Chapter 28
Downloaded by UNIV MASSACHUSETTS AMHERST on September 28, 2012 | http://pubs.acs.org Publication Date: September 10, 1998 | doi: 10.1021/bk-1998-0705.ch028
Analysis of Aroma-Active Components of LightActivated Milk Keith R. Cadwallader and Cameron L . Howard Department of Food Science and Technology, Mississippi Agricultural and Forestry Experiment Station, Mississippi State University, Box 9805, Mississippi State, MS 39762 Milks of varying fat levels (skim, 2% and whole) were exposed to fluorescent light at 200 FC. Control (not exposed) and light-activated flavored (LAF) milks were subsequently evaluated by sensory and instrumental methods. Results of sensory evaluation demonstrated that flavor of LAF milk is impacted by the fat level of the milk. The combined results of gas chromatography-olfactometry (GC-O) of volatiles isolated by static (SHS) and dynamic (DHS) headspace sampling and by vacuum distillation-solvent extraction (VDSE) from control and LAF milks of varying fat contents revealed that odorants of low, intermediate, and high volatility are involved in LAF. Furthermore, these compounds are derivedfromboth lipid and non -lipid precursors. Specific components involved the aroma of LAF milk can be quantified by DHS.
Freshly pasteurized and homogenized milk of good quality has a subtle but distinctive clean and fresh flavor. Badings (7) stated that there are three basic elements responsible for the flavor of milk: (1) pleasant mouthfeel due to presence of macromolecules such as colloidal proteins and fat globules, (2) sweet and salty taste due to lactose and milk salts, respectively, and (3) a weak and delicate aroma due to numerous volatile compounds present at near or below their odor threshold levels. Patton and coworkers (2) were first to study the aroma constituents of fresh milk and found dimethylsulfide to be an important constituent. Since then, carbonyl compounds, alcohols,freefatty acids, and various sulfur compounds also have been found to play important roles infreshmilk flavor. These compounds are derived mostly through normal metabolism of the cow or from the feed by either direct transfer or release during digestion (7,5). During and after processing, the mild flavor of milk can be negatively impacted by many processes and chemical reactions (5). These include both oxidative and hydrolytic rancidity, thermal degradation, packaging interactions, microbial contamination, and exposure to light. Probably the
©1998 American Chemical Society In Flavor Analysis; Mussinan, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
343
Downloaded by UNIV MASSACHUSETTS AMHERST on September 28, 2012 | http://pubs.acs.org Publication Date: September 10, 1998 | doi: 10.1021/bk-1998-0705.ch028
344 most common and severe flavor problem encountered in milk is caused during its exposure to light (4). This flavor defect has been studied for over 40 years and is commonly referred to as "sunlight", "oxidized", "light-induced", and "lightactivated" (5). Exposure of milk to light results in the development of off-flavor and causes the destruction of several key nutrients such as riboflavin, ascorbic acid and the essential amino acid methionine (5). Factors affecting the extent of off-flavor formation or nutrient loss include wavelength and intensity of light, duration of exposure, type of packaging material (e.g., light transmission properties), and product temperature (5). Extensive work has been carried out on the effects of light on theflavorand nutritional quality of milk and numerous reviews have been published (5-8). Patton (9) was first to propose the involvement of two mechanisms in the development of light-activated flavor (LAF) in milk, but it wasn't until some time later that these mechanisms were confirmed (10-12). The key player in both mechanisms is the photocatalyst riboflavin. Photoreduction of riboflavin in milk results in the Strecker degradation of methionine to form the potent odorant 3-(methylthio)propanal (methional), and also leads to the photogeneration of superoxide anion (1,3,13). Superoxide anion can subsequently undergo dismutation to form singlet oxygen (and other "activated" oxygen species) which can initiate oxidation of polyunsaturated fatty acids, leading to formation of numerous volatile carbonyl compounds (75). It is the combination of the products of the two types of reactions that give typical LAF in milk (7, 