WO2015092387A1

WO2015092387A1 - Improvements in frying technology

Info

Publication number: WO2015092387A1
Application number: PCT/GB2014/053722
Authority: WO
Inventors: Andrew CLAY
Original assignee: Oil Preservation Technologies Ltd
Priority date: 2013-12-16
Filing date: 2014-12-16
Publication date: 2015-06-25
Also published as: WO2015092388A2; GB201322146D0

Abstract

A method of preserving cooking oil, including by reducing the extent of foaming and/or the build up of free fatty acids in the cooking oil during its use in the frying of food, comprises providing in a cooking vessel a body of cooking oil and locating in the cooking oil a solid, calcium-containing filtering medium. Food is located within the oil and the oil is heated to effect frying of the food. The filter medium contains a minor amount of microsilica. Additionally or alternatively, an air entraining agent is added to the filtering medium during its manufacture so as to create numerous pores in the structure of the medium. Additionally or alternatively, there is located in the oil means for reducing the extent of flow of oil over the surface of said filtering medium during the food frying.

Description

IMPROVEMENTS IN FRYING TECHNOLOGY

FIELD OF THE INVENTION

This invention relates to improvements in technology relating to methods for the in situ treatment of cooking oils and fats.

INTERPRETATION

In this patent specification references to oils should be interpreted as being references to animal, vegetable, nut or synthetic oils and fats (which are generally solid at room temperatures). References to fryers herein should be taken as including any frying equipment, chamber, pan, tank, commercial, domestic or industrial fryer.

BACKGROUND TO THE INVENTION

Deep fat frying has become one of the most popular methods of cooking in domestic, restaurant and industrial establishments throughout the World. Because of the high temperatures involved (typically 160 to 200°C) it is relatively quick, cooks food right through to the middle, generates a distinctive crust on the food and perhaps most importantly produces rich and complex flavours and food textures, which are very appealing to the consumer.

Frying, whether carried out in oils or fats, however also has a number of well-known disadvantages.

Cooking oil is expensive: high end olive oils are more expensive per litre than petrol or diesel and the price of even lower end cooking oils is comparable to that of petrol or diesel. Cooking oils have to be replaced frequently as the oils degrade during the cooking process, as more fully explained hereinafter. Also cooking oils (and their breakdown products) are absorbed by the food cooked in them which therefore necessitates the operator of a fryer to regularly keep the oil or fat topped up by the addition of extra cooking oil or fat. Cooking in oil therefore comes at a relatively high price compared to boiling in water or roasting in air. The frequent changing of cooking oil in kitchens, restaurants and food preparation industrial sites is also a labour intensive and laborious task, which is costly and increases equipment down-time. Unfortunately it is not possible to extend the life of cooking oils and fats merely by filtering out the food debris which frequently accumulates within them. During use cooking oils and fats do not remain unaltered but begin to chemically breakdown.

Cooking oils and fats are commonly referred to as triglycerides but are in fact

triacylglycerols : i.e. triesters of glycerol (1, 2, 3 propanetriol, which is commonly referred to as glycerine) and three fatty acids. The fatty acids do not need to be of the same type and frequently are not). Common chain lengths for the fatty acids, as determined by gas liquid chromatography, are 12 to 24 carbon atoms with 16 and 18 being particularly favoured. The breakdown of such triglycerides is complex, dependent on numerous factors and is subject to numerous feedback effects but involves three well-understood basic mechanisms: oxidation, polymerisation and hydrolysis.

Oxidation

Oxidation occurs when air comes in contact with frying oil, (see for example Josephson and Lindsey 1987, Journal of Food Sciences, 52, 328 and Fischer and Muller 1991, Potato Research, 34, 159). Oxygen from the air reacts with the two unsaturated carbons at the double-bond via a free radical initiated reaction. The oxidation reaction is promoted by high cooking temperatures (typically 190°C and above), the presence of metals (including in particular copper and iron) and the presentation to the air of a large surface area of the oil as well as exposure to UV light, which promotes free radical formation. Initially hydroperoxides are produced but these are unstable and at frying temperatures they rapidly break down (via e.g. fission, dehydration and free radical formation) to produce a wide array of secondary oxidation products including polymers, acids, alcohols, esters, aldehydes, methyl ketones, lactones, alcanes, aromatics and other hydrocarbons, (see Belitz and Grosch 1999, Food Chemistry, 2^nd edition, Springer- Verlag, Berlin, p. 211).

Some of these secondary oxidation products are volatile and give rise to both pleasant rich flavours but some are also associated with rancid and offensive flavours. For example only 0.08 ppm of pentane is sufficient to reliably produce rancidity, (Warner et al. (1974) Journal of Food Science, 39, 761). Non-volatile compounds, such as core aldehydes, remain in the oil and are absorbed by the food. Polymerisation

When cooking oil breaks down, the resulting products form both volatile low boiling point and non-volatile compounds. The non-volatile higher boiling point compounds remain within the frying oil and readily polymerize at frying temperatures above 190°C or in isolated hot spots within the fryer. Such polymerisation products then bond together to form larger clusters, which accumulate as an insoluble layer on the surface of the oil, thus preventing water vapour, evaporating from food cooking in the oil, escaping from the oil's surface and thereby producing dangerous foaming, which can lead to fires and personal injury of kitchen staff. The presence of the impermeable polymer layer in turn promotes more hydrolysis in what can become a runway feed-back driven process. Polymerisation also leads to an increase in the viscosity of the oil which reduces its ability to effect heat transfer and promotes yet more polymerisation. The increase in viscosity also increases the amount of energy required to effect cooking and thus increases energy bills.

