WO1996013171A1

WO1996013171A1 - Treatment and production of potato slices for preservation

Info

Publication number: WO1996013171A1
Application number: PCT/GB1995/002508
Authority: WO
Inventors: Peter George Howard
Original assignee: Retep Foods Limited
Priority date: 1994-10-29
Filing date: 1995-10-25
Publication date: 1996-05-09
Also published as: EP0784435A1; AU3704495A

Abstract

A method of treating and packing a vegetable foodstuff such as chipped potatoes comprising the steps of blanching the foodstuff by treatment in an aqueous liquid at a suitably high temperature, treating said foodstuff in a solution containing one or more non-toxic or food acids, subjecting said foodstuff to a frying process and sealing the treated foodstuff in a sterile container containing an inert gas and oxygen. The treated and packed foodstuff having a pH of not less than 4.8.

Description

TREATMENT AND PRODUCTION OF POTATO SLICES FOR PRESERVATION

This invention relates to a method of manufacture of partially prepared vegetable foodstuffs and more particularly, although not exclusively, to products such as sliced potatoes, the storage of same at ambient temperature and to hermetically sealed containers of such potato slices.

Foodstuff products such as prepared and sliced potatoes have increased in popularity with approximately

10 to 20 million portions of such products consumed . in the UK annually. This consumption includes both domestic use and commercial use.

When developing a process for food production and/or food preservation it is vital that there is no pathogenic risk to the consumers of the foodstuff as a result of microbial contamination. One of the most important microorganisms in this regard is the Gram positive anaerobic bacterium, Clostridium botulinum . This bacterium produces an exotoxin which is liberated from intact bacterial cells into the surrounding medium during growth. The exotoxin causes specific damage to the nervous system. Consumption of foodstuff contaminated with this toxin results in a form of food poisoning known as botulism. Botulism can be fatal as a result of cardiac arrest or respiratory failure. This bacteria is commonly found on vegetation and it is therefore vital that the presence of such microoganisms in food products and/or the processes of producing same be minimised as far as possible.

Accordingly, a need has been identified for a cost effective method of producing such foodstuff products on a large scale and of equal importance a cost effective method of storing such products without any reduction in quality.

Previous methods of preserving such foodstuff as partially prepared potato slices usually utilise such processes as freezing or chilling in which case the foodstuff is not sterile. Under such conditions the growth of spoilage microorganisms present in the foodstuff being controlled to a greater or lesser extent according to the initial load of such organisms and/or their spores in the foodstuff and the temperature of storage. Both freezing and chilling methods require very high initial capital expenditure for freezing or chilling apparatus and very high costs for energy use in the chilling or freezing process, such costs also being incurred throughout the storage and distribution chain.

The chilled potato slice has the further disadvantage of a relatively short life of a maximum of 21 days at chill temperature before bacterial growth makes the foodstuff unacceptable for consumption.

Other methods used for the preservation of such foodstuffs are well known and include the use of sulphur dioxide as a preservative. However, health concerns about sulphur dioxide mean that such methods are no longer acceptable.

It has also been previously proposed that food products could be treated with acid in order to preserve them. International patent application number PCT/W093/19609 teaches subjecting the potato product to a long acid blanch at a high temperature prior to packing in a hermetically sealed container in the absence of oxygen. Whilst this process provides a means of preserving the sliced potato, it has been discovered that consumers find the taste of the finally cooked product unacceptable, detecting an acid after-taste.

Accordingly, a need has been identified for improvements in such methods or generally. It is an object of the present invention to attempt to provide improvements in relation to one or more matters discussed above; and/or to improve matters generally.

According to the invention there is provided a method of processing and preserving vegetable foodstuff as defined in the accompanying claims.

