AU5554894A - Preserving fruit and vegetables - Google Patents

Description

PRESERVING FRUIT AND VEGETABLES

This invention relates to a method of preserving cut fruit and vegetables and to preservative solutions and preservative compositions for this purpose. It involves the treatment of cut fruit and vegetables, such as apples for example, so that they retain their appearance for up to six weeks, so that the fruit preserved in this manner can be used in the food processing industry.

There is a necessity in the food processing industry to prepare fruit and vegetables by cutting, slicing or dicing and sometimes peeling the fruit or vegetables, so that the cut fruit or vegetables can be used later in cooking or baking, or for further processing. When fruit or vegetables are cut or peeled, the cut surfaces normally quickly discolour and begin to spoil. Various processes and methods have been used in the past to treat cut or peeled fruit and vegetables to preserve them for a period, such as a number of weeks.

In the past, cut fruit and vegetables were preserved by being contacted with sulfur dioxide (SO₂) in aqueous solution, usually by dipping the cut fruit in a solution of sodium metabisulfite, which is also known as sodium disulfϊte (Na₂S₂O₅). However in recent times there has been some health concerns raised over treatment of food with sodium disulfite, because of the sulfur dioxide residues in food. There has also been increasing consumer interest in, and demand for, pure food which is unadulterated, or which only contains substances which occur naturally in fruit and vegetables. Fruit preserved with sodium disulfite will normally keep for 1 to 3 weeks, depending on the amounts used.

There has therefore been a move away from treating cut fruit and vegetables with sulfur dioxide, and an alternative process is to blanch the cut fruit and vegetables, by immersing the cut fruit in boiling water for a short time and then immediately afterwards in cold water. However, this is expensive, and also the food treated in this matter does not generally keep for very long. Also, fruit treated in this manner, is generally inferior in quality to fruit that has not been blanched, especially when subjected to further processing.

It is also known to treat cut fruit and vegetables with various food acids such as citric acid or ascorbic acid, sometimes together with sodium or calcium chloride and cut food treated in this manner will sometimes keep for several days. For example, a preservative sold under the trade name "Snow Fresh" by Monsanto Chemical Co of St Louis, USA, contains sodium acid pyrophosphate (SAPP), calcium chloride, ascorbic acid, and citric acid. This will give cut vegetables, such as apple, a shelf-life of 5 days. (Food Processing, Sept. 1988, pp 98-99).

It is believed that it is known to use a formulation consisting only of ascorbic acid, low levels of citric acid, sodium chloride and low levels of calcium chloride, to preserve fruit for a period of a few days.

It therefore would be advantageous to have an improved method of treating cut fruit and vegetables to allow the fruit to be kept for several weeks, ideally without the use of any SO₂, and the present invention has arisen in order to find a solution to this.

The present invention in one broad form relates to a method of preserving cut fruit or vegetables so that they retain their natural appearance, this method comprising soaking the cut fruit or vegetables in a dilute aqueous solution containing:

(a) sodium and/or potassium ions,

(b) calcium ions,

(c) chloride ions,

(d) ascorbic acid or ascorbate ions, (or their isomers or derivatives) and

(e) citric acid or citrate ions, or malic acid or malate ions, (or their isomers or derivatives) for a few minutes, and preferably at a temperature that is at or below room temperature. In one preferred form the method described above also includes: (f) sorbic acid or sorbate ions, (or their isomers or derivatives). The sorbic acid can be included in the soaking mixture, or preferably may be sprayed onto the treated fruit after the dipping step is completed, in a dilute aqueous solution. Sorbic acid or its salt is particularly preferred when treating fruit.

Mixtures of the various ingredients within each class (a) to (f) are also possible, such as the use of sodium as well as potassium ions, for example, or the mixture of citric acid and malic acid, or else using citric acid and sodium citrate. It is particularly preferred that both sodium and potassium ions be present.

The invention further comprises a preparation for use in the method of preserving cut fruit or vegetables, the preparation comprising compounds or substances including:

(a) sodium and/or potassium ions,

(b) calcium ions,

(c) chloride ions,

(d) ascorbic acid or ascorbate ions (or their isomers or derivatives), and

(e) citric acid or citrate ions, or malic acid or malate ions (or their isomers or derivatives), either in solid form or in solution, either concentrated or at a sufficient concentration for treating cut fruit or vegetables. As one option the preparation described above also includes: (f) sorbic acid or sorbate ions, (or their isomers or derivatives). Preferably sodium and potassium ions are both present. Fruit or vegetables treated in accordance with the above method or with the above preparation is also part of the present invention.