Hydrolysis

Hydrolysis is caused by the reaction of water (a weak nucleophile) with the ester linkage in the triacylglycerol molecule to produce initially a diaglyceride and a free fatty acid, which then further breakdown to produce various compounds including lactones. The presence of free fatty acids is frequently associated with a characteristic rancid or acidic flavour.

The production of free fatty acids in cooking oils is additionally problematical for several reasons.

Firstly free fatty acids are one of the main constituents of smoke haze and are both a fire and a health hazard. The smoke point of an oil is the temperature at which it is seen to start smoking under specified test conditions. The flashpoint of an oil is the temperature at which volatile products are produced in sufficient concentration and quantity to allow ignition. The fire point of an oil is the temperature at which the rate of production of volatile products is sufficiently high to support continuous combustion of the gases emerging from the surface of the oil.

High levels of free fatty acid are associated with reduced smoke, flash and fire points and are thus a significant fire hazard. For example Weiss (Food Oils and Their Uses, Wesport, The AVI Publishing Co. 1983) found that a free fatty acid composition of 0.04% was associated with a smoke point of 218°C, a flashpoint of 327°C and a fire point of 366°C whereas for the same oil increasing the free fatty acid content to just 1% percent lead to the smoke point decreasing to 160°C, the flashpoint decreasing to 307°C and the fire point dropping to 360°C.

In addition to being a fire hazard, increases in the concentration of free fatty acids (and their break down products) in the cooking oils also has deleterious effects on the preparation of food cooked in the oil.

Fatty acids and some of their breakdown products, having both distinct hydrophobic and hydrophilic regions, act as effective surfactants. The effect of the concentration of surfactants in cooking oil on the properties of the food cooked in such oil is well-known (see e.g. Blumenthal MM A New Look At The Chemistry And Physics Of Deep Fat Frying: Food Technology, 1991, 45:2, 68 -71, 94). When for example chips are cooked in fresh unused cooking oil they are light in colour and do not have the rich complex aromas associated with fried potatoes. The oil, during this "break in" phase has only low levels of surfactants (such as free fatty acids), which means that the oil has a relatively high surface tension which prevents the oil having close contact with the food. The heat from the oil is not effectively transferred across the oil/wet-food barrier and the food is in part boiled rather than fried as the steam emerging from the food pushes a substantial amount of the oil away from its surface. As the oil is used further the amount of free fatty acid and other surfactants increases resulting in improved food quality. During the so-called optimum phase chips cooked in the oil are golden brown in colour and have a significant crust but with relatively low levels of oil being absorbed by the food, which is cooked through to the centre. For example fresh French fries will typically consist of about 10% by weight of oil, when cooking during the so called optimum phase. However as the oil is subject to both further hydrolysis and oxidation, the increase in free fatty acids and other surfactants decreases the surface tension significantly and ensures that the oil can rapidly bridge the otherwise immiscible oil food barrier. This results in the surface of for example chips having a characteristic dark and spotted appearance. Excessive contact with the oil rapidly dries the surface of the food thus trapping moisture in the food and inhibiting heat penetration deeper within the food's centre which therefore typically is undercooked. The resulting greasy chip with an oil content by weight of typically in excess of about 20%, with a dark spotted exterior and undercooked centre, is familiar to many who have eaten at down market fast food establishments, which do not change their cooking oil often enough.

The absorption of excessive amounts of cooking oils by food cooked in the oil also significantly increases the calorific value of the food, thus giving many consumers extra calories they do not need and promoting obesity and the numerous health problems associated with it.

Further the absorption of excessive amounts of cooking oil by food has other important consequences for health. Hydrogenated vegetable oils and fats are widely used in cooking due mainly to their increased stability and shelf -life and their greatly increased resistance to rancidity.

However such oils contain increased amounts of trans-fatty acid side chains on the glycerol backbone, which are a material health hazard. After ingestion most of the initial digestion of cooking oils is accomplished in the stomach via specialist pancreatic enzymes (lipases) and bile secretions. The resultant fatty acids and glycerol are then absorbed by cells lining the intestines called enterocytes, where they are re-esterified into triglycerides and transported to the liver as chylomicrons. When the chylomicrons reach the liver, the fatty acids are repackaged into triacylglycerols and phosphatidylcholine and thence into lipoproteins.

High levels of trans fatty acids in the diet are associated with raised serum levels of low density lipoprotein (LDL) cholesterol and with lower levels of high density lipoprotein (HDL) cholesterol in humans. Raised serum LDL and reduced serum HDL levels are associated with coronary artery disease, increased risk of stroke and elevated blood pressure as they decrease the health of the endothelium, the cells lining the arteries of the body which are essential for good cardiovascular health. Studies in humans further demonstrate that trans fats increase inflammation in the body, a potent risk factor for cardiovascular disease, diabetes, and other diseases. Studies in primates have

demonstrated that trans fats cause weight gain, especially increasing abdominal fat, which has the greatest metabolic consequences, and worsens insulin resistance, the precursor to diabetes.