Thus, in accordance with a broad aspect of the invention by providing for more acidic treatment of potato slices for a shorter period of time than has been previously proposed and following this with a par-frying step which may be longer than is conventional, together with the step of packaging the product in a relatively hot condition and ensuring the presence of oxygen in the final packed product, notable advantages are provided in an embodiment. These include storage of the product at ambient temperature without the need for capital equipment costs related to refrigeration and/or freezing as have been previously required and the avoidance or reduction of an acidic after-taste.

In an embodiment of the invention there is provided a method of processing vegetable foodstuffs such as potatoes into food products such as prepared slices and the preservation of such foodstuffs comprising the steps of blanching the potato foodstuff by treatment in aqueous liquid at a suitably elevated temperature, treating the foodstuff in a solution of pH between 0.5 and 3.0 and preferably 0.8 to 2.5, which solution contains one or more non-toxic or food acids, subjecting the foodstuff to a frying process which has been found to reduce the water activity in the product, maintaining the foodstuff at a temperature of between 50°C and 95°C and preferably 75°C and sealing the treated, hot foodstuff in a sterile container containing an inert gas, and oxygen. The foodstuff having a pH of between 3.0 and 7.0 and preferably no less than 4.8.

In the present specification it is understood that references to preserving the foodstuff means extending the shelf life of said foodstuff and reference to sliced refers to discrete pieces of whole potato as opposed to powdered potato and includes all the usual forms of slices, including those which are employed to form "chipped" potatoes.

In the embodiment, the potatoes are peeled and sliced to the required size. The slices are then blanched in hot water, for example at a temperature from about 65 degrees centigrade to about 95 degrees centigrade from a time of about 10 to about 20 minutes. This has the benefit of removing excess reducing sugars from the potato slice which can cause undesirable browning or blackening of the finished chip.

Such blanching also has the beneficial effect of removing excess starch from the potato. Starch has a buffering effect on food acids and by removing some of the starch it is made possible to reduce the concentration of acid required in the next stage of production. The sugar blanch creates some breakdown of the cell structure of the potato, facilitating penetration of acids in the next phase of the process.

This particular step in the process also has the advantage that water used in the sugar blanch may be re-used, reducing the requirements for clean water or effluent spray and disposal. The blanched chips are preferably, but not essentially, sprayed with water to remove surface starch and bacteria or bacterial spores adhering to the surface.

Following the blanching process, chips are dipped briefly for 15 to 60 seconds and preferably 30 seconds in a acidic solution at a temperature of 40 to 95 degrees centigrade and preferably 70 - 80 degrees centigrade. The pH of the solution is in the range 0.5 to 3.0 and preferably 0.8 to 2.5. The acids used are either hydrochloric, orthophosphoric, acetic, tartaric, malic or citric acids. These acids may be used either singly or in combination. They are all permitted to be included in foodstuffs by the Miscellaneous Additives in Foods Regulations 1974.

Typically, the best results are achieved by using hydrochloric or orthophosphoric alone or in a mix of the two acids or one of those in combination with acetic, tartaric, malic or citric acids. The most preferred treatments from the point of view of reducing the possibility of acidic off-flavours being detected in the final product are either a blend of orthophosphoric and tartaric acid when the pH of the solution is in the range of 1.0 - 3.0 or preferably 1.6 -2.5, or hydrochloric acid when the pH of the solution is in the range 0.5 - 2.0 and preferably 0.8 to 1.4.

The time for which the chips are dipped in the acid may vary from about 15 seconds to about 60 seconds. The concentration of acid required to produce the required concentration within the chip is directly related to the size of the chip, the larger the chip, the higher concentration and thereby lower pH of dip required.

If the preferred acid, hydrochloric is used, it is preferred that the chips be cooled to 50 to 60 degrees centigrade by spraying with water not warmer than 20 degrees centigrade before dipping in the acidic solution. If chips are dipped when at a temperature of about 70 degrees centigrade or above the hydrochloric acid gelatinizes any remaining surface starch. When subsequently fried, chips so gelatinized become very hard and blistered on the surface which is considered to be an unsatisfactory quality feature.

