This invention relates to a method of removing water from a water-coated surface of a food slice, in particular to form a snack food. This invention also relates to a method of producing a cooked snack food slice.

It has been known for many years to produce potato chips from slices of potato which are fried in oil, usually vegetable oil. The potato slices are cut by a cutter, typically a high speed rotary cutter. The slice thickness is typically from 1 to 1.5 mm. The cutting operation tends to produce a starch on the cut surface, which may be raw starch, suspended starch and/or an aqueous starch solution. The slices are then passed through a water bath to remove excess surface starch prior to the frying step.

The starch is removed because otherwise the starch may cause the potato chips to stick to each other during frying. Also, starch may build up in the fryer which may cause blockages or reduce the fryer efficiency or performance.

The washing process may be carried out in any of a variety of devices which use cold or warm water and agitation to separate and wash the slices (see U.S. Patent Nos. 3,223,024, 4,251 ,895 and 4,272,554 for exemplary processes). Coincidentally, the designs usually provide a means whereby slices too small to be included in the commercial product are selected out, and the starch and fragments are removed as the process water is changed. Many processes also use water to cool and lubricate the sheer and/or to convey slices from the slicer to the wash tank. The wash water may be filtered to remove the starch and recycled. The wash water must eventually be discharged and replaced and must usually be treated to meet discharge permit specifications (effluent pollution control requirements).

Wash water adheres to the surfaces of the washed potato slices. If the potato slices are fried immediately after washing, this surface water may constitute a significant amount (e.g., about 15 wt%) of the total water removed by evaporation during frying. Added water in a hot oil fryer also contributes to loss of oil due to the "steam distillation" of the oil in a two-phase system. These effects may significantly reduce the energy efficiency of the frying process and may also reduce product quality and/or uniformity.

Ί Potato chips exhibit specific organoleptic properties, in combination with visual appearance, to the consumer. The consumer desirous of purchasing a potato chip has a clear expectation of these product attributes in the product. There is a need for any modifications to a potato chip frying process not to have a negative impact on the quality of the potato chips, in particular the organoleptic properties and the visual appearance.

There is a need for a method for efficiently and reliably removing surface water from a food slice prior to frying in oil, particularly in the manufacture of potato chips.

There is also a need in the potato chip manufacturing art to provide a potato slice de- watering method which provides a lower surface water content on a potato slice which is introduced into a fryer so that a resultant potato chip is produced which has a consumer acceptance on parity with conventional fried potato chips.

The present invention accordingly provides a method of removing surface water from a water-coated surface of a food slice, for example to form a snack food, the method comprising the steps of:

i. providing a food slice having at least one surface which is coated with a water-containing layer on the food slice surface; and

ii. applying at least one air jet onto the water-containing layer at an air velocity at the water-containing layer of from 60 - 130 metres per second to blow at least a proportion of the water-containing layer from the surface of the food slice.

The present invention further provides method of removing water from a water-coated surface of a food slice, the method comprising the steps of:

a. providing a belt assembly comprising an upper endless belt and a lower endless belt, the upper and lower endless belts defining a product flow path therebetween;

b. feeding a plurality of water-coated food slices along the product flow path by rotation of the upper and lower endless belts, wherein the product flow path has a height, defined between the lower and upper surfaces of the respective upper and lower endless belts, which is greater than 200% of the maximum thickness of the food slices, wherein water-coated surfaces of the food slices form at least one water-containing layer on the food slice surfaces; and

c. applying at least one air jet onto the water-containing layer to blow at least a proportion of the water-containing layer from the surfaces of the food slices, wherein the at least one air jet is applied to the food slices when the food slices are in the product flow path and between the upper and lower endless belts

The present invention further provides a method of producing a cooked snack food slice, the method comprising the steps of:

a. washing a food slice with water to provide at least one water-coated surface of the food slice;

b. removing water from the at least one water-coated surface of the food slice according to the method of the invention; and

c. cooking the food slice to form a snack food.

Preferred features are defined in the dependent claims.