For all these reasons the amount of trans-fatty acids absorbed in the diet should be kept at low levels. One way of achieving that is to reduce the amount of cooking oil absorbed by fried food. Various ways have been suggested to prolong the useful life of cooking oils. Some of these involve the step of removing the cooking oil from the fryer, followed by the step of subjecting it to one or more treatment methods to remove the contaminants before finally returning the treated oil back to the fryer. Other methods provide for at least the complete cessation of the cooking process, treatment and then the recommencement of the use of the oil.

Oil Removal and Treatment Methods

US-A4112129 (Duensing et al., Johns Manville) discloses a method of filtering the cooking oil through a composition comprising by weight (i) 47 to 59 parts diatomite, (ii) 28 to 36 parts synthetic calcium silicate hydrate and (iii) 12 to 24 parts synthetic magnesium silicate hydrate.

US 4681768A (Mulflur W Jospeph et al) discloses a method for the continuous filtration of cooking oil with a filter made from synthetic calcium silicate. The method involves removal of the oil from the fryer, passing it through the filter and then passing it back into the fryer. GB 2006729 (Johns Manville) discloses a method for filtering used cooking oils to remove free fatty acids, which uses synthetic calcium silicate but does not disclose an in situ solution suitable for unadapted fryers.

US 5870945 discloses a filter cartridge for fitting to a fryer, which includes a mesh housing for containing filtering material which is used to treat the cooking oil outside the fryer prior to its return to the fryer.

US 4112129 A discloses a method for extending the life of cooking oil by removing free fatty acids which involves treating the oil with a composition of synthetic calcium silicate hydrate and synthetic magnesium silicate hydrate. US 4112129 A states that the method can be used with conventional cooking oil filtration systems but does not disclose an in situ solution suitable for unadapted fryers which do not have a filtration system.

EP 0226413A discloses a filter container provided with a removable filter bag but which cannot be used during the cooking operation.

US 6210732 discloses a method of extending the life of cooking oil by the use of a blend of finely milled citric acid and calcium silicate powder, which is added to the hot oil, left for a certain length of time and then removed by filtration. The US 6210732 invention cannot be used during the cooking process.

WO 91/11914A discloses a still further treatment method for used cooking oils which uses an amorphous silica and alumina composition, which is either added to the hot oil and then filtered out or put in a container which is permeable to the oil but not the treatment composition. The invention disclosed cannot be used during the cooking operation.

US 4330564A discloses a method of treating used cooking oil with a composition including a porous carrier, water and a food compatible acid, with the resultant residue being removed by filtration. The invention disclosed cannot be used the cooking operation. US 3947602A discloses a method of treating cooking oil with a food compatible acid and a suitable adsorbent such as activated carbon. The invention disclosed cannot be used during the cooking operation. US 5391385A discloses the treatment of cooking oil with a mixture of 60-80%

amorphous silica and 20 to 40% alumina, the mixture being placed in a permeable container which is then placed in the oil, the container being permeable to the oil but not to the mixture so that the adsorbent is not released into the oil and no filtration is required. All the above treatment methods either require removal of the oil from the fryer and its filtration before reuse and/or cannot be carried out during the normal frying operation with standard frying equipment, which does not include in-line filtration equipment and a pump.

In Situ Treatment of Cooking Oil

Other methods are known for the treatment of cooking oil in the vessel where cooking takes place.

US 4764384A discloses a method of treating used cooking oil with filtering media comprising synthetic amorphous silica, synthetic amorphous magnesium silicate and diatomaceous earth.

US 5354570A discloses a method of frying food in cooking oils with a porous rhyolitic powder which selectively reduces the concentration of certain surfactants, whilst the cooking process is on-going.

JP 07-148073A discloses a method of treating cooking oil using finely pulverized zeolite stones which are inserted into a permeable bag which is itself placed into the fryer, with or without food also being present. The above methods either require the addition of powders to the oil, which is undesirable as they may contaminate and change the texture and taste of any food cooked therein or require the a further container to be added to oil, which will often be problematical during use of the fryer. The WO 2008/015481 and WO 2009/019512 Inventions

WO 2008/015481 and WO 2009/019512 ("the BBM Patents") (BBM Technology

Limited) disclose the use of cementious hydraulically set mixtures of white OPC clinker and white OPC, in the form of standalone briquettes, blocks, pellets or balls, which do not substantially leach calcium or magnesium into the cooking oils. The use of such filters has the advantage that it does not require existing frying equipment to be modified, the filters just being put into the oil. No filtration systems or pumps are required. Such filters can be used in situ in the oil during use and can prolong the life of the cooking oil so treated typically by up to 100 percent by removing free fatty acids and other polar compounds. They also reduce the amount of cooking oil absorbed by the food and thereby the amount of trans fatty acids digested.