In a continuous production situation, the initial pH of the dipping solution is preferably maintained at a constant level by injecting acids, for example a pH of 1.00 when treating chips of 12 mm cross section or a concentration of 0.8 per cent if using 28 per cent hydrochloric acid. If a blend of more than one acid is used a concentrated acid mix is injected to the dip, that concentrate being, for example a mix of 50 parts water, 50 parts tartaric acid, 50 parts orthophosphoric acid. The constant level of pH may be maintained, for example, by use of a dosing pump with automatic pH controller. If batches of chips, rather than continuous production, are made in the same dip solution, acid may be added to return dip to previous pH after each batch is dipped.

Following the dipping procedure chips may be dried to remove moisture. Drying may be effected by passing hot air over the chips. The drying can improve the quality of the finished chips and extend the life of frying oil by reducing hydrolysis. The drying process can also reduce the weight of the chips by about 10 per cent. This step also assists to reduce the water activity in the packed product. This has the advantage of assisting to maintain the sterility of the packed product by discouraging the development of spoilage microorganisms such as CJbotu.Ii.nu.^**..

Next, the chips are par-fried. The length of par- frying time and the temperature of par-frying is determined in part by the initial dry matter of the potato, partly the size of chip, and in part by the market for which the product is required. For example, a flash par-fried potato chip may be finally fried for about 45 to 90 seconds at approximately 160 to 180 degrees centigrade to give equivalent quality to the normal frozen potato chip. A par fried chip may be fried for about 3 to 6 minutes at about 130 - 160 degrees centigrade. Such a chip has a higher dry matter than the former and can, therefore, be fried for final consumption in a shorter time and with lower shrinkage and fat uptake. It is more costly to produce due to the higher shrinkage in the longer fry. Oven chips, those for preparing by the final user in the oven, are fried for 5 to 7 minutes at about 140 to 160 degrees centigrade. The step of frying also assists to reduce the water activity in the packed product with the subseqent advantages of reducing the development of microbial spoilage organisms. The par-frying step, in addition to assisting to produce a partially prepared product for the market place, it has also been found to have the unexpected benefit that the frying stage has the advantage of acting as a further heat treatment step in the production of the sliced potato product, thus assisting to reduce the development of spoilage microorganisms.

We have found that subjecting the chips which have been previously treated in phosphoric and/or tartaric acid to a glycerol treatment, acts to reduce the water activity in the chip and also assists to impart an acceptable sweet taste to the product. The inclusion of salt to the acid mix also contributes to a reduction in water activity with the attendant advantages discussed previously.

After frying, excess fat is removed and the foodstuff filled, whilst still hot, preferably at a temperature of 75 degrees centigrade, into a container. Whilst any sort of container may be used which can be hermetically sealed, one manufactured from a thermoplastic material is preferred. As will be well understood by those skilled in the art, the gas and vapour permeability of the container will depend on the gas and vapour permeability of the materials used in its construction, and in general the shelf life of the treated foodstuff of the invention will be reduced if it is exposed to oxygen permeating into the container. In general, as will be appreciated, it is preferred to use very low oxygen permeable materials such as containers made of laminates of nylon and polyethylene, laminates of polyester and polyethylene, and extrusions of ethylene vinyl acetate co-polymerwith lineal low density polyethylene, or other laminates with equally low gas permeability. Standard shelf life is for example 4 to 6 weeks. However if a shorter shelf life is required eg 3 weeks it is also possible to pack in much cheaper polyethylene films. For the longest possible life, for example 3 to 6 months the treated foodstuff may be packed in containers made from metallized films or aluminium foil laminates, which have even lower permeability, but these have the disadvantage of being opaque and thus preventing visual inspection by the purchaser of foodstuffs packed therein. In general, the packaging material selected will be that which provides the shelf life required by the user at the lowest packaging cost.

