or below their glass transition temperature.

The invention also provides a process for the preparation of a frozen plant tissue product, the process comprising the steps of: a) freezing a plant tissue product to a temperature lower than or equal to its glass transition temperature; and b) storing the product at a temperature lower than or equal to its glass transition temperature; characterised in that, prior to freezing, the product is not subjected to a process which inactivates lipoxygenase.

Preferably, the frozen plant tissue product is stored for at least one week, more preferably for at least one month and most preferably for at least three months.

Optionally, the plant tissue product is pasteurised prior to freezing. Suitable pasteurising heat treatments are 600C for 43 mins or 700C for 2 minutes. The application of ultra high pressure, high intensity light or mano-thermo-sonication are also suitable pasteurisation treatments. These treatments do not inactivate enzymes such as lipoxygenase and peroxidase,

but do kill microorganisms, so that the product is microbiologically safe to eat upon thawing.

In contrast, a blanching heat treatment is, for example, 92"C for 80 to 90 seconds; such a treatment inactivates lipoxygenase and peroxidase.

For practical and economic reasons, the plant tissue product is preferably frozen and stored at a temperature which is no lower than 10"C below its glass transition temperature, more preferably, no lower than 5"C below its glass transition temperature.

The present invention provides a product which is frozen and stored under controlled conditions.

The product may be frozen to a temperature lower than or equal to its glass transition temperature and stored at the same or a different temperature lower than or equal to its glass transition temperature.

Preferably, the plant tissue product does not include any added preservative prior to freezing.

As the temperature of a plant tissue product is reduced during freezing, ice forms, so the concentration of the unfrozen media increases. Consequently, the viscosity of the unfrozen matrix between the ice crystals increases. At a certain temperature, known as the glass transition temperature, the viscosity of the matrix becomes so high

that molecular diffusion is greatly inhibited. The glass transition temperature is different for distinct plant tissue products and is likely to vary according to variety, maturity etc.

The glass transition temperature (Tg) may be measured by any suitable method. Such methods are given in, for example, J Food Eng (1994) 22 483-494 in the article entitled "The Glass Transition in the Freezing Process" by D S Reid, W Kerr and J Hisu and Zeitschrift fuer Naturforschung section C Biosciences 49 (3-4) (1994) in the article entitled "Glassy state in plant cuticles during growth" by P Luque and A Heredia.

For the purposes of the present invention, the inventors used Differential Scanning Calorimetry to measure glass transition temperatures.

The trace from a Differential Scanning Calorimeter of an aqueous solution shows a large peak corresponding to the heat input required to melt the ice in a sample. Traditionally, in prior art disclosures, a small step immediately prior to the large step was taken as the glass transition temperature.

However, the article entitled "Differential Scanning Calorimetric Study of Frozen Sucrose and Glycerol Solutions" J Chem Soc Faraday Trans (1992) 88 (6) 789-794 by S Ablett, M J Izzard and P J Lillford, showed that a small heat capacity change is sometimes observed before this small step and it is now generally believed that this small heat capacity change is the true glass transition temperature.

Without wishing to be bound by theory, it is believed that when a plant tissue product is frozen and stored at or below its glass transition temperature, diffusion reactions do not happen over a reasonable timescale and thus reactive species, such as enzymes and substrates, are prevented from coming together and causing spoilage reactions eg off flavour development, colour changes and rotting.

Some advantages of frozen storage at a temperature below the glass transition temperature have previously been recognised (see, for example, D S Reid, "Optimizing the Quality of Frozen Foods" in Food Technol July 1990 p 78-82). Storage below the glass transition temperature has previously been utilised to reduce tissue structure disruption during freezing via the prevention of formation of large ice crystals in pre-blanched vegetables. However, the prior art consistently teaches that at least a minimum blanching step is required to inactivate enzymes such as lipoxygenase.

There is no teaching in the prior art that the blanching step can be removed in totality when freezing to below the glass transition temperature.

Preferably the plant tissue product is selected from the group consisting of vegetables, fruit, leaf tea, herbs and includes, for example, peas, green beans, potato, spinach, broccoli, sweetcorn and tomatoes. The plant tissue product preferably consists of only fruit and/or vegetables.

Examples of glass transition temperatures are as follows:

Peas -38°C Broccoli -20°C Parsley -26 to -290C Green Beans -22 to -240C Spinach -29°C Sweetcorn -100C carrot -260C The inventors have also measured the glass transition temperatures for several vegetables disclosed in the prior art documents mentioned above: cep mushrooms -26 to -27°C potatoes (Bintje variety) -16 to -18"C turnip -35 to -390C Detailed Description of the Invention Examples of the products and processes of the invention will now be described to illustrate, but not to limit, the invention.

