Sensory-Directed Analytical Concentration Techniques for Aroma

Chapter 15 Sensory-Directed Analytical Concentration Techniques for Aroma—Flavor Characterization and Quantitation

Downloaded by FUDAN UNIV on March 12, 2017 | http://pubs.acs.org Publication Date: September 13, 1991 | doi: 10.1021/bk-1991-0473.ch015

Kent Hodges The Dow Chemical Company, Analytical Sciences, 1897 Building, Midland, MI 48667

"The amount of an odor needed to cause a "nuisance" differs with each person and each separate time the judgement is made. The effects of odors are more psychological than physical." "Because of the complexity of odors, the odor transmission systems, and odor detection and recognition capabilities, no equipment has been developed to satisfactorily detect and measure odors recognized by the nose." Quotes such as these occur in a number of olfactory instructional texts. Their content could disincline any scientist who has been involved in trace analytical measurements for many years, and now has the mandate to characterize and quantify aromas and flavors. Most odors and tastes are quantifiable using modem day analytical instrumentation, provided at each step of the analytical separation, concentration and characterization, sensory analysis is employed. The food packaging industry today is actively involved in measuring odor and taste as itrelatesto maintaining the integrity of food packaged in various types of containers. Figure 1 identifies some of the interactions that typically could occur between a food, beverage, or pharmaceutical and its container. Trace concentrations of residual solvents, monomers, plasticizers, inhibitors or mold release agents, that are an integral part of the containers manufacturing, could conceivably migrate to the food, thereby imparting an off taste or flavor. In other cases the flavor ingredients of the food can migrate into the container causing the loss of some of its natural aroma profile. Environmental contamination, such as oxygen, water or other volatile odorants could conceivably migrate through the plastic, i f appropriate barrier resins are not selected, thereby, adulterating the food and causing it to take on a bad odor or taste. These are the concerns of the food packaging analytical scientist. Challenges in food packaging today are not necessarily set by the food processor or baker. Todays "olfactory literate" consumer demands convenient, easy to prepare, quality food items with good taste. In addition the containers must be readily disposable, recyclable, convenient to use and aesthetically pleasing. These demands on the food processor or baker challenge a fabricator or 0097-6156/91/0473-0174S06.00/0 © 1991 American Chemical Society

Risch and Hotchkiss; Food and Packaging Interactions II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

15. HODGES

Aroma—Flavor Characterization & Quantitation


converter. The demand concurrently is accepted by the resin supplier who must offer innovative, high-quality plastics or other materials such that the fabricator can meet these challenges. Todays successful packaging design requires close interaction between the food processor, fabricator, and resin supplier for the following reasons. The food processor understands intimately the chemistries involved in the manufacture and storage of his food. The fabricator is part artist, part engineer, capable of taking resins and molding them into containers which protect and are very aesthetically pleasing. The resin supplier is a plastics chemist, fully understanding the properties of his starting material. Today, these different, complex disciplines must work together to insure the finished product meets the demands of the consumer. But Why are These Demands so Great Today? When I first became involved in the measurement of food packaging interactions, 10 years ago, I surveyed the literature to determine what had been written over the past 20 years. In the '60's homo-polymer and co-polymer films and containers were used essentially for short-term, less than one-week, storage under what we might consider low stress conditions, (i.e., refrigeration). During the '70's co-polymers and laminated films became more sophisticated, because the packaging applications demanded longer shelf life (i.e., several weeks). Microwaving introduced a higher stress to the packaging. During the '80's a technology explosion has occurred with multi-layered, barrier containers; some containing 25 different laminated layers to impart characteristics and aesthenics to the package. Shelf life has also been extended, often many months and even years in length. And with the advent of dual-ovenable containers and susceptors, another degree of stress has been imposed. Indeed, very delicate, sensitive food matrices such as cottage cheese, water, orange juice, and even wine are successfully packaged in plastic containers today. Who would have guessed in the '60's that we would be packaging the F D A leachables test media, 30% ethyl alcohol (vodka) in a plastic container? So the demands to produce high quality resins which perform well in the food packaging industry will continue to be extremely challenging. To meet this challenge at Dow, we have established four emphasis groups to develop a database for food packaging interactions. One group covers the design and engineering as well as materials of construction of such films and containers. Another group concerns itself with the measurement of barrier properties. These are essentially analytical measurements which determine the solubility and diffusivity of migrant molecules through and into plastic films. Product and packaging interactions are covered by another group in which the relationship between aroma and flavor performance and trace analytical measurements are correlated. The fourth group is a food sciences laboratory in which food integrity can be measured, as a function of time for a given packaging application. Common measurements such as moisture, texture and the fate of nutrients that are in a given food product when it is placed in a plastic container, can be determined. A fifth group has recently been added, a sensory testing lab with trained panelists for determining product performance. Analytical aroma/flavor characterization and quantitation require a knowledge of the three vectors that contributed to "Total Aroma Perception": Concentration, Potency, and Hedonic (Pleasantness or Unpleasantness).