After filling the containers with the hot food product, the air in the container is evacuated as far as possible and replaced by an inert gas or combination of inert gases, for example, nitrogen and carbon dioxide. It is important that the packed product should contain no added liquid and include oxygen, preferably in the region of 2 per cent to help control the risk of the development of C .botulinum . In a further embodiment of the invention, excess fat is removed from the chips after the frying stage, the chips are then cooled to below 4 degrees centigrade and packed. If the product is to be packed loose in boxes and maintained at chilled temperature for the life of the product (usally in the region of 7 days) no further processing is required.

If it is desired to keep the product for a period longer than 7 days, the product is filled into a plastic container, usually a bag formed on a vertical form fill machine. The bag is flushed to remove at least 95% of the oxygen and then gas flushed with food grade nitrogen or, preferably, with a mixture of 60% nitrogen and 40% carbon dioxide. The carbon dioxide assists to inhibit the growth of bacteria. The final pH of the product is carefully monitored and maintained below 5.4.

The advantages of this embodiment of the invention include extending the shelf life of chilled potato chips treated as hereinbefore described, particularly those chips packed loose in boxes, from 7 days to in the region of 3 weeks.

The process also enables the shelf life of chips packed in sealed, gas flushed containers and chilled, to be extended from 21 days to in the region of 5 weeks.

A further advantage of the process is that it enables chilled packed chips to have a shelf life of in the region of 2 weeks at chilled temperatures followed by one week shelf life at ambient temperatures. This allows for a chilled distribution of the product but a purchaser with limited freezer or chilled space can store a product treated by this process at room temperature.

It is a feature of the invention that the blanched acid treated foodstuff has either been sterilised, (for example by heat, during the process as a result of the dipping and frying stages and the sterility thus created is maintained by "hot filling" the product into the container) or, an environment has been created inside the container in which any bacterial spores which may remain in the foodstuff after the treatment will not grow. Sterilisation is achieved by a combination of time, temperature and pH. By continued research and development we have discovered that by decreasing the pH of the acid dip it is possible to correspondingly lower the temperature of the dip and the exposure time of the foodstuff to the acid dip required to achieve a product which is either bactericidal or bacteriostatic. This has the advantage that the process requires less energy which represents a cost saving. It also provides the additional benefit that the resultant pH of the treated foodstuff is less acidic and therefore more paletable to the consumer.

A major factor influencing sterility is the water activity within the pack. It has been found that by reducing the water activity in the foodstuff by the addition of salt to acid dip and/or by frying, the food product can be packed and stored safely at the higher pH.

When developing a process which depends on heat process technology it is important to demonstrate that there is no pathogenic risk to consumers of the foodstuff. Heat penetration studies have shown that in, for example, a slice of 14 mm cross section which is fried for 5 minutes at 140 degrees centigrade the whole slice receives a Botulinum cook through 90 per cent of its volume. The remaining 10 per cent lies in the core of the slice. Undamaged vegetable tissue is sterile and resists penetration of organisms. A Botulinum cook is one which ensures that any spores of C. botulinum which may be present are reduced in number to 10¹² which is the accepted standard of protection against the botulisum hazard in the food industry. The only spores of the organism which may not have been destroyed are those which may have penetrated to the very centre of the slice chip via bruised or diseased tissue.

The fried potato slice has been found to have a water activity of 0.97 to 0.98 depending upon length of fry. It is recorded in the literature that C . botulinum spores will not grow at pH 5.2 if water activity is 0.97 and nor at pH 4.8 if water activity is 0.98 at pasteurization temperatures as reached at the centre of the slice. Thus, in the very unlikely event of spores reaching the centre of the slice, they will not grow and form toxin in the conditions created. Previous work has shown that the inclusion of 2 per cent oxygen in the sealed container inhibits the development of C . botulinum spores.