The present inventors have found that the provision of such an air jet treatment, subsequent to a water washing treatment, which may be conventional, at a particular air flow velocity can surprisingly achieve lower free surface water content in the resultant food slice immediately prior to cooking by frying. The free surface water content of the potato slices is lowered, which can improve the energy efficiency of the frying process and can also improve product quality and/or uniformity when the potato slices are subjected to cooking by a frying process.

The present invention is at least partly predicated on the finding by the present inventor that although some free surface water can be blown by low velocity air off the surface of a water- coated food slice, such as a potato slice, low velocity air blowing alone cannot remove sufficient free surface water, because the food slice has "hard to remove" free surface water on its surface. That "hard to remove" free surface water can be removed by blowing the free surface water off the food slice surface using air jets at a selected air jet velocity. The air jets are controlled to provide a low free water content in the food slices so that the food slices can be cooked by frying without excessive water evaporation during the frying process.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 schematically illustrates a potato slice to be de-watered in an embodiment of the method of the present invention;

Figure 2 schematically illustrates the relationship between free surface water film thickness and air velocity of an air jet applied to a potato slice to be de-watered in an embodiment o the method of the present invention;

Figure 3 is a schematic side view of an apparatus for de-watering potato slices, prior to cooking by frying, for use in an embodiment of the method of the present invention;

Figure 4 is a schematic plan view of a lower belt for use in belt assembly in an embodiment of the method of the present invention; and

Figure 5 is a schematic plan view of an upper belt for use in belt assembly in an embodiment of the method of the present invention.

Figure 1 schematically illustrates a potato slice 2 to be de-watered in an embodiment of the method of the present invention. The potato slice 2 is coated with free surface water to form a water-containing surface layer 4. The free surface water is present in a "hard to remove" fraction which is primarily an interior free surface water layer and an "easy to remove" fraction which is primarily an exterior free surface water layer.

Although the present invention is described with particular reference to potato slices, the method of the invention may be employed to de-water any food slices coated with surface water. For example, the food slices may comprises slices o other vegetables, for example root vegetable such as carrot, parsnip, beetroot, sweet potato, etc. Any of these vegetable slices may be used to firm a snack food. Alternatively, the food slices may be fruit slices for the manufacture of any fruit-containing food product, including purees and beverages, or snack food produced from the fruit slices. The food slices may be planar and have opposite planar surfaces. Alternatively, the food slices may have a non -planar cross-section, for example they may be crinkle cut or ridged. The food slices may have any plan shape, which may be regular, for example rectangular, square, triangular, hexagonal etc, or irregular, for example the shape of slice cut from a peeled vegetable, such as a potato, or fruit, such as an apple.

Figure 2 schematically illustrates the relationship between water film thickness and air velocity of an air jet applied to the water-coated potato slice 2 to be de-watered. The "easy to remove" fraction is present at high film thickness, d, and can be removed by low velocity air jets. At these film thicknesses, viscous forces are dominant and the water can be removed by relatively low velocity air, optionally together with low vacuum pressure which can suck the water droplets away from the potato slice surface. However, at smaller film thicknesses, i.e. closer to the surface of the potato slice, such low velocity air is insufficient to remove the water, and such water is a "hard to remove" fraction. The present inventor has found by extensive experimental research that with regard to the "hard to remove" fraction, surface tension forces are dominant and that the free surface water can be removed by employing high velocity air jets to remove the free surface water from the surface of the potato slice. The high velocity air jets remove the free surface water so that the resultant potato slice has controUably low free surface water content.

An embodiment of an apparatus for de-watering potato slices, prior to cooking by frying of the potato slices to form potato chips, according to one aspect of the method of the present invention is illustrated in Figures 3 to 5.

