Ultrasonic velocity in triacylglycerols or oils is a measure of the speed of sound in these food components. It is related to its fatty acid composition and the supramolecu- lar lipid structure. This is one of the most important vari- ables in predicting the velocity of sound in an emulsion. The speed of sound in oils decreases monotically with temperature (134), is similar for oils and water at 20°C (79), and is lower in solid fats than oils. Ultrasonic veloc- ities for some triacylglycerols as well as for some com- mon oils are given in Tables 9.28 and 9.29, respectively.
The ultrasonic attenuation coefficient is a measure of how much ultrasound is dissipated in an oil per unit dis- tance. This coefficient has little direct value beyond predict- ing the ultrasonic properties of an emulsion. However, in this application, a precise value is essential. The attenuation coefficient increases with frequency at a single temperature (79). Attenuation and its frequency dependence tend to decrease with increased temperature. Ultrasonic attenuation coefficients for some common oils are given in Table 9.29.
IV. OPTICAL AND SPECTROSCOPIC PROPERTIES
A. COLOR
Fatty acids and triacylglycerols, are colorless and essen- tially transparent to visible light. Natural fats and oils, however, often contain pigments that partially absorb transmitted light. Most of these pigments are removed from fats and oils by the refining and bleaching process,
Fats: Physical Properties 9-15
TABLE 9.27
HLB Values of some Commercial Nonionic Emulsifiers
Emulsifier HLB
Sorbitan trioleate 1.8
Sorbitan tristearate 2.1
Mono- and di-acilglycerols 3.2–3.5
Glycerol monostearate 3.8
Sorbitan monooleate 4.3
Sorbitan monostearate 4.7
Sorbitan monopalmitate 6.7
Sorbitan monolaurate 8.6
Polyoxyethylene sorbitan monostearate 9.6
Polyoxyethylene sorbitan monooleate 10.0
Polyoxyethylene sorbitan trioleate 11.0
Glycerol monostearate 11.0 Polyoxyethylene monostearate 11.1–16.0 Polyoxyethylene monolaurate 12.8 Sodium oleate 18 Sucrose monoester 20 Potassium oleate 20
Sodium stearoyl lactylate 22
Sodium lauryl sulfate 40
Source: Refs. 41, 115. TABLE 9.26
Interfacial Tension of Oils
Oil Interfacial Tension (dyne/cm) at 70°C
Cottonseed 29.8
Peanut 29.9
Soybean 30.6
but some of them remain and most oils have their specific color, which is a consequence of the type and amount of natural pigments present. Therefore, some oils are natu- rally darker than others (68).
Carotene is the predominant red/yellow color pigment in soybean, safflower, and sunflower oils, among others. Carotene becomes colorless when subjected to the tempera- tures encountered in the edible oil processing steps. The oxi- dation products and other pigments found in these refined oils can be removed by adsorption on diatomaceous earth in the edible oil processing step referred to as bleaching. Most
of the red color found in cottonseed oil, on the other hand, comes from a minimal residual level of gossypol and gossy- pol derivatives, especially a complex gossypurpurin. While some of the pigments that contribute to oil colors can be removed by adsorption on bleaching earths and any carotene is rendered colorless by heating, the gossypol can only be removed by alkali refining. The level of color removal that can be achieved is, to a great extent, dependent on the hand- ling and storage of the seed and crude oil prior to refining.
In addition, poor grade of crude oil or improper pro- cessing and handling may produce oils and fats which are darker than usual. Thus, insufficient refining may leave residual phospholipids that will darken the oil during deodorization. Vegetable oils and shortening will also darken after being stored for a long time or at elevated temperatures due to the oxidation of tocopherol to toco- quinones. During frying, oil darkening is further compli- cated by the polymerization of oil and interaction between the oil and other components of the food being fried (95).
Color may be determined by a number of procedures. The Lovibond method determines color by matching the color of the light transmitted through a specific depth of liquid fat or oil to the color of the light originating from the same source, transmitted through glass color standards (140). Results are given in red and yellow units describing the combination that matches the sample color. By using this methodology, the maximum accepted values for edi- ble oils are collected in Table 9.30. Other methods that also determine color by comparison with permanent color standards or glasses of known color characteristics have also been described (142,143).
Color can also be determined spectrophotometrically. In this case the oil or fat is dissolved in the required sol- vent and the transmittance or the extinction of the solution is then determined at the specified wavelengths with ref- erence to pure solvent (144). These absorptions may be expressed as specific extinctions (the extinction of 1% solution of the oil in the specified solvent, in a thickness of 1 cm), conventionally indicated by K (145). K values are usually employed to define olive oil quality (146).
TABLE 9.30
Lovibond Colors of Edible Oilsa
Fat or Oil Red Yellow
Coconut 1.0 10 Cottonseed 2.5 Palm 3.0 Palm kernel 1.5 Peanut 2.0 25 Safflower 1.5 15 Soybean 1.0 Sunflower 2.0 20
aMaximum accepted values are indicated.
Source: Refs. 71, 73, 99, 105, 106, 141. TABLE 9.29
Ultrasonic Properties of Fats and Oils
Ultrasonic Attenuation Ultrasonic Velocity (m/s) Coefficient (at 20°C) Fat or Oil 20°C 40°C 70°C 2 MHz 5 MHz Butter 1359 Castor 1494 1457 11.0 Coconut 1362 Corn 1470 1403 1308 Cottonseed 1405 Grapeseed 1309 Linseed 1414 Olive 1466 1401 1302 6.5 1.94 Palm 1459 1399 1298 Palm kernel 1368 Peanut 1466 1405 1308 3.6 1.0 Rapeseed 1468 1411 1308 3.6 1.0 Safflower 1472 1408 1310 4.0 1.18 Sesame 1403 Soybean 1470 1405 1309 4.9 Sunflower 1472 1407 1311 Source: Refs. 79, 135–139. TABLE 9.28
Ultrasonic Properties of Triacylglycerols
Ultrasonic Velocity (m/s) TAG a 20°C 40°C 70°C LaLaLa 1357 PPP 1290 PSP 1292 SSS 1301 POP 1389 1293 PPO 1390 1295 POS 1392 1297 PSO 1393 SPO 1394 SOS 1302 OOO 1463 1397 1304 LLL 1474 1407
aAbbreviations: TAG, triacylglycerol. Fatty acids in TAG: La, lauric;
M, miristic; P, palmitic; and St, stearic. Source: Refs. 108, 135, 136.