In this process regard is paid to the above factors and the permitted maximum pH of a macerate of fried chips is increased to about 5.4 for chips fried for about 5 minutes or for longer times and to about 5.0 for those fried for less than 5 minutes in which case the heat process given in frying is much less severe. This reduces the concentration of acid within the chip, thereby reducing an acidic or off-flavour being detected by tasters.

Hence if a 12 mm chip is dipped for 30 seconds in a solution of 10 mis per litre of 28 per cent hydrochloric acid, the chip then being dried and fried for 5 minutes it has a pH of 4.S. If the strength of the solution is reduced by 25 per cent to 8 mis per litre, the pH of the chip is increased to 5.2.

It has been found by experiment that if potatoes of about 19 per cent dry matter or lower are processed as described above, the chips made have a hard texture. Whilst it is preferred to use potatoes of dry matter exceeding 20 per cent, it has also been found that if a variation is made in the process described above, chips of acceptable quality can also be made from potatoes of lower dry matter. The process is varied by applying additional heat process to soften excessive hardness in the low dry matter chips. This is achieved by blanching in hot water at 95 degrees centigrade or thereabout for about 7 minutes immediately after the sugar blanch and before washing and dipping. Alternatively such a blanch may be done in non-pressurised steam, the blanching times being about 6 minutes.

For chips with a lower dry matter flash fried as described above the shorter frying time of up to 90 seconds may be found to be inadequate to ensure sterility of the chips after packing, in which case chips are steamed in non pressurised steam for about 4 to 7 minutes and preferably 6 minutes between the dipping in acid as described above and the frying. If such a procedure is adopted it is not possible to dry chips before frying.

The present process utilises a long fry to obtain bacterial kill preceded by a very short acid dip at low temperature and low pH to help keep the product stable after packing. It was found that it was possible to decrease the pH in the blanch solution and then to reduce the time this product is exposed to the acid by utilising the frying step, normally used to produce the partially cooked product required by the consumer, to complete the bacterial kill usually achieved during the long acid blanch step. In this process a drying step is included which combined with the long fry greatly reduces the water activity. It was found that utilising these steps enabled the process to achieve the required bacterial kill and results in a stable product.

A further advantage of the process is that it is not necessary to further heat treat the packaged contents with, for example, steam to achieve pasteurisation as is conventional.

A still further advantage provided by the process is that the process can produce a food product having a pH of 4.6 - 5.4. which does not suffer from the disadvantage of being considered as having an unacceptable taste. As a consequence of the higher pH the product must be packed in the presence of oxygen, preferably in the region of 2 per cent, to avoid the possibility of a C.jbotuIinu-71 hazard.

The following examples illustrate the invention

EXAMPLE 1

Potatoes are peeled, hand trimmed to remove all eyes and passed through a chipping machine to give chips of 12 mm cross section and silvers removed. 10 kgs of chips were prepared in this manner. The chips are then blanched for 15 minutes at a temperature of 75 degrees centigrade, washed in running water and then dipped for 30 seconds in a solution of 30 litres water, 240 mis 28 per cent HCI, pH of solution 1.00. If required, the chips are then dried until weight was reduced by 10 per cent by blowing air at about 70 degrees centigrade and then fried for 5 minutes at 140 degrees. They were immediately filled into pouches made from a laminate of nylon and polyethylene. They were then put into a vacuum sealer and evacuated to 980 mbars, nitrogen was flushed into the pack until the vacuum was reduced to 400 mbars, leaving 27 oxygen and the packs were then sealed by a double impulse seal. After sealing and resting for 24 hours the treated potatoes were checked for acidity. The acidity was found to be 5.2. After storage at ambient conditions the potatoes were examined and found to be in acceptable condition.

After dipping one batch of chips, the pH of the dipping solution was returned to pH 1.0 by adding concentrated hydrochloric acid and a further batch was then prepared.