However the use of the filters disclosed in the BBM Patents is associated with foaming in use. Foaming is not the same as mere bubbling. During foaming water vapour produced during the cooking process is trapped just below an impermeable polymer layer at the top of the oil and cannot escape so that, as the volume of trapped gas increases, the oily impermeable layer of bubbles rises up the sides of the fryer and can bubble over, causing either injury, fire or damage to property as well as an unwelcome interruption to busy kitchen staff.

Foaming is encountered with the filters the subject of the BBM Patents particularly in cooking environments where a lot of hydrolysis takes place due to large amounts of wet food being cooked (for example frozen chips).

It may be possible to prevent (or at lease reduce) such foaming in practice by very tightly controlling the manufacturing process parameters associated with the manufacture of such filters but such tight control is not straight forward to achieve outside laboratory conditions, adds very considerably to the cost of production and limits the scale on which such manufacture can be economically carried out.

Without wishing to be bound by any theory it is thought that the relatively high calcium content of the filters disclosed in the BBM Patents promotes the release of calcium ions into the oil. Indeed the free fatty acids in cooking oil are expected to react with the calcium hydroxide in such filters to produce fatty acid salt soaps, the build-up of which in the oil tends to lead to the formation of an impervious layer of calcium soap on the surface of the oil, which causes foaming.

This effect has been noted, for example, in US 5,597,600, column 2, lines 15 to 30 of which state that:-

"Most industrial fryers use Silasorb (Celite Corporation, Denver, Colo.) which is a synthetic calcium silicate, as their filter medium because it is very effective in lowering free fatty acid concentration. Silasorb lowers the free fatty acid (FFA) concentration of the oil by a combination of adsorption and neutralization. The use of Silasorb, however, often darkens the oil.

In addition, the product of the neutralization of a fatty acid with an alkaline metal is a fatty acid soap. The amount of soap formed is dependent on the amount of alkaline metal present, and the initial percentage of free fatty acids in the oil. When the soap level is high, the oil foams. The use of Silasorb in order to lower the free fatty acid concentration sometimes results in uncontrollable foaming." Although the actual concentration of such calcium fatty acid salts in cooking oils, exposed to the filters disclosed in the BBM Patents may be very low in absolute terms (below 3 ppm and possibly even below 1 ppm) it can still give rise to foaming in practice as the polar calcium ions tend to become concentrated at the surface of the oil, from which they are pushed out.

There is therefore a need for an in situ treatment method for cooking oils, which combines the advantages of the filters of the type disclosed in the BBM Patents, but which does not lead to the production of undesirable foaming in use. SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a method of preserving cooking oil, including by reducing the extent of foaming and/or the build up of free fatty acids in the cooking oil during its use in the frying of food, the method comprising providing in a cooking vessel a body of cooking oil, locating in said body of cooking oil a solid, calcium-containing filtering medium, locating in said body of cooking oil means for reducing the extent of flow of oil over the surface of said filtering medium during the food frying, locating food within said body of oil and heating the cooking oil to effect frying of the food.

Accordingly, the invention provides a method of treating cooking oil during frying operations which comprises in situ treatment of the oil with a solid filter treatment material of the type exemplified in the BBM Patents. The flow of oil over the surface of the filtering medium may be modified by at least partially enclosing or encasing the filtering medium to reduce the frequency of occurrence or eradicate foaming.

The filtering medium may be in any suitable form, for instance, a single block or loose material in granular form or in the form of beads.

Preferably, the filtering medium is at least partially surrounded by a foraminous barrier. More preferably, the barrier extends laterally adjacent said filtering medium.

Preferably, the barrier forms at least part of a container or enclosure within which the filtering medium is located. The container may be, for instance, of a box-like

construction having two foraminous side walls. The base and top of such a container may both be continuous, that is to say, unperforated.

The container may be an entirely separate item to the cooking vessel. Alternatively, the container may be at least partly integral with the cooking vessel.

The filtering medium may contain a minor amount of microsilica.

An air entraining agent may be added to the filtering medium during its manufacture so as to create numerous pores in the structure of the medium.

The present invention also provides a method of preserving cooking oil, including by reducing the extent of foaming and/or the build up of free fatty acids in the cooking oil during its use in the frying of food, the method comprising providing in a cooking vessel a body of cooking oil, locating in said body of cooking oil a solid, calcium-containing filtering medium and locating food within said body of oil and heating the cooking oil to effect frying of the food, the filter medium containing a minor amount of microsilica. The present invention further provides a method of preserving cooking oil, including by reducing the extent of foaming and/or the build up of free fatty acids in the cooking oil during its use in the frying of food, the method comprising providing in a cooking vessel a body of cooking oil, locating in said body of cooking oil a solid, calcium-containing filtering medium, locating food within said body of oil and heating the cooking oil to effect frying of the food, an air entraining agent being added to the filtering medium during its manufacture so as to create numerous pores in the structure of the medium. In another aspect, the present invention provides a deep oil or fat fryer having a depression or recess for releasably holding, or providing at least a part of, an enclosure for a filtering medium, said enclosure being provided with means for reducing, in use, the extent of flow of oil over the surface of said filtering medium during food frying.

