METHOD AND ARTICLE FOR DETERMINING FREE ACID CONCENTRATION IN LIQUID

Abstract: An article (10, 30 or 50) for testing liquid for free acid includes a substantially nonreactive, neutral and nonbuffering, porous, absorbent support material having at least one portion which contains an active composition. The active composition contains (1) an effective amount of acid-base indicator capable of changing colour in a pH range on the order of 6-10, (2) a predetermined amount of base compound reactive with free acid and being present in an amount equivalent to a known concentration of free acid, and (3) about 30-99.5 parts by weight of non-volatile material of a substantially colourless, substantially neutral, hemectant organic solvent which is substantially non-volatile under ambient conditions. An organic liquid to be tested is contacted with the article; and any colour change in the article after the passage of sufficient time is observed, to determine free acid in the liquid, in a rapid, convenient manner.

Description: - - D E S C R I P T I O N METHOD AND ARΗCLE FOR DETERMINING FREE ACID CONCENTRATION IN LIQUID Technical F eld This invention relates to a method and test st for use in estimating the free acid content n l iquids s as coo king o l . Background Art Great quantities of cooking oil and fats are utilized daily fay cooks and food processors to prepare f Typical cooking methods involve immersing the food tα be cooked, e.g. , in a wire basket, into a heated vat of coo oil and permitting the food to remain therein until cook Cooki ng- under such circumstances gives the food its own distinctive cooked flavor. Unfortunately, as cooking oils and fats are us there is a progressive build up of undesirable constitue such as free fatty acids. Cooking oils and fats used fo frying foods are generally composed of glyceridas with minor amounts of free fatty acids. The formation of additional free fatty acids has generally been attribute to the chemical break down of the glycerides to form fat acids, mainly caused by hydrolys s and oxidation, produci short chain compounds such as aldehydes, ketones and aci If degradation proceeds to any significant extent, the degradation products impart an unacceptabl e flavor to the cooked foods. As the concentration of fre e fatty acid builds, the cooking oil progressivel assumes undesirable charac- tsristics such as fuming, darkening, smoking, forming gum and residues and imparting distinctive interfering flavor to the food be ng cooked. BAD ORIGINAL Some cooks and food processors merely add new cooking oil or fat to the degraded material to extend th useful life of the cooking oil . While this is temporaril effactive, the problem will be repeated as additional degradation takes place. Other cooks merely discard the entire batch of cooking oil or fat and replace it wi-th a fresh supply thereby prolonging the time before additiona corrective action must be taken. Many cooks and some food processors discard their cooking oil after a schedul use time to preserve the integrity of the flavor of their cooked food notw thstanding the fact that degradation ma not have occurred to an extent to even remotely interfere with the food. The latter method causes considerable was and the unnecessary expenditure of considerable man hours if the cooking oil or fat is discarded prematurely. Various methods have been devised to quantative or qualitatively determine the degree of degradation and/ fatty acid content, but these generally have one or more problems associated therewith. One method involves monitoring the taste of the food and s when the food begin to assume an off taste, utilize that as a point at which the oil is replaced or rejuvenated. Since this method causes the flavor of the food to vary, it is undesirable. Other methods include monitoring the color of the oil, since it darkens with degradation, either by visually ob¬ serving it or by using photoelectr c devices zo make the observation. These methods are undesirable because they generally require the removal and handl ng of small volum of hot oil. Additionally, the photoelectric examination requires relatively expensive equipment which must be maintained and is subject to failure. Another method in¬ volves titrating small samples of the oil with a standard basic solution. The titration method is undesirable because it also requires removing samples of typically ho oil and also requires the preparation of standard titrati solutions and the maintenance of certain laboratory equip ment for use therewith, typically net found i- most restaurants and some food processing .facil ties. Seme test methods have been devised util izing test papers, bu they have generally been unacceptable because they have not been sufficiently accurate, are difficult to use, or gi-ve misleading results. Illustrative of the prior art dealing with testing oils, fats and the