In one preferred form the dilute aqueous solution in the method contains (1) sodium chloride (NaCl), (2) calcium chloride (CaCl₂), (3) potassium chloride (KC1), (4) ascorbic acid, and (5) citric acid; and preferably (6) potassium sorbate (KC₆H₇O₂) either in the solution or preferably sprayed onto the fruit after the dipping, and especially when preserving fruit.

However, food industry substitutes for the ions and substances referred to above can be used in place of the actual substances referred to, or else, particular isomers such as an optical isomer may be used, or a derivative of the substance retaining the general activity of the substance, which is also useable in the food processing industry as an alternative to the substance. For instance, in place of, or in addition to (L-) ascorbic acid can be used D-araboascorbic acid.

For instance, potassium chloride can be used in place, or in addition to, sodium chloride, and in a preferred embodiment of the invention both may be present. Also for example, malic acid can be used in place of, or in addition to citric acid. The invention requires that at least one member, of each of the groups (a) to (e), and optionally (f), be present in order for the fruit or vegetables to be preserved.

The ingredients (a) to (c) are preferable present in the form of sodium chloride, potassium chloride and calcium chloride. However, other salts of sodium (or potassium) and calcium can be used instead of the chloride, provided some chloride salt is also present.

Ascorbic acid or its salt should be present also. Any optical isomer can be used of ascorbic acid such as L-ascorbic acid which is also known as vitamin C. Otherwise, iso-ascorbic acid (erythorbic acid) or D-arabinoascorbic acid may be used. Any other substitute for ascorbic acid commonly used in the food industry may also be used in addition to or in place of ascorbic acid, provided it has the same effect. A salt of ascorbic acid such as sodium ascorbate may be used in addition to or in place of the acid.

Citric acid, malic acid or the salts of citric or malic acid should also be present. A mixture of citric acid and malic acid (or their salts) may also be used, and commonly known food industry substitutes for citric or malic acid may also be used in addition to or in place of these substances.

Potassium sorbate, any other salt of sorbic acid, or sorbic acid itself, or a mixture of these is a preferred ingredient. Potassium sorbate (the potassium salt of 2,4-hexadienoic acid) helps to act as a mold or yeast inhibitor in the preservative formulation. The soaking or spraying of the potassium sorbate may, if desired, take place separately before and/or after the dipping or spraying of the solution containing ingredients (a) to (e). Alternatively, it can be included in the composition of (a) to (e). The presence of the sorbate is especially preferred when preserving fruit, which mostly contain sugars and other substances which encourage yeast or mold growth.

A formulation only consisting of ascorbic acid, low levels of citric acid, sodium chloride and low levels of calcium chloride, does not form part of the present invention.

The pH of the solution used in the invention is determined according to the fruit or vegetable to be preserved. Generally, the pH is about 1 to 1.5 less than the pH of the fruit; for example, with cut apples which have a natural pH of about 2.9 to 3.8 depending on the type of apple, then preferably the pH of the preservative solution is selected to be about from 1.8 to 2.8. The pH can be outside these limits if desired, especially depending on the pH of particular fruit or vegetable being preserved, and on the desired taste of the preserved product, which may also be affected by the level of acidity present. The pH is normally adjusted by changing the amount of the acid ingredient(s) present, or by selecting the ingredient compounds as acid or as salts; for example by using either citric acid or a citrate salt. The pH will normally be more acidic than the natural pH of the fruit or vegetable being treated.

The cut fruit or vegetable may be any type of fruit or vegetable for which preservation is required and suitable fruits include stone and pome fruits especially apples, pears, apricots or peaches and vegetables such as potatoes, tomatoes and so forth. Other fruits and vegetables which may be treated include bananas, pineapple, berries, asparagus, as just some examples. The fruit to be preserved are generally in pieces, and may have been peeled, cut or sliced and diced. The invention can also be used with whole fruits or vegetables, especially if they have been damaged, or subjected to rough handling.

A dilute aqueous solution of the preserving ingredients is prepared in a large tank, and the solution is preferably chilled below room temperature. The temperature of the water is below room temperature, preferably less than 20 °C and most preferable at a temperature between 10° and 14°C.