In addition, the present invention provides a calcium-containing filtering medium for use in preserving cooking oil, the medium containing a minor amount of microsilica.

Furthermore, the present invention provides a calcium-containing filtering medium for use in preserving cooking oil, the medium having had added to it during its manufacture an air entraining agent so as to create numerous pores in the structure of the medium.

In a still further aspect of the invention there is provided a deep oil or fat fryer having a base or sidewall formed with a depression or recess comprising or for receivably holding or interfacing with an enclosure or cover or comprising (at least in part) such an enclosure or cover so as to reduce the frequency of occurrence or eradicate foaming.

The invention still further provides a method for reducing the frequency of occurrence or eradicating the foaming associated with the use of the filter means of the type disclosed in the BBM Patents by the addition of microsilica or similar sized silica particles to the pastes from which such filters are made. The invention yet further provides a method for improving the performance of the filter means of the type disclosed in the BBM Patents by the addition of a suitable air entraining agent to the pastes from which such filter means are made. BRIEF DESCRIPTION OF THE DRAWINGS

How the invention may be put into effect will now further be described by reference to the accompanying drawings.

Fig 1 shows an enclosure with a single piece filter.

Fig 2 shows an enclosure with multiple loose filter particles.

Fig 3 shows a fryer with a side wall formed for connectably receiving an enclosure of the invention.

Fig 4 shows a fryer with a base with a recess for receivably connecting to an enclosure of the invention.

Figure shows a filter made according to the WO 2008/015481 and WO 2009/019512 inventions with a build-up of fatty acid salt soap on its surface.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is applicable to the in situ treatment of cooking oils or fats for use in all types of fryers having a capacity between 2 to beyond 100 litres. It may be used with the hydraulic cement filter products such as those described in WO 2008/015481 and WO 2009/019512.

Encasement or partial covering

In one embodiment of the invention (depicted in Fig. 1) there is provided an enclosure lcomprising a substantially continuous upper wall , in the form of a slidable lid (not shown), a substantially continuous lower wall 3 and further having four side walls 5, at least two of which 7,9 are perforated with holes or are otherwise partial so as to allow diffusion of the cooking oil between the outside of the enclosure and the inside in a direction substantially perpendicular to the upper and lower walls. The lower and upper walls should not be perforated or partial for the invention described herein to work to best effect. The upper sidewall is in the form of a lid which can be securely placed onto, against or round the lower sidewalls. The lid may be hinged or otherwise connectably attached to the lower sidewalls so as to allow relative movement but may also simply detachably securely fit onto the said sidewalls of the said enclosure. It will be readily apparent to the man skilled in the art that the invention herein described is not limited to enclosures which are in a cuboid or rectangular cuboid form: for example numerous polygonal, cylindrical, oval or triangular forms of container will also work satisfactorily as would containers made from side walls with sides of different lengths or shapes.

The side walls of the enclosure may (but do not need to be) welded or spot welded or riveted or otherwise connectably joined together. The side walls may be made as separate pieces or in one piece. The side walls can be made out of food grade stainless steel (grade 316) or out of any material which provides an acceptable degree of rigidity,

impermeability, resistance to bacterial growth, which is non-toxic and has a melting point in excess of the temperature range at which frying typically takes place (160 to 210°C). The said enclosure or cover is used to substantially enclose or cover filter media 9 of the type disclosed in the BBM Patents. In this example the filter medium 9 is a block having holes 11 extending therethrough. In use, the upper and lower walls of the enclosure or cover should ideally be parallel to the upper surface of the filter medium 9 and the perforated side walls should be substantially perpendicular to the surface of the filter medium 9.

Fig. 2 shows an enclosure 13, similar to enclosure 1 of Fig 1. and showing the lid 15 in slid back to reveal the filter medium 17, in this example being in the form of spherical balls. The invention also extends to include covers which do not substantially encase the filter media but nonetheless significantly reduce the flow of cooking oil over the surface of the said filter media, caused by thermal convection currents. The cover may form a partial enclosure for the filter or it may comprise for example a single plate or more than one plate.

The invention extends also to fryers and frying equipment having a base and/or sidewall formed with a depression or recess comprising or for receivably holding or interfacing with an enclosure or cover or comprising (at least in part) such an enclosure or cover. Examples of such equipment are depicted in Figs. 3 and 4.

It has surprisingly been found that by the use of an enclosure or cover of the type described herein that all or substantially all of the benefits obtained by the use of the filters disclosed in the BBM Patents can be retained whilst at the same time the foaming phenomenon frequently experienced during the use of such filters is either completely eradicated or else occurs very much less frequently.

Without wishing to be bound in any way by any particular theory it is thought that the partial or complete covering or enclosure of the said filters reduces the frequency of foaming or eradicates it completely as it decreases the concentration of calcium ions which leach out of the filter by reducing the rate of flow, due to thermal convection, of cooking oil across the surface of the filter. As the cooking oil typically contains between approximately 0.1 to 2 percent free fatty acids, depending upon, inter alia, the type of oil, what has been cooked in it, the frying temperature and the surface area of the oil exposed to air, this in turn reduces the rate at which calcium and in particular calcium hydroxide and calcium silicate hydrate minerals are attacked by the free fatty acids in the cooking oil.