like for degradation, fatty acid content, or similar purposes are the following U.S. Patents : Bergstro et al (2,770,530), Seemaπ et al (3,030,190), Apter (3,615,225) and Ell ott et al (.2, 953,4 disclose various "wet" methods of testing oils which requ the removal of small amounts of oil from the sain batch. Eiseman (3,238,020) discloses an oil test str p containin an indicator and an aliphatic polyhydroxy compound. Davi (3,420,635) discloses a fruit ripeness tel ltale formed of a sheet of plastic having thereon a color-changing com¬ position including an acid-base indicator and a solid absorbent for C0 2 such as calcium hydroxide to detect whe the fruit is ripened and a small amount of hydroscopic material such as calcium hydroxide to detect when the fru is ripened and a small amount of hydroscopic material suc as calcium chloride to insure the presence of mo sture. Pickup et al (.3,580,704) disclose testing free fatty acid content in oil with paper treated with an acid-indicator and a non-aqueous, non-volatile liquid solvent in which t oil and indicator are soluble. Matsushita (.4, 098,575) discloses a test paper impregnated with potassium iodide to determine the peroxide value of oils in fats. Summary of the Invent on The present invention provides an article for quantitatively -estimating the concentra on of free fatty a in organic l iquids such as cooking oils, motor oil and th " l ike. The article of the invention may also be used to quan atively estimate the free ac content of aqueous l iquids. The article is preferably in the form of a stri wr icπ can easily be submersed in the c o c i n g o l π c -smo v e o w i t h " a p r e d e t e rm i n e d vo l ume o _i ι i - : i o 6C - aπd therefore does not require removal of test samples o the oil, thereby avoiding liquid handling steps. The article of the invention provides a rapid, convenient mea of determining the degree of fatty acid content in cookin oil , providing results in less than an hour. The article of the present invention comprises a substantially noπreactiye, neutral and nonbufferi ng, porous absorbent support material having at least one portion thereof which contains a composition comprising an effective amount of acid-base indicator capable of changing color in a pH range on the order of 6-10, a predeterm ned amount of base compound reactive with free fatty acid and being present in an amount equiv alent to a known concentration of free fatty acid, and about 30-99.5 parts by weight* of a substantially colorle substantia ly neutral, hu ectaπt organic solvent which is substantially non-volatile under ambient conditions. A preferred article of the invention includes specially separated test areas, each of which contain the composition, but with varying amounts of base compound i'n each to provide the" capability of identifying a range of concentrations of free fatty acid in the organic liquid. The method of the invention comprises contacting an organ liquid to be tested with a test article described above a observing any color change in the article after the passa of sufficient time. Drawiπq The invention may be further understood by reference to the drawing, wherein FIG. 1 is a top plan of an embodiment of the article of the invention; *based upon the weight of the non-volatile const uents o the solvent, indicator and the base comoound. BAD ORIGINAL FIG. 2 is a side view of the article of FIG. 1 FIG. 3 is a top plan view of another embodimen of the article of the invention having the capabil ity of determining the fatty acid content over a range of conce t a ons: FIG. 4 is a side view of the article of FIG. 3 FIG. 5 is a top plan view of yet another embod ment of the article of the invention having a similar capability as the article of FIGS. 3-4; and FIG. 6 is a side view of the articl e of FIG. 5 Description of the Preferred Embodiments Referring now to FIGS. 1-2, there is shown an article in the form of a strip 10 for estimating the fre fatty acid content in organic liquids such as cooking oi Strip 10 is formed of a substantially noπreactive, neutr and non-buffering support material which may be a woven a πonwoven material . The preferred porous support materi is formed of porous paper such as filter paper (.e.g., Whatman's #1 filter paper). Other useful support materi include πoπwoven glass f lter material, open-celled foam and woven fabrics such as woven cotton cloth. The suppo material absorbs a controlled amount of oil in a given reasonable period of time, typically less than 1 minute, preferably less than 5 seconds. While the support . material preferably is ri the form of an elongate strip, it may also have other forms, e.g., a tube, disc, rp or the lita. The support material is impregnated with a composition comprising an effective amount of acid-base indicator capable of changing color in a pH range on the order of 6-10, a predetermined amount of