3,5-8). Early investigators employed distillation-based methods (i.e. vacuum distillation followed by either solvent extraction or static headspace sampling of the distillate) for the analysis of volatile constituents of LAF milk (13-15). Distillation methods have the advantage of allowing for isolation offlavorcompounds with a wide range of volatilities and recently have been applied in milkflavorstudies (16-17). A major disadvantage of these techniques, however, is that they are time consuming and generally required a large amount of product and, therefore, are not well suited for the routine analysis of LAF milk. Several researchers recognized this problem and began using dynamic headspace sampling (DHS)(76V27). This method is both rapid and sensitive, requires minimal sample preparation, and can accommodate small sample sizes. Furthermore, with DHS no solvent is required, insuring that only sample volatiles are analyzed. However, the technique is not without limitations, e.g., it will only allow for the determination of sample components of high and intermediate volatility. Therefore, the suitability of DHS for the routine analysis of LAF milk would depend on the volatility of the key odor-active components. However, considerable confusion currently exists in regards to the identities of the predominant odorants in LAF milk. The objectives of our study were: (1) to identify odor-active components of LAF milk by employing several complimentary isolation techniques and gas chromatography-olfactometry and (2) to evaluate the suitability of DHS for the routine analysis of LAF milk by focusing on the key odorants involved in the development of this off-flavor.
In Flavor Analysis; Mussinan, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
345
Downloaded by UNIV MASSACHUSETTS AMHERST on September 28, 2012 | http://pubs.acs.org Publication Date: September 10, 1998 | doi: 10.1021/bk-1998-0705.ch028
Experimental Milk. Fresh pasteurized and homogenized milk of varying fat levels (i.e., skim, 2%, and whole) was obtained from a processor in Kosiusko, MS. Milk, in standard 1 gallon plastic containers, was transported on ice to the Department of Food Science and Technology and stored in a walk-in cooler (4°C) until needed. During transport and storage, care was taken to prevent any exposure of the milk to light. Prior to analysis, milk quality (i.e. lack of LAF) was assured by sensory testing by a 3-6 member expert panel employing standard ADSA dairy score card techniques. For development of LAF, milk in clear flint glass bottles (French square, Qorpak no. 7905) equipped with PTFE lined caps was exposed to fluorescent light at 200 FC. The light source consisted of four 48 in. 34 W fluorescent bulbs (Supersaver Coolwhite, Sylvania, Louisville, KY). Two sets of light banks (Power Products Co., Philadelphia, PA), each comprised of two bulbs horizontally aligned and spaced 3 in. apart on wooden supports, were placed facing each other to create a "light box". A shelf was placed in the center of the light box and the light banks were oriented so that light meter readings taken at the center of the shelf (facing each light bank) registered approximately 200 FC. Sample bottles were vertically placed in random order along the shelf. Light exposure was conducted in a walk-in cooler at 4°C. Chemicals. Aroma compounds listed in Tables I-IV were obtained from the following commercial sources: nos. 1-7,9,14,17,19, 21, 25 (Aldrich Chemical Co., St. Louis, MO); no. 11 (Bedoukian Research Inc., Danbury, CT); and nos. 8 and 15 (Lancaster Synthesis, Inc., Windham, NH). Standard no. 13 was obtainedfromDr. R. Buttery (USDA, ARS, WRRC, Albany, CA). Compound no. 16 was synthesized according to Ullrich and Grosch (22) and compound no. 18 was preparedfrom1nonen-3-ol (Lancaster Synthesis, Inc.) by oxidation with pyridinium chlorochromate (Aldrich Chemical Co.)(25). 2-Methyl-3-heptanone (internal standard) was purchasedfromAldrich Chemical Co. Vacuum distillation-solvent extraction (VDSE). The apparatus used for distillation is shown in Figure 1. Prior to use, clean glassware was baked at 160°C for at least 2 h. A 25 mL aliquot of milk was placed in a 250-mL round bottom flask (a) and the system connected as shown. The two receiving tubes (bl and b2) were placed in liquid nitrogen and allowed to cool for 5 min. The sample flask was kept at room temperature and vacuum (c) was applied (
