PROCESSING OF NUTS.

PROCESSING OF NUTS"

This invention relates to the processing of nuts, and in particular it relates to the processing of nuts such as chestnuts, almonds, cashew nuts, hazelnuts, pecan nuts, pistachio nuts, macadamia nuts, walnuts and even coconuts which have a harder outer shell layer, together with a softer, thin, inner skin layer which is in fact connected to the flesh of the nut.

At the present time, Italy, France and Japan are the major producers of chestnuts worldwide, with a world production of these nuts being in excess of 400,000 tonnes per annum. The production of chestnuts in Australia constitutes only a very small percentage of the total worldwide production, and at present is approximately 500 tonnes per annum, however production in Australia is predicted to rise dramatically in the next few years as a result of increased plantings to approximately 3,000 tonnes per annum. The most common species of chestnut grown in Australia is Castanea sativa, the European or Spanish chestnut-

Chestnuts are harvested when the spiny burrs containing the nuts have opened, and, depending on the stage of development of the chestnuts, either fall from the burr or else remain encased in the burr when the burr falls from the tree. In the latter case, the nuts are separated from the burrs on harvesting. In Australia, chestnuts are presently harvested over a period of around 8 to 10 weeks beginning in late March, and the nuts should be collected as soon as possible after they fall from the tree as they contain a high percentage of water and thus there is a high chance of spoilage (mould growth and sheen and flavour losses) of the nuts if they are allowed to remain on the ground. Furthermore, the fruit of the nut deteriorates quite rapidly on keeping so that the present practice is for the nuts to be marketed as soon as possible after they have been harvested. At present, the demand in Australia is balanced with supply so that marketing of the nuts can take place as soon as they become available after harvest so that deterioration can be avoided, however, in future when an increased supply of chestnuts becomes available in Australia, it may be necessary to store the nuts after harvest so that they can be marketed over an extended period. As the result of the deterioration of the fruit on storage, there will in the future be the need for an effective method for the post harvest storage of chestnuts whereby the quality of the fruit will be retained over an extended period of time.

Another major problem in the marketing of nuts such as chestnuts arises from the fact that the nuts must be processed so as to remove the shell and skin before the flesh of the nut becomes available for use. The method most used at present to "peel" the nuts comprises a mechanical steam/roast treatment to loosen the outer shell followed by a separate abrasion step to remove the inner skin from the flesh of the nut. In general terms, methods proposed for the peeling of chestnuts include both chemical and mechanical methods (see for example, Oh, Y.T., Park, N.K. & Ko, Y.H., Plant Eng.27(2) pp 98-101 (1988)). In French Patent No. 1-561,968 (1969) there is disclosed a treatment process for the peeling of chestnuts which comprises steaming the nuts at 100 °C and baking at 900 °C for a couple of seconds, followed by rubbing of the nuts in a corrugated perforated drum. Chemical methods which have been proposed for the peeling of the chestnuts include treatment of the nuts with perchloric acid (2%) and nitric acid at 50 °C/30 min. as well as the use of starch or protein hydrolysing enzymes (see Japanese Patent No. 16342/69). Treatments using microwaves have also been proposed. It is an object of the present invention to provide an improved method for peeling raw nuts, particularly chestnuts. A further object of the present invention is to provide an improved method for the storage of nuts, particularly chestnuts, so as to extend the post-harvest shelf-life thereof.

In accordance with a first aspect of the present invention, there is provided a method for peeling nuts, in particular chestnuts or almonds, which comprises the step of exposing the nuts to a source of sulphur dioxide for a sufficient time to facilitate the removal of the shell and/or skin layers thereof, and thereafter removing the shell and/or skin layers from the nuts.

In this aspect, the present invention also extends to nuts, in particular chestnuts, which have been peeled by the method as broadly described above.

This aspect of the present invention is particularly applicable to the peeling of nuts which have two layers, including a harder, outer shell layer together with a softer and thinner inner skin layer connected to the flesh of the nut. The treatment of the nuts in accordance with the method of the present invention results in softening of the hard outer shell layer, so that the softened outer shell layer can be readily peeled away. In addition, the inner skin layer is also softened and the connection between the inner skin layer and the flesh of the nut is loosened. As a result, the softened inner layer requires only slight rubbing for it to also be removed from the flesh of the nut.