base compound reactive with free fatty acid being present in a amount equivalent to its known concentration of free fatt acid present in the oil absorbed into the support materia and about 30-99.5* by weight ( . based upon the weight of tπ ncn-volatile const uents) of a su stantially colorless, substant all neutral, humectant organic solvent which is The article of FIGS. 1-2 will be capable of changing color after a particular fatty acid content has been achieved in the organic liquid being tested and present in the support material. The amount of fatty ac at which the color change takes place is determined by t amount of base compound in the article. The amount of base should be adjusted to correlate to the desired conc tration of free fatty acid. That is, if the amount of b is selected to provide a color change at IS fatty acid, color change will take place at IS and higher concentrat of free fatty acid. The preferred articl e according to the present invention has the capability of revealing a range of fre fatty acid content, for example, in increments of 1% fro 0 to 10%. Such articles are shown in FIGS. 3-6. Referr to FIGS. 3-4, there is shown an article in the form of strip 30 having test areas 31, 32, 33, 34, 35, 36 and 37 each containing the impregnant composition described abo except having varying amounts of base compound to reflec a varying range of free fatty acid content. The test areas are separated by a barrier stripe 38 which prevent migration of liquids from one area to the other to preve interference between areas. FIGS. 5-6 show yet another bodiment of the article of the invention in the form of strip 50 bearing test area segments 51-57, each containi varying amounts of base material to reflect a range of fatty acid -content. The area segments are bits of base material which are impregnated as described above and adhered to a carrier strip 58. Other modifications of t article according to the present invention may be made without departing from the scope of the claims. The support material may be formed of any sub¬ stantially unrεactive, i.e., pK neutral, non-buffer ng, and chemically non-reactive porous, absorbent woven, noπwoven, or the like material which has the capability o holding and retaining a sufficient amount of the active ingredients which will be contained therein. The suppor material is preferably, oi1 -absorbing and light colored or colorless. Suitable support materials may be formed porous paper such as filter paper, preferably Whatman's filter paper, πoπwoven glass filters, open-cal led foams, w.ebs of polymeric icrofibers, and woven cotton cloth. The impregnant solution which is contained in support material is formed of humectant solvent, indicat and base compound. The humectant solvent is a substanti non-volatile, substantially colorless, substantially neutral l iquid that is soluble in water and capabl e of solubil izing the indicator and preferably the base compo Suitable humectant solvents include mono and dihydrσxy aliphatic polyethylene glycol compounds such as those available under the trade designation "Carbowax" 200, "Carbowax" 400 and "Carbowax" 500, d po lypropyl ene glyco methyl ether, and diethylene glycol ethyl ether and surfactants such as alkylphenoxy polyethoxy ethanols e.g availabl e under the trade designation "Triton" X-100. T humectant sol vent, corπpri ses about 30-99.5% of the non¬ volatile content of the impregnant composition. The base compound may be any organic or inorganic base compound with sufficient reactivity to react with the fatty acids or inorganic acids. Some examples of useful base compounds include sodium carbona sodium bicarbonate, potassium carbonate, potassium bicarbonate, tetramethyl guanidine, guanidine carbonate and amino silane (available under the trade designation "A-1100" silane from the Union Carbide Company). The amount of base co pouad ' may vary from 0.4% to 75% based upon the non-volatile constituents of the impregnant composition. The amount of base compound is controlled, depending upon the amount of oil imbibed upon the support, to provide in the articl e an amount which permit quantification of the amount of acid being measured. The acid-base indicator may be any organic acid base indicator dye or combination thereof which is capabl of providing a color change, preferably a v e ry distinct color change, in the pH range of 6-10. Suitable indicato include -cresol purple, neutral r e d . Thymol bl ue, phenol Fo r strips which are capable of reveal ng a ra of acid concentrations of the type shown in FIGS. 3-4, a continuous stripe of barrier material is util ized to def test areas to prevent migration of the ingred ents from one test area to another. The barrier material is any suitable material for providing this function. Examples suitable barrier materials include polys il oxaπes , poly- acrylates and methacryl tes, epoxy resins and