The use of such an enclosure or a cover may also decrease the extent to which calcium hydroxide is dissolved in the water which exists in the cooking oil typically as a suspension or emulsion. Although the amount of water in cooking oil is generally relatively low (typically a few hundred parts per million in used oil), the solubility of calcium hydroxide in water is generally inversely related to its temperature (i.e. it decreases as the temperature of the water rises) and given that many cooking oils will be at or near room temperature for prolonged periods when not in use then it is likely that the water in the oil does materially dissolve the calcium hydroxide in the filters. Calcium hydroxide is a major component of the filters disclosed in the BBM Patents, being one of the main reaction products of the main cement minerals alite and belite with water. It typically accounts for between twenty to thirty five percent by weight of the filter products disclosed in the BBM Patents.

Thirdly the cover may also simply protect the filter from water dropping down from the basket or other food containment means, when high water content food is added to the oil: for example solidly frozen chips contain a lot of ice which has a relatively high latent heat capacity and therefore the melting of which materially reduces the temperature of the cooking oil possibly allowing, given that the water has a higher density than the cooking oil, some of the emerging water to simply fall downwards on to the filter.

Enclosures or covers of the type herein described also prevent food debris from covering the filter or at least reduce the extent to which that happens. Such food debris may accelerate the degradation of the surface of the said filters. A further advantage of using an enclosure or cover of the type described herein is that it makes it practical to use relatively large numbers of smaller separate filter elements. This is advantageous as the use of a plurality of small filter elements materially increases the surface area of the filter media and promotes their efficacy during operation. Addition of Silica fume/microsilica

It has further been found that adding relatively small amounts of sillica fume or microsilica to the paste out of which filters of the type described in the BBM Patents are made also materially ameliorates foaming. Sillica fume (also known as microsilica) is a by-product of the production of silicon or ferrosilicon alloys, which is achieved by the reduction of quartz (the mineral) in an electric furnace. Some SiO is lost as a gas, which is then rapidly oxidised by the oxygen in the air to give a very fine white or grey particulate solid. European patent (UK) No. 0,289,240 further discloses another way of making very small glass particles, which are also suitable for use in the invention disclosed herein.

Microsilica suitable for the invention herein described can be obtained from either Metalmin & Co (UK) Ltd of Livermore House, High Street, Great Dunmow, CM6 1 AW, Essex, or from Glassflake Limited of Forster Street, Leeds, LS10 1PW, West Yorkshire. High quality silica fume consists of spherical particles of glass typically with a diameter of about 100 run and having a surface area of 15 to 25 m²g-^J. Glass flakes made using the technology disclosed in European patent (UK) No. 0,289,240 may be made with very tightly controlled dimensions and even higher surface areas and are particularly suitable for the present application.

It has been known to incorporate silica fume into cement for many years. The

incorporation of silica fume into cement has several effects due to its strong pozzolanic activity. It accelerates the hydration reactions of the clinker phases (particularly alite). The fine microsilica particles fill the spaces between clinker grains thereby producing a denser paste and stronger overall cement. The addition of microsilica further reduces the amount of free calcium hydroxide produced by the hydration of alite and belite and promotes the production of calcium silicate hydrate gel. Although the use of microsilica has been known to be beneficial for the production of cement in the construction industry the effect of the addition of microsilica in filters of the type disclosed in the BBM Patents to reduce foaming has not hitherto been shown. The effect is further described by reference to example 4 below. Without wishing to be bound by any theory it is thought that the reduction in foaming demonstrated in example 4 is caused by the greater resistance to acid attack of CSH gels relative to calcium hydroxide and their lower solubility in water dissolved in the oil.

Air entraining agents

It is still further been found that adding relatively small amounts of air entraining agents to the mixture used to make the filters of the type described in the BBM Patents produces filters which have greatly improved performance in terms of their ability to reduce the amount of free fatty acid build up in cooking oils. For many years entrained air has been deliberately incorporated in concrete and cement mixtures in parts of the world which experience freezing so as to reduce the damage caused by repeated cycles of freezing and thawing. Chemical additives known as air entraining agents are used to produce a stable system of discrete air voids, which are typically extremely small being between ΙΟμηι and 1 mm. There are normally more than 1 million such bubbles in one cubic inch of treated paste or more than 60,000 per cubic centimetre. The entrained air void system in cement can be viewed and determined from the examination of a cut and polished section of a hardened cement sample, using microscopic techniques in accordance with ASTM C457 (standard test method for microscopical determination parameters of the air void system in hardened concrete).

Such air voids provide empty spaces within the cement or concrete and are known to act as storage sites for freezing water moving in the capillary pores, thereby relieving the pressure generated during freezing and preventing damage to the cement or concrete. However it has surprisingly been found that the addition of relatively small amounts of air entraining agents to mixtures of the type used to make the filters of the type described in the BBM Patents produces very much more porous filters, which absorb very considerably much more free fatty acid than filters without such air entraining agents. There are a number of well-known types of air entraining agents. Typically these are surfactants and include for example wood derived acid salts, wood rosin, tall oil, vegetable oil acid salts such as the alkanolamine salt of coconut oil, synthetic detergents such as alkyl-aryle sulfonates and sulfates such as dodecylbenzenesulfonate. The surfactant used in the invention disclosed herein must be non-toxic. The man skilled in the art will readily be able to identify numerous such non-toxic air entraining agents suitable for use in the invention herein described.