Suitable sources of sulphur dioxide which may be used in accordance with the method of the present invention include not only gaseous sulphur dioxide itself, but also other sources including metabisulphite, bisulphite and sulphite salts, such as sodium or potassium metabisulphite, bisulphite or sulphite. These salts may be used in solid form, such as powder, granules or the like, or as solutions. Sulphur dioxide and the above described sources of sulphur dioxide have previously been used in the area of food technology as anti-oxidants or anti-browning agents as well as antimicrobial agents; however, so far as can be ascertained, these agents have not been used prior to the present invention in a method of peeling nuts as broadly described herein.

In another aspect, the present invention provides a method for extending the post-harvest shelf-life of nuts, particularly chestnuts, which comprises the step of treating the nuts after harvest with a source of sulphur dioxide.

In this aspect of the present invention, the nuts are stored in the presence of the source of sulphur dioxide for an appropriate time before use or marketing. The sources of sulphur dioxide which may be used are as broadly described above.

In addition to the use of a metabisulphite salt or other source of sulphur dioxide, the method in this aspect of the present invention may also include the use of an anti-transferant to prevent ingress and egress of water in the fruit, thereby retaining the moisture content thereof. Such anti-transferants have previously been used in the storage of soft fruits such as pears, apples, plums and cherries. In accordance with this aspect of the present invention, the use of the anti-transferants on nuts such as chestnuts either prior to or after treatment a source of sulphur dioxide not only provides extension of the post-harvest shelf-life, but also prevents moisture loss, in at least some cases and development of mould and fungi, in the stored nuts. As a result, the nuts undergo minimal deterioration in quality on storage.

Suitable anti-transferants include, for example, Semperfresh (available from Colin Campbell (Chemicals) PtyXtd., which is a mixture of sucrose esters, sodium carboxymethyl cellulose and mixed mono- and diglycerides of fatty acids), and chitosan (which is the deacetylated product of the alkali treatment of chitin, obtained from crustaceans such as crabs, lobsters, shrimps, and the like) .

Prior to the present invention, methods which have been proposed for the storage of nuts such as chestnuts include storage at low temperatures of 0- 1 ^βC, or storage in a controlled atmosphere such as nitrogen containing carbon dioxide and/or oxygen. Storage at low temperatures is not efficient or practical, whilst storage in a controlled atmosphere is in general only effective whilst the nuts are maintained in the controlled atmosphere, and once the nuts have been removed from the controlled atmosphere they are subject to normal deterioration, often even accelerated deterioration. In storage trials which have been conducted in accordance with the present invention wherein chestnuts were stored with sodium metabisulphite powder, it has been found that the nuts can be stored for a period in excess of two months and periodic sampling of the nuts to assess moisture content and mould development has not detected any unacceptable deterioration during storage.

In the foregoing description, emphasis has been placed on the peeling and storage of chestnuts, however it will be appreciated that the techniques of the present invention are also applicable to the processing of other nuts having a similar shell and skin structure, particularly almonds, cashew nuts, hazelnuts, pecan nuts, pistachio nuts, macadamia nuts, walnuts and even coconuts.

Further details of the present invention and of the results which have been obtained during trials of the techniques of the present invention are set out in the following Example.

EXAMPLE 1

MATERIALS:

Chestnuts (Castanea sativa) were harvested in Stanley, North East Victoria, late in March 1991, and handled in the usual manner, involving hand harvesting, removal of burr, and mechanical polishing followed by grading according to size.

Within 24 h of harvesting, the chestnuts (about 27 kg and 8 kg of "hard" and "soft" peeling chestnuts, respectively) were transported overnight to the laboratory in hessian sacks. The internal temperature of the sacks was recorded at 17.5 °C (average of 4 readings).

Within two hours of arrival at the laboratory, the sack of "hard peelers" (27 kg) was halved into another hessian sack for ease of handling. Each sack was then placed into a black plastic bag to prevent water uptake when the bags were packed in ice (keeping the nuts at about 0 °C) in a large polystyrene bin with draining holes.