175

176

FOOD AND PACKAGING INTERACTIONS

Let's consider trying to measure them or put some quantitative number on "Total Aroma Perception."


The log of odorant concentration is directly proportional to sensory response. As long as we are above the threshold recognition concentration this relationship appears to fit for most odorants. This also points out one of the fundamental challenges we have in the packaging industry. A 100-fold reduction in odorant concentration would only have a 2 unit reduction on the sensory response scale! Therefore, if some package component causes a food product to taste bad, we must reduce its concentration by one hundred fold before its affect would be below threshold recognition concentration. Figure 2 shows the large range of aroma potency for some organic compounds the human nose can detect The odor threshold for compounds such as octane and nonane are approximately 1000 ppm. These could be classed as odorants with "weak potency." A compound s u c h ^ vanilla which is detectable by the human nose at approximately a ppt or 10 times lower would be classed as possessing a "strong potency." litis variance in odor potency creates one of the analytical separation scientist's dilemmas characterizing odorants in packaging. Solvents used in the manufacture of plastics are selected so as not to contribute taste or odor to the finished package. They have a "weak potency." Sometimes trace levels of these solvents are retained, yet the analytical scheme must separate them from the compounds which are truly impacting the odor or taste problem of the package. y

The hedonic tone or pleasantness/unpleasantness impression was for many years thought to be dimensionless. Andrew Drevnicks of the Institute of Olfactory Sciences, has made an attempt to quantify this hedonic tone (Figure 3). Over 250 different types of odors were ranked in terms of pleasantness or unpleasantness by a panel of 350 people. The data was normalized to a scale covering from 44, which is very pleasant, to -4, which is very unpleasant With some falling in the neutral hedonic rating. We might argue whether the methodology can distinguish between something which is +1 or -1 as agreeable or disagreeable, but I don't think we have any trouble recognizing the difference between the aroma of "fresh baked bread," which has one of the highest universal hedonics, and something which smells like "burnt rubber" or "cadaverous." I think we would all agree packaging films that have a "cadaverous" odor would not sell on the open market. Concentration techniques for aromas involve a variety of approaches. One technique, headspace gas chromatography, is often described as a viable technique for odor characterization, however, contemporary analytical equipment facilitates measurements only to low ppm. Recalling the sensitivities of human olfaction we described earlier, we need to have detection capabilities on the order of at least a ppb and often a ppt, to truthfully characterize the presence of an odor. Cryofocus, headspace, capillary gas chromatography, or purge and trap techniques, can be applied to measure low ppb by focussing as much as 50-100 milliliters "on-column." Odorant selective concentration techniques such as vacuum steam distillation must be employed to get the 5,000fold concentration needed to detect these odors. It is very important in all concentration efforts that sensory analysis be performed on the concentrate,



HODGES

RESIDUALS (SOLVENTS, MONOMERS, PLASTICIZERS, INHIBITORS, MOLD, RELEASE AGENTS, ETC.)


FLAVOR SCALPING

ENVIRONMENTAL CONTAMINATION OXYGEN, WATER, GASOLINE, NEAREST NEIGHBOR

(BARRIER PROPERTIES)

Figure 1. Typical food packaging affect odor/taste performance.

Odor Threshold

interactions which can

Compound

3

octane, nonane

2

10

ethanol, acetone

10'

toluene, ethylacetate, methylethylketone

10°

vinyl acetate, acetic acid

10'

styrene, mesityloxide, methylmethacrylate

10

2

chlorophenol, eugenol. butanoic acid

3

butylacrylate, 2-nonenal, ethylmercaptan

10 10

4

l-octen-3-one, amylmercaptan, ethylacrylate

5

10

l-nonen-3-one

10*

vanillin

10

Figure 2. Example of the wide range of sensory threshold detection concentrations expressed i n parts per m i l l i o n (v/v) f o r some organic compounds.