How the invention may be put into effect will now be further described by reference to the following examples. EXAMPLE 1

Filters were made with Alborg white cement clinker and Alborg White OPC. Alborg white clinker is made using an extremely pure limestone originating from a marine deposit which is now located in Denmark. A typical composition of the Alborg clinker used is:- Si0₂ 25.0%

A1₂0₃ 2.00%

Fe₂0₃ 0.30%

CaO 69.0

This gives a calculated Bogue composition as follows :- C₃S 65.0%

C₂S 21.0%

C₃A 5.0%

C₄AF 1.0%

CaS0₄ 0%

The equivalent typical figures for the OPC used in this experiment were:-

S0₃ 2.03%

Si0₂ 24.4%

Fe₂0₃ 0.34%

CaO 68.6%

MgO 0.58%

CI 0.01%

Ti0₂ 0.09%

P₂0₅ 0.30%

K₂0 0.16%

Na₂0 0.19%

Thus giving a calculated Bogue composition (corrected to take into account a free lime content of about 3%):

C₃S 66.04% C₂S 20.1%

C₃A 4.64%

C₄AF 1.04%

CaS0₄ 3.45%

Importantly the clinker has a very low free iron content which is important as iron is a powerful pro-oxidant trace metal, (see for example Sonntag 1979 in Bailey's industrial Oil and fat Products, New York, John Wiley and Sons, Vol 1. pp 152). The mix to make the filters comprised three parts of clinker to one part of OPC. Thorough mixing of the clinker and OPC using an industrial mixer took place and then sufficient water was added to give a good paste.

The paste was then placed into moulds of the type depicted in figures 1 to 4 of

WO/2013/121206 and as further described in that patent application and were then allowed to cure until fully set. Thereafter they were dried in an industrial oven for 72 hours at 130°C to remove excess water and were then sealed in water impermeable wrapping. The dimensions of the filter were approximately 15 cm x 2 cm x 9 cm with 15 holes each with a diameter of approximately 2 cm: (these figures ignore slight tapering of the filter to facilitate extraction from the mould).

Two filters of this type were then located in an electric fryer containing 18 litres of oil. Oils develop acidity during frying due to oxidation, hydrolysis and other decomposition routes. Different oils have different oxidation and hydrolysis rates. The free fatty acid level in any given oil in a fryer is determined by the starting FFA level, the subsequent rate of FFA generation and the amount of its elimination by distillation and absorption by food. It is essential therefore if meaningful data on FFA levels are to be obtained to use the same oil throughout all experiments. The trials described herein were all conducted with the same batch of KTC vegetable oil, which contains E900 (PDMS anti -foaming agent). The filters were always located below the heating element and left in situ for five days during routine frying operations of eight hours a day (mainly chips). The temperature of the oil was always kept below 190°C. Five trials were conducted and the results averaged.

The filters were subjected to ICP-MS analysis as was an unused filter from the same batch. The unused filter had levels of calcium, aluminium and iron levels of 45%, 2% and 0.2% respectively. These values act as a positive control for the used filters.

After five days frying the five filters had on average lost 80% of their calcium in the outer 6 mm layer: calcium levels were down to 9% from the 45% seen with the unused filter. The values for iron and aluminium also showed a loss of over 50%, showing that the outer structure of the filter had started to break down.

Eight of the ten filters were covered with a dirty brown waxy layer containing calcium salt soaps, an example being shown in Fig. 5, which were found to have melting points between 150 to 190°C. The deposit was found to contain 16% calcium. The thickness of the waxy layer varied considerably from at most about 2 mm to a very thin layer considerably under 1 mm thick. This may be an indication that as the calcium leached out of the filters, at least some of it accumulated in the brown waxy layer.

Six out of ten filters foamed on average after 3.5 eight hour days of frying. All the foaming filters were covered with the waxy deposit. Although the life of the oil had been extended in all cases (from about two days without the filters) the occurrence of foaming was unacceptable to the users and led in all cases to the oil being immediately discarded.

EXAMPLE 2

The frying trials from experiment 1 were repeated but on this occasion the filters were encased in enclosures of the type illustrated in fig 2 made in accordance with the inventions disclosed herein. The location of the filter in example 2 was the same as for example 1 but in this instance only one of the filters showed the waxy brown layer, which was very thin and not continuous. Additionally the amount of calcium depletion in the upper 6 mm layer was considerably less than was seen in example 1: the depletion was only 20% (from 45% with the unused filter to 36% for the used filter). The upper layer was also less friable and the material much more tightly secured into the matrix of the filter as was clearly visible under an electron microscope.

Filters enclosed in an enclosure of the type described herein suffered materially less surface degradation than filters which were simply inserted into the cooking oil.

None of the ten filters used in this second trial foamed during the five day tests and in four out of five tests the oil was still fit for use at the end of the five day trial, which represented approximately a one hundred percent increase in the life of the oil.

