Determination of Calcium in Cottage Cheese by Titration

Abstract

The calcium concentration of cottage cheese has been determined by the use of disodium ethylene diamine tetraacetate as a calcium binder and calcein as an indicator. This method of analysis compares favorably with the atomic absorption procedure. Most mineral analyses reported in the literature involve the use of flame photometry or atomic absorption spectrophotometry procedures which are time consuming and require equipment not normally found in laboratories of milk or cheese plants. A back-titration procedure for the determination of calcium and magnesium in biological fluids was developed by Kamal (2), and refined by Ntaili-anas and Whitney (3) for use in milk. This procedure uses disodium ethylene diaminetetraacetate (EDTA) to bind calcium. An excess of EDTA is added and the excess is measured by titration with a known strength of calcium solution. When the equivalence point between excess EDTA and added calcium is achieved, calcein, the indicator, will change from brown to green. The titration vessel must be viewed against a black background for visualization of this color change. The objective of this study was to explore the use of this titration procedure for the determination of calcium in cottage cheese. MATERIALS AND METHODS The method of Ntailianas and Whitney (3) was modified and used to evaluate the calcium concentration. Cottage cheese was blended in an Osterizer and approximately 2 g weighed into a 150-ml beaker. To the beaker was added 10 ml of 0.05N EDTA and 70 ml of water. The mixture was stirred by use of a magnetic stirrer and 8 N KOH was added to bring the pH to 13. The mixture was poured into a 100-ml volumetric flask and made up to volume with distilled water previously adjusted to pH 13 with KOH. One 25-ml portion was taken for spectro-photometer analysis using the method described by the manufacturer of the equipment (I). Three 25-ml portions were put into 50-ml Erlenmeyer flasks, three drops of 0.2% calcein were added to each flask, and with the flask on a magnetic stirrer, 0.05 N calcium chloride was added until a permanent green color appeared as viewed against a black background. A burette of 5 ml capacity divided into 0.01 ml divisions was used. The concentration of calcium in the titrant was determined previously by atomic absorption spectrophotometry. The concentration of calcium in cottage cheese was calculated as: (a X b)-(c X d) = milliequivalents of calcium per gram of e sample where: a = ml of EDTA in titration vessel, b = normality of EDTA, c = ml of calcium used in back titration-ml used for blank, d = normality of calcium standard, and e = grams of sample. Meq per gram of cheese X 2004 = mg Ca/100 g cheese. The ml calcium standard used for the blank was determined by titrating against the reagents only. Data were analyzed for significance of effect by the Stu-dent's " t " test (5). The accuracy of the titration procedure was tested statistically by using a minimum variance quadratic unbiased estimation procedure (4). RESULTS Known quantities of calcium were added to known quantities of cottage cheese after which the total calcium concentration was determined by the titration procedure. In a series of 5 samples, a calcium spike ranging from 0.046 to 0.092 meq/g was recovered at 100% in all case except one, which showed a 98% recovery. Forty-five samples titrated in triplicate showed an estimated standard deviation between samples of 14.2 mg Ca/100 g cheese, and an estimated standard deviation within samples of 3.6 mg Ca/100 g cheese. The average and standard deviation of the results by titration was 83.1 +/-14.2, mg/ 100 g and that by atomic absorption was 82.0 +/-13.2 mg/ 100 g. The two averages were not statistically different (P >.05).