A primary endless belt conveyor 10 in the form of a belt assembly 12 having a substantially horizontal orientation is provided. An inlet end of the conveyor 10 communicates with an exit of a water flume 14 (illustrated schematically) comprising part of a washing unit for the potato slices 2. The conveyor 10 carries a succession of the potato slices 2. The conveyor 10 has a translational speed of from 0.1 to 0,5 m/second, typically about 0.2 m/second. An outlet end 13 of the conveyor 10 communicates with an output conveyor 15 which conveys the de-watered potato slices 2 for further processing, such as cooking by frying. The potato slices 2 have been randomly delivered onto the conveyor 10. The potato slices 2 are delivered onto the conveyor 10 in a slice distribution so as to have at least about 50% of the slices being single slices, i.e. not overlapping with an adjacent slice. In addition, at least 50% of the overlaps are no more than 50% of the area of each of the respective overlapping slices. Also, for each overlap no more than two slices 2 are stacked one upon the other on the conveyor 10. This substantially provides a monolayer of potato slices 2 across the length and width of the conveyor 10.

The potato slices 2 typically have a thickness of 1 to 2 mm, more typically about 1.35 mm (53 thousandths of an inch).

The potato slices 2 have been pre-treated in water in a washing step which is conventionally used in potato chip manufacture to remove surface starch from the cut slice surface. After the washing step and prior to the de-watering step the potato slices 2 have about 10 to 30 wt% free surface water, typically from 15 to 25 wt%, free surface water based on the dry weight of the final potato chip produced from the potato slice 2. In this specification the "dry weight of the final potato chip" assumes 1-2 wt%, typically about 1.5 wt%, water content in the total weight of the final cooked and dried potato chip, prior to final seasoning of the potato chip.

The belt assembly 12 comprises an upper endless belt 16 and a lower endless belt 18, which define a product flow path 20 therebetween. A plurality of the water-coated potato slices 2 are fed along the product flow path 20 by rotation of the upper and lower endless belts 16, 18. The product flow path 20 has a height, defined between the upper and lower surfaces 22, 24 of the respective upper and lower endless belts 16, 18, which is greater than 200% of the maximum thickness of the potato slices 2. Typically, the height of the product flow path 20 is from 220 to 300% of the maximum thickness of the potato slices 2.

The upper endless belt 16 comprises a plurality of interlinked metal links 26 having an open area of from 75 to 85% and a link depth of from 4 to 6 mm. Preferably, the open area is from 80 to 85% and the link depth is about 5 mm. The plurality of interlinked metal links 26 form a rectangular grid structure in the upper endless belt 16. The lower endless belt 18 comprises a plurality of interlinked metal links 28 having an open area of from 60 to 75% and a link depth of from 5 to 8 mm. Preferably, the open area is from 65 to 70% and the link depth is about 6 mm. The plurality of interlinked metal links form a rectangular grid structure in the lower endless belt 18,

The lower endless belt 18 can extend at its upstream end into a wash tank for collecting washed food slices 2 thereon. Alternatively, as illustrated a further lower belt 19 is provided which extends into water flume 14, or a wash tank, for collecting washed food slices 2 thereon and then delivers the washed food slices 2 onto the lower endless belt 18, and this embodiment provides the advantage that the lower endless belt 18 is relatively dry, and has a coating of water limited to water originating from only the food slices 2, since the lower endless belt 18 has not extended into the washing unit.

As the potato slices 2 are carried on the upper surface of the conveyor 10, air is blown downwardly and upwardly onto the potato slices 2 in a continuous manner at an air-blower station 34.

At the air-blower station 34, at least one air jet 38 from an air jet nozzle 40 is applied, as an air knife, onto the water-containing layer at an air velocity at the water-containing layer of from 60 - 130 metres per second. The at least one air jet 38 blows at least a proportion of the water-containing layer from the surface of the potato slices 2. Preferably, the air velocity is from 85 - 130 metres per second. The at least one air jet 38 is applied at an angle of from 75 to 105 degrees to the surface of the potato slices 2. Typically, the at least one air jet 38 is applied at a substantially perpendicular orientation to the surface of the potato slices 2.

The air jet treatment is carried out on opposite major surfaces 44, 46 of the potato slices 2, with the opposite surfaces 44, 46 being treated substantially simultaneously.