The iced bags were then stored in the refrigerator (2 °C) for about 1.5 weeks until transported under ice to another laboratory where a number of storage trials were initiated at various time intervals, meanwhile the chestnuts in black plastic bag - hessian sacks were stored at 0 °C ± 1 °C.

The "easy peelers" (8 kg) were stored in ice (0 °C) as described for the

"hard peelers" for about 2 weeks. The remaining chestnuts were then transferred to the blast freezer (-40 °C) for storage after being cryogenically (liquid N) frozen.

TRIALS:

A. Storage at room temperature (18-20 ^β C)

One hessian sack (2 kg) of chestnuts was labelled and stored at ambient storage temperature (18-20 °C). Each week, a sample of 20 chestnuts (about 100-200 g) was withdrawn and monitored. This was the reference sample for the trials as it represents the typical conditions for storage and handling by wholesalers and retailers. B. Treatment with Sodium Metabisulphite

Polyethylene bags (13) were filled with 22 (about 150 g) whole chestnuts each and weighed. Into each bag, sodium metabisulphite (5 g) was added, the bag sealed and re-weighed. The bags were stored at 1 ± 1 ^CC in an ambient atmosphere and at regular intervals a bag was withdrawn and the quality of the chestnuts monitored.

In comparative trials, chestnuts (about 300 g) in polyethylene bags were stored in refrigerated air (1 °C ± 1 °C), controlled atmosphere (2.5% C0₂, 2.5% 0-₂, N₂) anti-transferant (Chitosan) and anti- transferant (Semperfresh plus sodium metabisulphite), respectively.

The quality characteristics recorded were: moisture loss, peeling ease, and microbiological quality (mould growth).

RESULTS: Peeling:

After the treated chestnuts had been exposed to sodium metabisulphite for one month, it was noted that the peeling procedure for raw nuts (which normally involves cracking the outer shell and then laborious hand and knife scraping of the inner skin from the raw nut) involved only peeling by hand of the outer shell and a gentle rubbing to remove the inner skin.

Moisture Loss: Within one week at room temperature, the moisture level of the reference sample had dropped from 55% to 40%, about a 27% loss of moisture, significant in terms of yield and cost. By contrast, the moisture level for the metabisulphite-treated samples had decreased from 55% to 51%, a 7.2% loss over a month storage period. It was also noted that moisture retention could be improved because the Chitosan, controlled atmosphere and Semperfresh samples had less than a 1% loss of moisture over the same period. Mould Growth: Whilst mould numbers increased from 500 (5 x lO g) per nut up to

10,000 - 100,000 per nut for the refrigerated, Chitosan and Semperfresh samples over a one month storage period, no mould growth was detected for either the control or the metabisulphite-treated samples.

The very low moisture loss in the reference sample would prevent the growth of mould in that sample. Significantly, there was a decrease in the mould level/chestnut for the metabisulphite-treated samples during storage.

EXAMPLE 2

MATERIALS:

Chestnuts (Castanea saliva) were harvested in Stanley, North East Victoria, April 1992 and handled as in Example 1. Other nuts of the same species, but different varieties, were obtained from a number of farms in the Monbulk region of Victoria. The nuts of the different varieties were kept separated for each trial undertaken.

TRIALS

Ai Interaction of sodium metabisulphite and chitosan. Solutions of chitosan (0.5%) and sodium metabisulphite (1% and 5%) were prepared. Small numbers (5-10) of nuts were treated by dipping in these solutions as follows:

Group 1- chitosan (for 10 sec) and then either 1% or 5% sodium metabisulphite for 5 min; Group 2- sodium metabisulphite (1%) for 5 min and then chitosan;

Group 3- sodium metabisulphite (5%) for 5 min and then chitosan;

Group 4- controls : no treatment or either agent singly. The treated nuts were then refrigerated (4 °C) and evaluated periodically for shell bleaching, peelability, inner flesh quality (colour and texture) and flavour.

Results:

With the exception of the untreated nuts, the flesh quality and flavour were ranked as excellent, however only the nuts treated with 5% sodium metabisulphite treated nuts showed slight improvement with respect to ease of peeling, after 10 days storage.