The cut fruit is allowed to remain in the slightly chilled solution for a few minutes; preferably about 1 to 10 minutes, more preferably 2 to 8 minutes, and most preferable about 2 to 3 minutes. The exact amount of time can be adjusted to give optimum results, and will vary according to the type of product used. For example, potatoes can be dipped for 5 to 12 minutes, pears for 5 to 8 minutes, and other fruits for about 2 to 5 minutes.

As one possible alternative to this, the aqueous preservative solution can be sprayed onto the cut fruit instead of, or perhaps in addition to, dipping in a treatment tank.

In a further embodiment of the invention, the preservative solution of the invention can treat the fruit under vacuum, which may enhance the treatment process. Ideally, the treated fruit or vegetable is stored before and/or after treatment at low temperatures; preferably less than 4°C, as in a cool room. In this situation, the treated fruit or vegetables will keep for three to six weeks, in accordance with the present invention. Storage in a cool room can add a week to the preservation period.

The ingredients are dissolved in water in an amount to give a solution having a concentration of between about 0.1 % to 3 % weight/ volume (w/v). For example, and with reference to the preferred ingredient compounds; the citric acid is preferably present in an amount of about 0.1 to 3 % w/v, ascorbic acid is present in an amount of about 0.1 to 3 % w/v, sodium chloride in an amount of about 0.1 to 3 % w/v, calcium chloride in also present in an amount of about 0.1 to 3 % w/v, and potassium chloride is also present in an amount of about 0.1 to 3% w/v. When used, potassium sorbate may be present in an amount of about 0.10 to 0.75% w/v.

More preferably the ascorbic acid is present in an amount of 0.25% to 2%, most preferably about 0.5%. The citric acid is more preferably present in an amount of 1 to 2 % , most preferably about 1.5%. The sodium chloride and potassium chloride are present more preferably in an amount each of 0.25% to 2%, most preferably about 0.25%, and the calcium chloride is more preferably present in an amount of 0.25 to 2%, most preferably about 1.5%. Potassium sorbate is more preferably present in the amount of about 0.25% to 0.6%, most preferably about 0.5%, and it is ideally applied to the fruit separately by dipping or spraying. If alternate ingredients are used, their amounts are adjusted to be similar to the indicated amounts for the preferred ingredients.

It is also possible to include a small amount of sulfur dioxide, although preferably this is not present so that the treatment procedure occurs with only safe ingredients which are naturally found in foods. Also the presence of sulfur dioxide with the other ingredients may reduce the preserving effect on the treatment using this invention.

The amount of ascorbic acid and citric acid, or the alternate use of sodium ascorbate and citrate for instance, can be adjusted to give a preferred acidity or pH to the resulting mixture. As mentioned before the pH is preferably about 1 to 2.0 less than the normal pH of the product, but the level of acidity of the treatment will affect the flavour of the treated food, and the actual amounts of the acid compound present (or instead, its salt), can be adjusted to produce a preferred flavour and give a desired tartness to the food.

Some preferred pH levels for different products are shown in the following table. The values for the selected pH range are approximate and can be varied.

It is also preferred that the acidity level is not too high, so as to change the appearance or the texture of the food. In general, and with reference to most fruits and vegetables, a pH of about 1.8 to 5 is preferred, more preferable the pH is between about 2 to 4. However a pH outside of these ranges is also satisfactory, depending on the desired acidity and taste of the product, and essentially neutral pH is also within the scope of the invention, for example when testing some vegetables like potato for instance. It is also preferred, when treating fruit like apple for instance, to avoid the so-called "pineapple effect", whereby too high a concentration of ascorbic acid for example, will cause the apple to change its colour and texture to be more clear and translucent than normal. However, sometimes the "pineapple effect" may be desired, and high ascorbic acid treatments can be used in this case. It is also useful to pre-dissolve any potassium sorbate used in the process.

It is preferred when making up the solution (using the preferred compounds) to add the citric acid, sodium chloride, potassium chloride, and calcium chloride salts to the chilled water first, and allowed these to fully dissolve. The ascorbic acid is added as a second step, and the potassium sorbate (if used) is added as a third step, or ideally, is sprayed as a dilute solution on the dipped fruit separately. Most preferably, the citric acid, calcium chloride, sodium chloride, and potassium chloride is added to the water in this order. Then, once the first ingredients have completely dissolved the ascorbic acid is dissolved. Finally the potassium sorbate may be added either predissolved or as a powder, or made up into a separate spray solution. This is preferable because of the precipitation affect of potassium sorbate on the calcium chloride; or otherwise additional calcium salt should be added to account for this.