EXAMPLE 2

As Example 1 except that the dipping solution 30 litres water, 160 mis hydrochloric acid, 80 mis Orthophosphoric acid.

EXAMPLE 3

As Example 1 except that dipping solution was 30 litres water, 200 mis hydrochloric acid, 60 grams malic acid.

EXAMPLE 4

As Example 1 except that dipping solution was 30 litres water 190 mis hydrochloric acid 28%, 70 grams citric acid.

EXAMPLE 5

As Example 1 except that dipping solution was 30 litres water and 1.5 litres of pre-mixed concentrate made by mixing water 50 parts, tartaric acid 50 parts, Orthophosphoric acid 50 parts.

EXAMPLE 6 As Example 1 except that potatoes were cut 14 x 14 is and then fried for 5 minutes at 140 degrees. The solution strength increased to 260 mis hydrochloric acid 28 per cent to 30 litres water. pH of fried chips was found to be 5.15.

EXAMPLE 7

As Example 1 except that potatoes were cut 8 x 10 mm, so called Steak Chips. Dipping solution as in Example 5. pH of fried chips after 7 days storage was found to be 5.05.

EXAMPLE 8

10 kgs chips were prepared as in Example 1 except that they were cut 10 x 10 mm. They were then blanched for 15 minutes at 65 degrees centigrade and then dipped in solution of 3 litres water, 320mls 28% hydrochloric acid for 30 seconds, dried 107 and fried for 75 seconds at 170 degrees. They were then filled and sealed as in Example 1. pH of chips after 24 hours was 4.90.

EXAMPLE 9

As Example 8 except that chips were cut 8 x 8 mm, so called Stringfellow Chips. Blanched 15 minutes at 65 degrees, dipped in solution of 30 litres water and 1.3 litres of concentrate as used in Example 5 and fried for 60 seconds. Chips were tested for pH after 24 hours which was 4.85. Pouches were opened after 14 days storage, chips fried for 2 minutes and found to be of excellent quality.

EXAMPLE 10

Chips prepared as in Example 1, were cut 10 x 10mm, dipped for 30 seconds in solution as Example 1, weight reduced by 10 per cent by drying and fried for 7 minutes at a temperature of 135 degrees. The pH of the chips was 5.00. After 7 days the pouch was opened, the chips spread on an oven tray and baked in an oven at 200 degrees centigrade for 15 minutes. They were considered to be of superior quality to the well known oven chips.

EXAMPLE 11

Chips were prepared as in Example 1. They were blanched for 15 minutes at a temperature of 75 degrees. They were then blanched in water at 95 degrees, dipped in acid solution as in Example 1, dried, fried and packed as in Example 1. Potatoes used in this Example were of 19 per cent dry matter.

EXAMPLE 12

Chips are made as in Example 11 except in that chips are blanched in non-pressurised steam for 6 minutes after the 15 minutes blanch at 75 degrees.

EXAMPLE 13

Chips are made as in Example 8 except that chips were blanched for 6 minutes between dipping in acid and frying, the drying process being omitted. Potatoes of 19% dry matter were used in preparing this Example.

EXAMPLE 14

A concentrate was made consisting of :

%

Water 37.5 Glycerol BP 37.5

Tartaric Acid 12.5

Orthophosphoric Acid 12.5

Potato chips were prepared and packed as Example 1 except that in 30 litres of water the concentrate was 900 mis giving a pH of 1.5. The final pH of chips was 5.2.

EXAMPLE 15

As above escept that the chips were left in the acid solution for 45 seconds. The final pH of the chips was 4.9.

EXAMPLE 16

A concentrate was made consisting of :

%

Water 60

Tartaric Acid 10

Ortophosphoric Acid 10

Salt 20

Potato chips were packed and prepared as Example 1 except that in 30 litres of water the concentrate was 1200 mis giving a pH of 2.15. The final pH of the chips was 5.2.