The nuts were examined periodically and with time the peeling improved. The peelability of the nuts of Group 3 (5% sodium metabisulphite followed by chitosan) was rated as good (+ +) after 2 weeks storage, with the inner flesh quality still rated as excellent. After some 6 weeks, the peeling rating of these nuts had improved to excellent ( + + + ) with the inner flesh quality still maintained at excellent. Significantly, the lower concentration (1%) of sodium metabisulphite was less effective with respect to peelability over the time period of the experiment, and the order of treatment with sodium metabisulphite and chitosan did affect the rate but not the trend of the improvement in peelability.

A similar experiment was conducted in which the nuts were undipped or dipped in chitosan (0.5% /5 min) and then exposed indirectly (see trial C) to 0.1 to 5.0 g of sodium metabisulphite (per about 10 nuts). After 2 weeks, even though coated with chitosan, the peelability of the dipped nuts treated with 5.0 g of sodium metabisulphite was rated as excellent, as was the quality of the inner flesh. Similar results were observed after 3 weeks with dipped nuts treated with 2.5 g of sodium metabisulphite.

B: Investigation of other factors which may affect peelability.

Factors investigated were : storage temperature (4 °C versus 8 °C)

: direct application of sodium metabisulphite : pre-slitting of nut before exposure to sodium metabisulphite.

In a preliminary trial, nuts (approx. 3 kg) were first dipped in chitosan solution (0.5% /5 min) before being divided into 5 groups and treated as follows:

Group 1- treated directly with sodium metabisulphite (2.5 g per 5 nuts) and stored at 4 °C; Group 2- longitudinal slit (= 2 cm), and then shaken as in (1) with sodium metabisulphite (2.5 g per nuts) and stored at 4 °C; Group 3- as for (2) but indirectly exposed to sodium metabisulphite (see Trial C); Group 4- as for (1) but stored at 8 °C.

The control group (Group 5) consisted of untreated nuts (no chitosan or sodium metabisulphite) with or without slitting.

Results

After 6 days, the peeling quality and inner flesh quality was ranked highly ^"(good + +) for Group 1. Over the same period, the nuts of Group 4 showed only slight improvement in peelability. However, after 15 days storage at 8 °C, the nuts of Group 4 were of unacceptable quality.

With respect to the effect of pre-slitting the nut, the peelability did improve as the nuts of Group 2 were ranked as excellent (+ + +) after 15 days storage with respect to peeling. However, the quality of nuts needs further investigation.

The effect of temperature (4 °C versus 8 °C) was evaluated for nuts indirectly exposed to sodium metabisulphite (see Trial C). The peelability and inner flesh quality was ranked as excellent (+ + +) for nuts exposed to 2.5 g of sodium metabisulphite and stored at 4°C for 15 days and 12 days at 8 °C. Thus the interaction of the method of application and the storage temperature affect the peelability and flesh quality.

C: Indirect exposure to sodium metabisulphite. Polyethylene bags were filled with varying amounts (0.1 g to 10 g) of sodium metabisulphite, and nuts (about 10 per bag) were placed on a perforated rack which was positioned in the bag such that there was no direct contact between the nuts and the sodium metabisulphite. The nuts were assessed periodically for shell bleaching, peelability, inner flesh conditions and flavour.

Results:

For nuts exposed to > 2.5 g sodium metabisulphite (per about 5-10 nuts), peelability reached the ranking of excellent, i.e. the inner skin "lifted off" from the flesh including the depths of grooves with minimal effort. The difference in treatment with various amounts of sodium metabisulphite was the time taken to reach the category "excellent" for peelability, and the subsequent rate of deterioration in peelability - as the level of sodium metabisulphite increased, the rate of deterioration in peelability increased once the peelability was at its optimum. Furthermore the quality deteriorated with extended exposure, i.e. the nut lost its crispness and became rubbery and instead of the typical creamy white flesh colour the nuts developed a purple tinge. Significantly, when the peelability characteristic was at its optimum, the inner flesh condition was also categorized as excellent.

It will be appreciated that many modifications and variations may be made to the processes specifically described without departing from the broad concept of the present invention as described herein, and the present invention extends to include all such modifications and variations.