When the cut fruit or vegetable is treated in accordance with the invention, it is found that the fruit will keep for as much as six weeks without appreciable- deterioration in its appearance or spoiling. For example when the invention was tested with cut apples, the following procedure was followed. The cut fruit is dipped for approximately five minutes in the solution at a temperature of between 10 and 14 °C in a tank which holds 1000 to 2000 litres, although normally the 2000 litre tank will only be partially filled, perhaps to contain about 1600 litres of the solution. The concentration of the components of the solution is selected to give the required amount of each ingredient in the resulting solution, for example an amount of 1 % w/v requires the addition of 1kg of the ingredient to 100 litres of water. The spray of the sorbate solution uses normal equipment for this purpose.

The water used is preferably partly pre-chilled, and to give the best results the water is preferably also distilled or filtered water that is free from any dissolved minerals and microorganisms. "Hard" water should not be used, if possible. The tank should ideally also have been carefully cleaned to ensure there is no unwanted contamination of the treatment solution, and to give optimum results.

The preservative preparation may be pre-packed in powdered solid form for sale, and preferably the ingredients will be separately packaged as follows: (1) citric acid, (2) the various chloride salts (3) ascorbic acid and (4) the potassium sorbate. This allows the ingredients to added to the water in this order, and the sorbate solution to be made up separately, if desired.

The invention is now described as reference to various examples.

EXAMPLE 1

Cut apple was treated in accordance with the invention. A solution was prepared in a 200 litre tank which contained approximately 160 litres of chilled water at a temperature between 1 and 4°C. Sodium chloride was added in an amount to constitute 1 % w/v (1kg per 100 litres of water) together with calcium chloride also at a concentration of 1 % w/v. Ascorbic acid at a concentration of 0.5% w/v and citric acid 1 % w/v was then added. Apple which had been sliced in thin slices was added to the tank and allowed to remain in the mixture for approximately 2 minutes. The apple was removed from the tank, drained and kept in a cool room at a temperature of less than 4°C. The apple was inspected and was found to be unaffected 3 weeks to as much as 6 weeks after treatment.

EXAMPLES 2-9

Further examples using differing amounts of the ingredients, ascorbic acid, citric acid, sodium chloride, potassium chloride and calcium chloride were prepared and tested in accordance with Example 1. The sorbate was sprayed separately onto the dipped apple, after treatment with the other ingredients. The results are shown in the following table:

EXAMPLE No.

The amounts shown are in %w/v (kg/ 100 litres of water).

The results of Examples 1 and 2 were good, and the results for Examples 6 and 7 were very good and the treated apple was still in good condition as much as 3 weeks after treatment. Examples 3, 4 and 5 were moderately good in that the apple kept up to 2 weeks without any significant deterioration, although the apple in Example 3 was slightly discoloured. The apple became somewhat translucent (the "pineapple effect") in examples 3 to 5, especially in example 5. Examples 4 and 5 included small amounts of sulfur dioxide arising from adding sodium disulfite, and the results indicate that the presence of sodium dioxide diminished the preservation effect of the solution at an early stage of 2 weeks. The results of examples 8 and 9 were extremely good. Fermentation was taking place in Examples 1 to 7, at around 3 weeks, whereas there was little or no activity with Examples 8 and 9.

The results of the solutions for Examples 7, 8 and 9 are also very good, and better than those described in Examples 1 to 6. Examples 8 and 9 show a formulation particularly useful in the commercial situation, when producing large quantities of up to 4 tonne of apple product, which is dipped in one solution tank. The sorbate prevents the growth of wild yeast, that otherwise may be a problem especially in the use of a large capacity tank, and high throughput of apple product. The yeast, and the unwanted fermentation that result, is less of a problem with small quantities of applies to be preserved.

EXAMPLES 10 to 14.