EXAMPLE 17

About I8kgs of potatoes were peeled, hand trimmed to remove all eyes, then passed through a chipping machine to give chips of 12mm cross section, slivers removed leaving lOKgs of chips. These were then given a "sugar blanch" for 12 minutes at a temperature of 70°c and dipped in an acid solution for 30 seconds at 70°c. This acid solution consisted of 30 litres of water and 250 is of 28% Hydrochloric acid. The pH of the solution being 1.0.

The chips were then dried until the weight was reduced 10% by blowing air at 70°C through the chips which were then passed through a fryer heated to 140°C. After 5.0 minutes in the fryer the excess fat was removed and the chips cooled to 2°C and filled into a pouch made from a laminate of nylon and polythene. The pack being gas flushed with a mixture consisting of 60% Nitrogen and 40% C02 and then sealed. The packs were then kept in chilled storage for two weeks, followed by ambient storage at one week.

During storage the packs were checked for acidity and water activity levels. The pH was found to be 5.2 and water activity was 0.97.

After dipping one batch of chips the pH of the dipping solution was returned to 1.0 by adding concentrated Hydrochloric acid and a further batch was prepared.

EXAMPLE 18

As example 17 except that the dipping solution contained 30 litres of water, 160 mis hydrochloric acid and 80 Mis Orthophosphoric acid.

EXAMPLE 19

As example 17 except that the dipping solution contained 30 litres water 190 mis hydrochloric acid and 70 gms citric acid. EXAMPLE 20

As example 17 except that the dipping solution contained 30 litres water 200 is hydrochloric acid and 60 is malic acid.

EXAMPLE 21

As example 17 except that the dipping solution was 30 litres water and 1.5 litres of pre mixed concentrate made by mixing water 50 parts. Tartaric acid 50 parts and orthophosphoric acid 50 parts.

EXAMPLE 22

As example 17 except that the potatoes were cut 14 x 14 mm and then fried for 5 minutes 29 seconds. The solution strength was increased to 260 is hydrochloric acid to 30 litres water. The pH of the fried chips was 5.15.

EXAMPLE 23

Chips were prepared as in example 17 but were cut to

10 x 10 mm but after dipping and drying were fried for 7 minutes at 135°C. The pH of the chips was 5.05. After 7 days the pouch was opened, the chips spread on an oven tray and baked for 15 minutes at 200°C.

EXAMPLE 24

Chips were made as example 17 except that the chips were blanched for six minutes between dipping in acid and frying, the drying process being omitted. EXAMPLE 25

Chips were prepared as example 17 except that the frying time was reduced to 3 minutes. An acid concentrate was added consisting of 50% water, 15% tartaric acid, 15% orthophosphoric acid and 20% salt at the rate of 2.0 litres. The resulting pH being 5.18 and the water activity was 0.97.

EXAMPLE 26

Chips were made as example 17 except that after chilling to 2°C the chips were filled immediately into cardboard boxes, with no other packaging. After three weeks at chill the product was found to be in good condition.

EXAMPLE 27

Chips were made as Example 17 except that after chilling on the production line, the chips were kept at ambient temperature for two weeks, and found to be in good condition.

EXAMPLE 28

The potatoes are peeled, trimmed and chipped as in Example 1.

The chips are then blanched for 15 minutes at a temperature of 75°C, washed in running water and dipped for 1 minute in a solution of 40 litres of water containing 1400ml acid solution, said acid solution comprising 84% water, 6% tartaric acid and 10% orthophosphoric acid. The chips are then fried for 4.5 minutes at 140°C. The pH of the prepared chips was found to be 5.2.

EMAMPLE 29

The potates are prepared as described in Example 9 and are treated as set out in Example 28 above except the chips are dipped for 30 seconds in the acid solution and are fried for 3.5 minutes at 140°C. The pH of the prepared chips was found to be 5.05.