178



PLEASANT

+4t-

Fresh Baked Bread 3.53 Floral 2.79

Vanilla 2.57

LU —I

< Ο (/)

Ο Ζ

NEUTRAL 0

Ο ο

LU Χ

Woody 0.94

Burnt Smokey -1.53

Oily, Fatty 1

Cardboard -.54

4

Sharp, Pungent, Acid -2.34 Burnt Rubber -3.01

Cadaverous -3.75

UNPLEASANT -4

Figure 3. Hedonic Tone (Pleasantness or Unpleasantness of an Odor). (Reproduced with permission from reference 1. Copyright 1984 Air Pollution Control Association.)


15. HODGES

Aroma-Flavor Characterization & Quantitation


thereby proving to the separation scientist that he has indeed isolated the odiforous fraction, without adulteration, before attempting any characterization. Cryogenic headspace samplers can be used to isolate the odorants from as much as 2 liters of headspace above a given packaging material. The odorants can then be transferred to the front of a capillary gas chromatographic column chilled to liquid nitrogen temperatures. The components are then separated and measured by any one of four or more different types of detectors. Olfaction plays a major role in deteraiining which of the component peaks are sensorially active. Sulfur or nitrogen specific detectors provide a measure of selectivity for certain types of aromas. Matrix isolation, fourier transform infrared spectrophotometers, as well as conventional quadrupole mass spectrometer, are used to provide structural information about the odorant molecule eluting from the column. This is essentially a screening technique, because often we do not obtain the sensitivity to low ppt, which is the concentration range in which we must make measurements. A modified Nielson-Kryger steam distillation apparatus is shown in Figure 4. A condenser is fixed atop a 2-liter resin pot and cooled to 3 * by means of a recirculating bath. Pentane or hexane can be placed in the condenser to extract the volatiles as they are steam distilled from the sample. After the steam distillation is completed, the extract is ready immediately for capillary gas chromatographic analysis. While steam distillation is most commonly employed to separate and concentrate volatile aromas from food matrixes, packaging films or containers can similarly be extracted. Capillary chromatograms of the extracts from the steam distillation of three packaging films is shown in Figure 5. A l l of the packaging films are identical except after exhaustive sensory testing, one has an odor intensity which is very weak, designated as a "one". Another has a strong waxy smell, which is ranked as a "three," and another is rated as intermediate, or a "two." In looking at the hydrocarbon response, the chromatograms are quite complex, and any hope of identifying the odorant molecule in this extract would be extremely difficult Figure 6 shows a simple silica gel class separation that we use to fractionate the extract and essentially separate the hydrocarbon fraction from the more polar compounds present in the sample which cause odor. After depositing the extract on a disposable pipette filled with silica gel, the first fraction is eluted with 2-mL of hexane. This fraction generally consists of saturated hydrocarbons. Next the bed is eluted with 2-mL of 10% methylene chloride in hexane to elute the unsaturates and aromatics. The next fraction is eluted with 2mL of methylene chloride. This fraction generally contains aldehydes and ketones. The fourth fraction is eluted with 2-mL of methyl-t-butyl ether and generally contains acids, unsaturated ketones, and unsaturated aldehydes. The fifth cut is eluted with 2-mL of methanol which are generally polyfunction^ polar type compounds. After reanalyzing the fractions by capillary gas chromatography, Figure 7, some very interesting observations can be made. The top chromatogram shows the concentrate before class separation, which from a sensory analysis was called "strong, waxy." The first fraction, which ironically looks virtually identical to the concentrate, has no odor! The second fraction has no odor, but the third cut has a very strong waxy odor. The fourth cut has a very slight floral aroma, while the fifth cut, which is the methanol eluant, also has no odor. With this


179



Figure 4. Schematic of a t y p i c a l simultaneous steam distillation/hexane extraction apparatus f o r aroma concentration.


HODGES


ODOR INTENSITY= 1 "BURNT", "WAXY"

luM i u ^

h [—• •—I—» • • • 1 • 1 li-l 1 _1. i 1 1 1 J L *. 1 1.

i

it * 4 1 t t i. A._A. 1..J 1

3 "STRONG WAXY"

2

> •—.—1—i—.—.—.—1—. . J _ 1 ..

. . . . 1

. 1. . .

1 . . . . 1 . . .

Sensory-Directed Analytical Concentration Techniques for Aroma

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