Example 1 Unblanched green beans with a glass transition temperature of -24°C were frozen in approximately 2 minutes using liquid nitrogen. They were stored at -280C for 5 months. All of the beans were firm and crisp and had a natural 'fresh' flavour and colour, when cooked and assessed by an informal panel.

Comparative Example A Blanched green beans with a glass transition temperature of -24°C were frozen in approximately 2 minutes using liquid nitrogen. They were stored at -280C for 5 months. The beans were less firm and crisp than those in Example 1, when cooked and assessed by an informal panel.

Example 2 The phospholipid content and free fatty acid content of a sample of hand-shelled raw (unblanched) peas was measured.

The peas had a glass transition temperature of -38°C.

Another sample of hand-shelled raw (unblanched) peas was blast frozen to around -20°C, rapidly frozen to -70°C and stored at -380C for 6 months. Upon thawing, the phospholipid content and free fatty acid content of the peas were measured.

The phospholipid measured was phosphatidylcholine.

Results The fresh hand-shelled peas had a phosphatidylcholine content of 150mg/lOOg and a free fatty acid content of Omg/l00g.

The frozen, raw hand-shelled peas had a phosphatidylcholine content of 194mg/100g and a free fatty acid content of Omg/lOOg.

Comparative Example B Two samples of hand-shelled raw (unblanched) peas having a glass transition temperature of -38°C were frozen using liquid nitrogen and stored at the required temperature for 6 months. One sample was stored at -20°C. The other sample was stored at -280C. Upon thawing, the phosphatidylcholine content and free fatty acid content of the peas were measured.

Results The peas stored at -20°C had a phosphatidylcholine content of 23mg/lOOg and a free fatty acid content of lllmg/lOOg.

The peas stored at -28°C had a phosphatidylcholine content of 76mg/lOOg and a free fatty acid content of 26mg/100g.

Conclusion From example 2 and comparative example B it can be concluded that peas stored at their glass transition temperature have a phosphatidylcholine content and free fatty acid content comparable to that of fresh, raw peas. When the peas are stored at temperatures above the glass transition temperature, phosphatidylcholine is broken down by enzymes to free fatty acids. The peas therefore lose the high levels of phosphatidylcholine and the low/zero levels of free fatty acids that are usual in fresh, healthy, normally-metabolising

plant tissue. This loss is detrimental to the quality of the peas.

Example 3 The Vitamin C content of fresh, raw (unblanched), hand- shelled peas was measured. Three samples of hand-shelled raw (unblanched) peas were frozen using liquid nitrogen. They were stored at -380C for 3, 6 and 12 months respectively.

Upon thawing, the Vitamin C content of the peas was measured.

Results The fresh peas had a Vitamin C content of 32 mg/lOOg.

The peas stored for 3 months had a Vitamin C content of 32.8 mg/lOOg.

The peas stored for 6 months had a Vitamin C content of 29.5 mg/bog.

The peas stored for 12 months had a Vitamin C content of 30.8 mg/bog.

Comparative Example C Fresh, raw, hand-shelled peas were blanched. Their Vitamin C content was then measured. Three samples of blanched, hand- shelled peas were frozen using liquid nitrogen. They were

stored at -38°C for 3, 6 and 12 months respectively. Upon thawing, the Vitamin C content of the peas was measured.

Results The unfrozen blanched peas had a Vitamin C content of 30 mg/lOOg.

The peas stored for 3 months had a Vitamin C content of 29.0 mg/bog.

The peas stored for 6 months had a Vitamin C content of 28.5 mg/bog.

The peas stored for 12 months had a Vitamin C content of 27.8 mg/lOOg.

Conclusion From example 3 and comparative example C it can be concluded that raw, unblanched, peas have a superior Vitamin C content than blanched peas. This superiority is retained during 12 months of frozen storage at the glass transition temperature.

Example 4 Three samples of raw (unblanched), hand-shelled peas were frozen using liquid nitrogen. They were stored at -38"C for 3, 6 and 12 months respectively. Upon cooking, the staleness

of their flavour was assessed by an expert taste panel on a scale of O to 10 where 0 is not stale and 10 is very stale.

Results The peas stored for 3 months had a staleness score of 1.

The peas stored for 6 months had a staleness score of 1.7.

The peas stored for 12 months had a staleness score of 1.8.

Comparative Example D The method of example 4 was repeated, except that the samples were stored at -240C.

Results The peas stored for 3 months had a staleness score of 2.75.

The peas stored for 6 months had a staleness score of 3.

The peas stored for 12 months had a staleness score of 3.1.

Conclusion From example 4 and comparative example D it can be concluded that the staleness of peas after prolonged frozen storage is lower when freezing and storing the peas at their glass transition temperature than when freezing and storing the peas at a temperature above their glass transition temperature.