the like. Add tionally, the barrier may be provided by the constructi shown in FI S. 5-6 by employing a barrier type backing f 58 upon which the test area segments are adhered. Such backing materials may be formed of thermoplastic sheet material such as polyethylene, polypropylene or the like The simplest form of the article of the invent may be prepared by mixing the humectant solvent, base compound, indicator and additional volatile solvent such as water or organic solvent, if required, to provide a solution, and impregnating the support material with the solution to provide " the desired composition within the base material and permitting the support material to dry. Articles of the type, depicted in FIGS. 3-4 may be prepared by applying parallel separata stripes of barrier material to the base material and impregnating t areas defined between the stripes with impregnant composi tions having varying amounts of base compound. The barrier stripes should be applied in a sufficient width prevent migration between areas. Typical barrier widths are on the order of 1 to 10 mm. Segments of support material , each having or subsequently being provided wit varying amounts of base compound in the impregnant com¬ position may be adhered to a barrier substrate by suitabl adhesive or by heat lamination to provide the article depicted in FIGS. 5-6. If the segments are to be heat laminated, it is preferred to impregnate the segments af lamination to avoid any heat induced alteration in their components. Add tionally, an articl.e according to the invention may be prepared by m xing the impregnant material s with paper or web- forming fibers and casting techniques. Other ways of producing the article of the invention will become apparent to those skilled in the a once apprised of the present disclosure. EXAMPLES The invention is further illustrated by the following examples, wherein all parts are by weight, unl otherwise specified. EXAMPLE 1 An 11 cm diameter disc of Whatman's #1 filter paper was treated by spraying with a solution consisting of Parts Ingredient m-cr sol purple indicator 0.04 ethylene glycol humectant non-volatile solvent avail¬ able under the trade desig¬ nation "Carbowax" 200 30.0 sodium bicarbonate base compound 0.7S water volatile solvent 59.ιε The paper was coated, to provide a wet coatinc weight of 2 15 mg per cm and thereafter dried at room temperature t 2 a constant weight of 3 mg per cm . A strip of the treat paper was dipped into oil containing known quantities of free fatty acid. The strip changed from blue to yellow approximately one hour after being dipped in oil containing free fatty acid to provide a concentration usi an acid number in excess of 5. No color change was note 'in the oil containing acid to provide an aci number less than 5. EXAMPLE 2 An article accord ng to the present invention prepared by laminating a 2 cm square piece of Whatman's # filter paper over the 2 cm square end p.ortic- of the polyethylene side o f a 2 cm x 10 cm strip of laminate f l consisting of a layer of polyethylene adhered, to a layer ■ zf polyester and available under the trade designation "Scotchpack" heat sealablε polyester film. Lamination of the ?=-- = .- to the film was accomol i shed with = "Ro ot" Mo l - i- by heating the juxtaposed paper film assembly therein between jaws heated at 150°C, holding the assembly there for 8 seconds at a jaw pressure of 0.24 kg per cm , to cause the paper to adhere to the polyethylene surface. The paper was then treated with the solution described i Example 1 to provide a wet coating weiαht of 15 mg per 2 cm and dried at room temperature to a constant weight t 2 provide a dry coating weight of 3 mg per cm . The . resultant strip performed substantially the same as the strip described in Example 1. EXAMPLE 3 An article of the type depicted in FIGS. 5 and according to the present invention having multiple test areas corresponding to acid numbers of 1, 3, 5 and 7 was produced by laminating four 0.5 cm by 1 cm pieces of Whatman's #1 filter paper on the polyethylene side of a 1 cm x 10 cm strip of the polyethyl ene:polyester laminat film described in Example 2 with the four strips of pape being separated by 0.5 cm and being parallel on their 1 sides with their 0.5 cm sides aligned along the 10 cm si of the laminate. The assembly was laminated in the heat sealer described in Example 2 at 150 β C for 10 seconds at 2 pressure of 0.24 kg per cm . The four segments of filte paper were treated respectively with the following soluti Ingredient Parts Parts Parts Par Acid No. 1 3 5 7 -cresol purple indicator 0.04 0.04 0.04 0. ethyl ene glycol humectant solvent available under the trade designation "Carbowax" 200 30.0 30.0 30.0 30. sodium bicarbonate base compound 0.275 0.5 0.78 1. water (volatile solvent) 69.59 69.46 69.18 53. Each of the coatinσ wei hts was approximately -, - 2 " ? \ -2 g per cm