EXAMPLE 3

At the end of each day of the final two trial runs from example 2, 50 gm samples of the oil were taken, after the oil had cooled, been filtered and were then flushed with nitrogen and kept at minus 20°C away from any source of light until analysed.

These were then compared with two additional trial runs in the same fryer with KTC vegetable oil wherein no filters were added.

The free fatty acid was measured for each sample (in accordance with the procedure laid down in IUPAC (1979), Standard Methods for Oils, Fats and Derivatives, 6^th edn, Pergamon Press, Oxford, reference 2) and the averaged results are tabulated below:-

Day FFA % from Example 2 FFA % from Example 3

(with enclosed filter) (with no filter)

0 0.15 0.15

1 0.25 0.30

2 0.30 0.45

3 0.35 0.58

4 0.40 0.65

5 0.45 0.81 It can be seen that the presence of the two filters within their enclosures materially reduced the rate of build-up of the free fatty acids. EXAMPLE 4

Example 1 was repeated but this time the composition of the filter was changed so that microsilica was added to the mix of example 1. The amount of microsilica added was ten percent by weight of the weight of the dry mix of clinker and OPC. The resulting filters were used without enclosures.

Only two of the ten filters foamed during the five day trials. The use of the microsilica appeared to have no material negative effect on the improvement in the life of the oil at all as all but one out of five oil samples were still fit for use at the end of the five day trial. The free fatty acid concentrations, which were measured (in the same manner as for experiment 3) each day for the last two trial runs are tabulated below (average figures):-

EXAMPLE 5

Example 1 was repeated but this time a small amount of the air entraining agent, microair 119 (manufactured by BASF) was added to the mixture. The amount of microair 119 added was approximately 25 ml to 6 kg of dry mix (OPC and clinker). It was surprisingly found that this had a dramatic effect on the filters thereby produced, which although having a volume of approximately 151 cm³ (the same as for the filters previously used in these experiments) weighed only 260 gms - about 20 gms lighter than the filters made in example 1. Electron microscopy revealed a network of very small pores in the filters. Such filters exhibited a markedly improved ability to reduce FFA build up. Below are tabulated the average free fatty acid concentrations for the last two trial runs for this set (five days frying per trial):-

Day FFA % for example 5 (with filter

containing air entraining agent)

0 0.15

1 0.25

2 0.27

3 0.30

4 0.35

5 0.40

Claims

1. A method of preserving cooking oil, including by reducing the extent of foaming and/or the build up of free fatty acids in the cooking oil during its use in the frying of food, the method comprising providing in a cooking vessel a body of cooking oil, locating in said body of cooking oil a solid, calcium-containing filtering medium and locating food within said body of oil and heating the cooking oil to effect frying of the food, the filter medium containing a minor amount of microsilica.

2. A method according to Claim 1, wherein an air entraining agent is added to the filtering medium during its manufacture so as to create numerous pores in the structure of the medium.

3. A method according to Claim 1 or Claim 2, wherein the method includes locating in said body of cooking oil means for reducing the extent of flow of oil over the surface of said filtering medium during the food frying, locating food within said body of oil and heating the cooking oil to effect frying of the food.

4. A method according to Claim 3, wherein the filtering medium is at least partially surrounded by a foraminous barrier.

5. A method according to Claim 4, wherein the barrier extends laterally adjacent said filtering medium.

6. A method according to Claim 4 or Claim 5, wherein the barrier forms at least part of a container within which the filtering medium is located.

7. A method according to Claim 6, wherein the container is of box -like construction having two foraminous side walls.

8. A method according to Claim 7, wherein the base and top of the container are both continuous.

9. A method according to any of Claims 3 to 8, wherein the container is at least partly integral with the cooking vessel.

10. A method of preserving cooking oil, including by reducing the extent of foaming and/or the build up of free fatty acids in the cooking oil during its use in the frying of food, the method comprising providing in a cooking vessel a body of cooking oil, locating in said body of cooking oil a solid, calcium-containing filtering medium, locating food within said body of oil and heating the cooking oil to effect frying of the food, an air entraining agent being added to the filtering medium during its manufacture so as to create numerous pores in the structure of the medium.

11. A method of preserving cooking oil, including by reducing the extent of foaming and/or the build up of free fatty acids in the cooking oil during its use in the frying of food, the method comprising providing in a cooking vessel a body of cooking oil, locating in said body of cooking oil a solid, calcium-containing filtering medium, locating in said body of cooking oil means for reducing the extent of flow of oil over the surface of said filtering medium during the food frying, locating food within said body of oil and heating the cooking oil to effect frying of the food.

12. A deep oil or fat fryer having a depression or recess for releasably holding, or providing at least a part of, an enclosure for a filtering medium, said enclosure being provided with means for reducing, in use, the extent of flow of oil over the surface of said filtering medium during food frying

13. A calcium-containing filtering medium for use in preserving cooking oil, the medium containing a minor amount of microsilica.

14. A calcium-containing filtering medium for use in preserving cooking oil, the medium having had added to it during its manufacture an air entraining agent so as to create numerous pores in the structure of the medium.