The air jet temperature is from 20 to 100°C. The temperature of the water-coated surface prior to air jet impact is from 10 to 40 °C, preferably from 15 to 25 °C. The air jet treatment is carried so as to provide, after the air jet treatment, a residual free surface water content on the potato slices 2 of from 1 to less than 8 wt%, preferably from 1 to 5 wt%, based on the total weight of the potato slices 2.

During the air jet treatment a bulk air flow is provided around the potato slices 2 to entrain dispersed droplets of water within the bulk air flow to convey the droplets away from the potato slices 2. The bulk air flow is induced by suction of air away from the potato slices which is provided by at least one port (not shown) in at least one vacuum plenum 50 located in the vicinity of the potato slices 2. In the embodiment, plural vacuum plenums 50 are provided, each opposite a respective air jet nozzle 40. The bulk air flow has an air velocity at the water-and-oil-containing layer of from 5 - 90 metres per second, for example from 50 to 80 m/s. Typically the vacuum plenum 50 applies a negative pressure of from -20 to -120 mbar, for example from -90 to -1 10 mbar. The at least one port is typically located at a distance of from 5 to 25 mm, for example from 8 to 15 mm, from the surfaces of the potato slices 2.

During the air jet treatment, the potato slices 2 are supported by and retained within the specifically configured belt assembly 12 which not only holds the potato slices 2 against being blown off by the air jet 38 but also permits water and air transmission therethrough to ensure effective water removal.

As described above, at least one air jet 38 from an air knife is applied onto the water- containing layer resulting from a prior washing process to blow at least a proportion of the water-containing layer from the surfaces of the potato slices 2.

The at least one air jet 38 is applied to the potato slices 2 when the potato slices 2 are in the product flow path and between the upper and lower endless belts 16, 18. The plurality of the air jets 38 are applied both downwardly and upwardly against the upper and lower surfaces 44, 46, respectively, of the potato slices 2. At least one of the plurality of air jets 38 is applied upwardly against the lower surface 46 of the potato slices 2 at an upstream location, with respect to the flow of the potato slices 2 along product flow path, relative to at least one the plurality of air jets 38 which is applied downwardly against the upper surface 44 of the potato slices 2. By providing that the product flow path 20 has a height, defined between the upper and lower surfaces 22, 24 of the respective upper and lower endless belts 16, 18, which is greater than 200% of the maximum thickness of the potato slices 2, in the absence of any net upward force on the potato slice 2 from an air jet causing the potato slice to be at least partially lifted off the lower endless belt 18, an upper surface of the potato slice is substantially free from contact with the upper endless belt 16. By providing essentially no or minimal contact between the upper surface of the potato slice 2 and the upper endless belt 16, this has been found to achieve an additional 4.5 wt% reduction in the water content of the potato slice 2, by removal of surface water using the air jets, as compared to constant contact of the upper surface of the potato slice 2 and the upper endless belt 16. Correspondingly, one or more lower air jets causes the potato slice to be at least partially lifted off the lower endless belt 18 at least once, and this has been found to achieve an additional reduction in the water content of the potato slice 2, by removal of surface water using the air jets, as compared to constant contact of the lower surface of the potato slice 2 and the lower endless belt 18.

After the removal of the free surface water from water-coated surfaces of the potato slices 2, the potato slices 2 are cooked, for example by frying in oil, to form a snack food, in this embodiment potato chips. In the frying cooking step, the bulk moisture content of the potato slices 2 is reduced from an average value of at least 75 wt%, typically about 80 wt%, based on the total weight of the potato slices 2 to an average final moisture content for the potato slices of 2 +/- 0.5 wt% based on the dry weight of the potato chip.

The oil typically comprises a vegetable oil such as sunflower oil, conventionally used for manufacturing potato chips. Alternatively the oil is any other vegetable oil, optionally at least one or a mixture of at least two of sunflower oil, rapeseed oil and olive oil.

In modifications to the illustrated embodiment, the number of air knives may be varied.