wet, drying to a constant 3 g per cm " weig at room temperature. The resul ant dried stri was dippe into a test oil sample containing fatty acid concentra io of less than acid number one. '.o color change in the str second strip was dipped into oil containing fatty acid provide an aci number greater than 1 but .les.s than 3. A color change from blue to yellow in the test area treated to correspond to an acid number of 1 was observ but no color change in the remaining treated test areas indicatinσ acid numbers of 3 or more were noted. The remaining test areas were found to perform in a similar manner when the free fatty acid concentration was incre EXAMPLE 4 A preferred article depicted in FIGS. 3-4 of drawing according to the present invention having the c ability, of detecting the range of fatty acid concentrat corresponding to acids number 1 , 3, 5 and 7 was prepare A 1 cm x 10 cm strip of Whatman's #1 filter paper was provided with barrier stripes consisting of a two part silicone resin available from the Dow Corning. Company a "Dielectric Gel" and applied by utilizing- a 1 cc hypode syringe fitted with a 25 gauge needle to provide parall stripes on the strip of filter paper parallel to the 1 . ends 2. mm wide and separated by approximately 4 mm to provide four 4 mm x 10 mm zones of untreated paper. Ea zone was treated, respectively, with the four impregnan compositions described in the previous exampl to provi test areas having respect vely the capability of detect acid numbers 1 , 3, 5 and 7. The test strip performed s stantial ly the same as described in Example 3.

Claims: Cla s An article for quant tatively determining the concentration of free acid in a liquid, said article comprising a substantially non-react ve, neutral, porous and absorbent, support material having at lea one portion thereof wHi.ch contains an active compo¬ sition characterized by comprfsfπg (.a ) an effective amount of acid-base indicator capable of changing color in a pH range on the order of 6-10; (_b) a predetermined amount o f base compound reactiv with acid absorbed in said support and being present in an amount equivalent to a known concentration of free acid; and Cc) about 30-99.5 parts by weight based upon the total weight of the non-volatile constituents o A ) , (b) and Cc) of a substantially colorles substantial y neutral, humectant organic solven which is substantially non-volatile under ambie conditions. - The article of claim 1 wherein said humectant solven is characterized by being selected from the group consisting of hydroxy aliphatic polyethylene glycol compounds and alkylpheπoxy polyethoxy ethaπols. . The article of claim 1 wherein said base compound is characterized by being selected from the group consisting of sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, tetra- methyl guanidine, guanidine carbonate and a ino silane. The article of claim 1 wherein said indicator is c6.aractari.zad by being selected from the group consisting of -cresol purple, neutral red, Thymol blue, phenol red and cresol red. The article of claim 1 wherein said support material is characterized by having a plurality of test area each containing said composition But with varying predetermined amounts of said base compound in each said test areas to provide amounts equivalent to a 10 range of known concentrations of free acids. 6. The article of claim 5 wherein said test areas are c6.aracteri.zed by being separated by barrier stripes which prevent the migration of ingredients from one test area to an adjacent test area. 5 7. The article of claim 5 wherein said test areas are characterized by.being individual segments of suppor material spacially adhered to a carrier strip formed of a barrier material which prevents migration of ingredients from one of said test areas to another. Q 8 A method of quantitatively determining the concentra tion of free acid in a liquid comprising (a) contacting the liquid to be tested with a test article comprising a substantially non-reactive neutral, porous, absorbent and non-bu fering 5 support material containing an active compositi and (b) observing any color change in said article characterized by said active composition comprising Q (1) an effective amount of acic-basa indicator capab e of chang n col r in a pH r nge on the or er of 6-10; (2) a predetermined amount of base compound ... . . .reactive with free acid aid being pr≥s in an amount equivalent to a kr.αwn concent tion of fre.e ' acid absorbed in said support material ; and (.3) about 3Q-9.9.5 parts by weight based upon t non-volatile constituents * of trie total wei of Ul, C21 and (_2\ of a substantially neutral, h-umectant organic solvent which i suδstantially non-volatile under ambient conditions * .

Inventors: MLINAR JERRY W, NEUMAYER JOHN J
Applicants: MINNESOTA MINING & MFG
Application year: 1981
Main classification: G01N33
IPC codes: G01N31, G01N33