Claims

CLAIMS : -

1 A method of processing and packing a vegetable foodstuff to preserve it which method comprises the steps of blanching the foodstuff by treatment in an aqueous liquid at a suitably elevated temperature, treating the foodstuff in an acid solution of pH 3.0 or less which solution contains one or more non-toxic food acids, for a time and at a temperature such that substantially no bacterial cells or spores can grow, drying the foodstuff and subjecting the foodstuff to a frying step for a time and at a temperature such that the water activity of the foodstuff is sufficiently reduced so as to be bacteriostatic or bactericidal; maintaining the elevated temperature of said foodstuff, sealing the treated foodstuff into a container, wherein air is replaced by an inert gas, and oxygen, the foodstuff contained therein having a pH of not less than 4.8.

2 A method of processing and packing a vegetable foodstuff in which the method comprises the steps of blanching the foodstuff by treatment in aqueous liquid at a suitably elevated temperature, treating the foodstuff in a solution of pH 3.0 or less which solution contains one or more non-toxic food acids, drying the foodstuff to remove excess moisture, subjecting the foodstuff to a frying process, sealing the treated foodstuff in a sterile container containing an inert gas and oxygen, said foodstuff contained therein having a pH of between 3.0 and 7.0

3 A method of processing and packing a vegetable foodstuff in which method comprises the step of treating the foodstuff in a solution of pH 3.0 or less. 4 A method according to any of the preceeding claims wherein said foodstuff is blanched in water heated from 35 degrees celsius to no greater than 95 degrees celsius for a period of 10 to 20 minutes whereby excess starch is removed from the foodstuff.

5 A method according to claim 4 wherein said blanched foodstuff is sprayed with water to remove any surface starch, bacteria or bacterial spores adhering to the surface thereof.

6 A method according to claim 1 or claim 2 wherein the vegetable foodstuff is dipped in the acid solution.

7 A method according to claim 6 wherein the acid solution is heated to from about 60 degrees celsius to less than 95 degrees celsius and the foodstuff is dipped therein from 15 to 60 seconds, the pH of the solution being maintained at from 0.8 to 2.5 throughout the period.

8 A method according to claim 7 wherein said acid dip comprises an acidic solution of a mixture of one or more of hydrochloric, orthophosphoric, acetic, tartaric, malic and citric acid.

9 A method according to claim 8 wherein the acid solution also contains salt.

10 A method according to claim 8 or claim 9 wherein the acid solution also contains glycerol.

11 A method according to claim 1 or claim 2 wherein the foodstuff is dried by hot air after removal from the acid.

12 A method according to claim 1 or claim 2 wherein the vegetable foodstuff is fried, said frying step reducing the water activity of the foodstuff to in the region of 0.97 - 0.98.

13 A method according to claim 12 wherein a suitable frying medium is heated to from about 125 degrees celsius to less than 190 degrees celsius and the foodstuff is dipped therein from 0.75 minutes to 7 minutes.

14 A method according to any of the preceding claims wherein the treated foodstuff is filled into a hermetically sealed thermoplastic material container.

15 A method according to claim 14 wherein said foodstuff is filled into the thermoplastic material container whilst at a temperature of 60 - 90 degrees celsius.

16 A method according to claim 14 wherein said foodstuff is cooled to a temperature of no more than 4 degrees centigrade prior to packing into a sterile container.

17 A method according to any of claims 14 to 16 wherein the container is not heat processed after filing.

18 A method according to any one of the preceding claims wherein the vegetable foodstuff is raw chipped potato.

19 A hermetically sealed container having therein a vegetable foodstuff which has been treated by the method of any of the preceding claims.

20 A hermetically sealed container within which is a sterile blanch vegetable foodstuff of pH 4.8 or more, having an extended shelf life at ambient temperature.

21 A hermetically sealed container within which is a sterile vegetable foodstuff having a water activity of 0.97 to 0.98.

22 A shelf stable foodstuff having a pH of no less than 4.8.