Stability and rancidity

T HE DETECTION of advanced stages of rancidi ty in a fat or fa t ty food has never been a problem for people with normal olfactory senses. The sharp pungent odors mixed with certain stale and musty odors have provided tell-tale evidence of rancidi ty throughout the ages. That this is not always regarded as undesirable is evidenced by the fact that some peoples have deliberately aged fats before using them. The p r e s e n t d a y p r e f e r e n c e s f o r p r o d u c t s w i t h f r e s h bland flavors and odors requ i re t ha t we have ways and means for evaluating keeping quali ty and incipient rancidity. The history of objective tests for rancidi ty and for keeping quali ty begins over 50 years ago with development of the Kreis test (12). Then the oxygen absorpL. R. Dugan Jr. tion methods for determining the resistance of a fat to rancidi ty were developed. The baking indust ry developed the oven test for rancidity, which became commonly known as the "Schaa l Oven Tes t " (10) although there is no published reference by the originator. Great strides forward were made with the development by Lea (14) and later by Wheeler (29) of quanti tat ive methods for peroxide determinatio~J. These were followed by the accelerated method known as the " S w i f t Tes t " or the "Act ive Oxygen Method" by King, Rosehen, and I rwin (11). The rapidi ty and reproducibil i ty of this method coupled with the organoleptic correlation of rancidi ty with developed peroxides made it extremely useful for evaluation of the resistance of a fa t to rancidi ty and especially for the evaluation of the relative effectiveness of antioxidants in fats. Many early workers recognized "the presence of aldehydes and /o r ketones in autoxidized fats and at tempted to develop objective tests based on them. I t was only af ter the development of quantitative colorimetric tests for carbonyl compounds in low concentrations that substantial agreement between carbonyl content and organoleptie evaluation of rancidity was accomplished. The most commonly used methods for assessing stability or rancidi ty of a fa t or f a t t y foods are the: active oxygen method, Schaal oven test, thiobarbituric acid test, carbonyl test, and peroxide value. The Active Oxygen Method. This method is based on the principle that the aging and rancidification of a fat is greatly accelerated by aeration in a tube held at a constant elevated temperature, While an organoleptic evaluation may be used to indicate the rancid point, the results are based on the peroxide content of the fat af ter correlation with organoleptic rancidity. The apparatus consists of a constant temperature bath and an aerat ing t ra in contrived to deliver a continuous flow of air to the sample at a constant rate. hl i t ial ly the heat was supplied by an oil bath surrounded by a hot water jacket maintained at a constantly boiling temperature. Recent refinements use an oil bath with a st irrer and a thermoregulator for precise temperature control. The aerating t ra in has a bottle with water through which air is passed into another bottle containing 2% potassium dichromate with 1% sulfuric acid. These bottles serve as scrubbers to pu r i fy the air before it is led into the manifolds, to which the sample tubes are connected. A tube leads from the scrubber to two cylinders connected in series and each filled with water to the height necessary to provide a suitable pressure head for forcing air through each tube leading into the sample tubes. The rate of air flow was established at 2.33 ec. per second through each sample. This flow rate is controlled by the pressure head and by inserting a piece of capillary tubing in the line leading from the manifold into the sample tube. The capillary tube is cut to length according to the size of the capillary to permit the established rate of air flow, The determinat ion may be made by measuring 20 cc. of liquid fat into three 8-in. by 1-in. tubes. One tube is heated to tempera ture in an external water or steam bath and then placed in the oil bath; and the air tube is connected to the tube which projects through the sample to near the bottom of the sample tube. The time of the beginning of air flow is recorded. At suitable time intervals of 1 or more hours, the remaining tubes progressively may be started. When the effluent air f rom the first tube has the characteristic rancid odors, a sample may be taken from each tube for a peroxide determination. By a proper choice of s tart ing intervals for the tubes containing a given sample, it is possible to bracket the end point of 20 milliequivalents of peroxide per 1,000 g. of fa t (animal fats) and then by periodic sampling from the remaining tubes accurately to determine the time for development of the necessary peroxide content. Variations of this method are practiced. A eommonly used variat ion is that reported by Riemenschneider et al. (23), which uses only one tube and periodic sampling of 0.2-g. samples for determination of the peroxide value. These workers introduced the all-glass aeration tubes in place of the rubber-stoppered test tubes and added a water condenser and extra air dist r ibut ing bottle to prevent entrained moisture in the air-distr ibuting system from clogging the capillary orifices. A major contribution to application of the A.O.M. to control work was made by Mehlenbacher (16), who found that the time for rancidification could be shortened by a factor of 2.5 by elevating the incubation temperature to 110~ ~t is obvious from this that temperature control is critical for reproducibi l i ty of results since only a small change in temperature may introduce a wide variat ion in end-point. There are numerous factors which can influence the rate of rancidification of a fat, and the temperature effect cited is one. Other factors are light, availabili ty of oxygen, surface factors, the presence of natural or synthetic antioxidants, the nature of the fat, and trace metal or metal contact catalysis. For reproduei-