Cut apple was treated in accordance with the invention. A solution was prepared in a 200 litre tank which contained approximately 160 litres of chilled water at a temperature between 1 and 4°C. Citric acid, and then sodium chloride, potassium chloride, and calcium chloride was added in an amount to equal the concentration shown in the following table. The ascorbic acid was then added. Apple which had been sliced in thin slices was added to the tank and allowed to remain in the mixture for approximately 3 to 5 minutes. The apple was removed from the tank, drained and then the potassium sorbate solution was sprayed onto the treated apple. The apple was then drained, shaken and dried, and packed into plastic bags, from which excess air was removed. The apple was kept in a cool room at a temperature of less than 4°C. The results of all examples were good, and of particular note were the results for Examples 13 and 14. Generally, the apple was inspected and was found to be unaffected as much as 4 weeks after treatment. The apple treated according to Example 13 was in good condition even after a further 2 weeks, whereas the fruit of Example 14 had shown noticeable deterioration at the same period.

EXAMPLE No.

The results from Examples 8 to 14 show that the spraying of potassium sorbate along with the addition of potassium chloride enabled the fruit to be kept for longer and in good condition. A 6 week or even longer period of preservation is therefore available in these conditions, especially when combined with cool room storage. EXAMPLE 15.

The procedure described in Examples 13 and 14 was repeated with cut peach. Melting flesh peaches were used in the experiment, which are harder to preserve than normal high quality non-melting flesh peaches. The results were excellent and similar to the results with apple, and the preserved peach kept in a cool room for at least 3 weeks.

EXAMPLE 16, (comparative)

As a comparison, the following solution was tested as a preservative: ascorbic acid (0.5%), citric acid (0.25%), CaCl₂ (0.25%), and NaCl (2%), in a similar manner as that used with the previous examples. The results showed the maximum refrigerated life of apple preserved with this mixture was only 5 to 7 days, before browning was apparent.

The invention has been described as reference to specific examples and preferred features, but obvious modifications may be made by persons in accordance with the described invention without departing from the spirit and scope of this invention.

Claims (13)

THE CLAIMS:

1. A method of preserving fruit or vegetable pieces so that they retain their natural appearance, comprising soaking cut fruit or vegetables in a dilute aqueous solution containing:

(a) sodium and/or potassium ions,

(b) calcium ions,

(c) chloride ions,

(d) ascorbic acid or ascorbate ions, (or their isomers or derivatives) and

(e) citric acid or citrate ions, or malic acid or malate ions, (or their isomers or derivatives), for a short period, then removing and storing the treated fruit of vegetable pieces.

2. The method according to claim 1, where said dilute aqueous solution contains sodium and potassium ions.

3. The method according to claim 1, where said dilute aqueous solution contains:

(f) sorbic acid or sorbate ions, (or their isomers or derivatives).

4. The method according to claim 1 , which has the additional step of spraying the treated fruit or vegetables with a dilute aqueous solution containing:

(f) sorbic acid or sorbate ions, (or their isomers or derivatives).

5. The method according to claim 1 or claim 2 where the following substances are used: sodium chloride, potassium chloride, calcium chloride, ascorbic acid, citric acid, and optionally potassium sorbate.

6. The method according to claim 1, whereby the soaking occurs for 1 to 10 minutes, and at a temperature that is at or below room temperature.

7. The method according to claim 6, whereby the soaking occurs for about 2 minutes, and at a temperature that is below room temperature.

8. The method according to claim 1 or claim 2, wherein the pH of the dilute solution is about 1 to 1.5 less than the natural pH of the cut fruit or vegetable.

9. A preparation for use in a method of preserving cut fruit or vegetables, which comprises compounds or substances including:

(a) sodium and/or potassium ions,

(b) calcium ions,

(c) chloride ions,

(d) ascorbic acid or ascorbate ions (or their isomers or derivatives), and

(e) citric acid or citrate ions, or malic acid or malate ions (or their isomers or derivatives), either in solid form or in solution, either concentrated or at a sufficient concentration for treating cut fruit or vegetables.

10. The preparation defined in claim 9 which includes sodium and potassium ions.

11. The preparation defined in claim 9 which also includes:

(f) sorbic acid or sorbate ions, (or their isomers or derivatives).

12. The preparation according to claim 10 or claim 11 where the following substances are used: sodium chloride, potassium chloride, calcium chloride, ascorbic acid, citric acid, and optionally potassium sorbate.

13. Fruit or vegetables when preserved according to the method of claim 1.