Aroma Characterization of Fresh and Stored-Nonfat Dry Milk - ACS

Chapter 8

Downloaded by MONASH UNIV on October 24, 2012 | http://pubs.acs.org Publication Date: October 10, 2002 | doi: 10.1021/bk-2003-0836.ch008

Aroma Characterization of Fresh and Stored-Nonfat Dry Milk Yonca Karagül-Yüceer1, MaryAnne Drake1,*, and Keith R. Cadwallader 2

1Department of Food Science, Southeast Dairy Foods Research Center, North Carolina State University, Box 7624, Raleigh, NC 27695 2Department of Food Science and Human Nutrition, University of Illinois, 1302 West Pennsylvania Avenue, Urbana, IL 61801

Determination of the chemical nature and sensory profiles of nonfat dry milk (NDM) is necessary to improve processing methods and storage conditions to maintain product freshness. Aroma-active compounds of NDM were identified by gas chromatography/olfactometry (GCO) and gas chromatography-mass spectrometry (GC-MS). Thermally induced volatiles Furaneol®, methional, sotolon, and maltol, free fatty acids, lactones as well as aldehydes and ketones were primary contributors to both desirable fresh and undesirable stale/stored aromas of N D M .

The primary dry milk product manufactured in the U.S. is nonfat dry milk. It is mainly produced by spray drying of milk. The main advantages of drying are reduction in transport and storage costs, and convenience of use in formulations. The drying process is a continuation of a concentration process to produce a stable, low moisture product with minimum sensory changes and specific functional properties (i). Based on pre-heat treatment prior to spray drying, three types of N D M are produced: low heat (not over 71°C for 2 min),

108

© 2003 American Chemical Society In Freshness and Shelf Life of Foods; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


109 medium heat (71-79 °C for 20 min), and high heat (88 °C for 30 min). Pre-heat treatment is an integral part of the drying process, because heat treatments affect the functional properties of the powder. These heat processes generate different degrees of protein denaturation distinguished by the level of soluble whey protein nitrogen. The product has a shelf life of 12-18 months (2). Nonfat dry milk should ideally have a clean, sweet and pleasant taste and be free of off-flavor defects. Cooked flavors may be present and vary according to heat treatment of the milk prior to evaporation and drying (3). Processing and storage of milk may change the flavor properties of the final spray dried product. Since nonfat dry milk is widely used both as an ingredient and for direct consumption, flavor quality is one of the most important factors to determine consumer acceptance or preference of dairy products. Because of consumer complaints, off-flavors are a major concern to the food industry. Undesirable flavors reduce the sensory quality and economic value of foods. Off-flavors can originate from several sources such as oxidation of lipids, enzyme decomposition, microbial growth or environmental sources. Studies on flavor volatiles in skim milk powder have been conducted. Volatile flavor and off-flavor compounds in spray-dried skim milk were determined by simultaneous distillation-extraction (SDE) (4, 5). Free fatty acids and lactones were major contributors to the flavor of skim milk powder. In addition, aldehydes, aromatic hydrocarbons and some heterocyclic compounds affected the flavor indirectly. B-Ionone, benzothiazole and tetradecanal were found to be responsible for a cowhouse-like off-flavor in skim milk powder (5). In other research, the contributors of sweet and milky odors of skim milk powder were investigated (6). Nonanoic, decanoic, and dodecanoic acids were found to be responsible for this attribute. Sensory properties were not addressed in these studies. Instrumental analysis of specific off-flavors in milk powder have also been studied. The role of Maillard reactions was investigated as an indicator of staling in nonfat dry milk (7). Constituents such as 2-furaldehyde, 2-furfuryl butyrate, alkylpyrazines and N-ethyl-2-formylpyrrole originating from nonenzymatic browning may contribute to the stale flavor. Early researchers (8) isolated carbonyl compounds responsible for the cereal-type flavor from instant and non-instant types of N D M . The compounds identified from instant N D M were formaldehyde, acetaldehyde, acetone, butanone, methylpropanal, 3methylbutanal, furfural, diacetyl, hexanal, and nonanal. Driscoll and coworkers (9) studied sensory properties of N D M during storage. Time, storage temperature and type of packaging were critical factors to provide desirable sensory qualities. This study provides information on the chemical nature of predominant aroma components of fresh and stored N D M . The aimss of the present study were to identify and compare the chemical nature of aroma active compounds of fresh and stored N D M s and to compare them with sensory evaluation results.

In Freshness and Shelf Life of Foods; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

110

Materials and Methods Chemicals


Aroma compounds (listed in Tables Π and ΙΠ were perchased from the following commercial sources including Aldrich Chemical Co. (Milwaukee, WI), Bedoukian Research Inc. (Danbury, CT) and Sigma (St. Louis, M O ) . Internal standards, 2-methyl-3-heptanone and 2-methylpentanoic acid, were purchased from Aldrich Chemical Co. and Lancaster (Windham, N H ) respectively. Odorant no. 11 was obtained from Dr. R. Buttery (USDA, A R S , W R R C , Albany, C A ) .

Milk Powders. Nonfat dry milks (n=100) were obtained from domestic producers. Powders ranged in age from approximately 3 months to 2 years old. Based on sensory analysis, six samples were chosen for chemical analysis. Three of them represented fresh, fluid-milk-type flavors and included different heat treatments. The other three samples exhibited stale/storage types of flavors. Samples were stored frozen (-20 °C) in Qorpak clear standard wide mouth bottles sealed with Teflon-lined closures ( V W R Scientific Products, St. Louis, M O ) until analysis.

Preparation of Extracts for AEDA Direct Solvent Extraction. M i l k powders (100 g) were hydrated with odor free water (500 mL) and blended with an electric hand-held mixer. The compounds 2-methyl-3-heptanone (5.44 μg/μL) and 2-methylpentanoic acid (6.18 μg/μL) in methanol were added (10 \iL) as internal standards for neutral/basic and acidic fractions, respectively. Each sample was extracted with diethyl ether (3 χ 300 mL) in 250-mL Teflon bottles with Tefzel closures ( N A L G E N E ; Rochester, N Y ) . Solid N a C l (180 g) was added to the milk to break the emulsion during extraction. Each sample was agitated on a Roto M i x (Thermolyne, Type 50800; Dubuque, IA) for 30 min at the highest speed, and then centrifuged at 3500 rpm for 30 min. After centrifugation, each sample was slowly stirred to break the emulsion. High Vacuum Distillation. The solvent extract was dried over anhydrous sodium sulfate (Na S0 ), and men concentrated to 100 m L at 35 °C using a Vigreux column (150 χ 15 mm; V W R Scientific Products). The extract was poured into 2

4


Ill a 1-L round bottom flask and frozen in liquid nitrogen. The two receiving tubes were placed in liquid nitrogen. Vacuum was applied (ca. 10 " Torr) to the system for 4 h (10). The sample flask was kept at room temperature for the first 2 h, then temperature was increased to 60 °C (in a water bath), and the process was continued for an additional 2 h. The distillate was washed with sodium bicarbonate (NaHC0 ) (0.5 M ; 2 χ 15 mL) and a saturated solution of sodium chloride in water ( 3 x 5 mL). The upper (ether) phase containing the neutral/basic volatiles was dried over anhydrous N a S 0 and concentrated to 0.5 mL under a gentle stream of nitrogen gas. The aqueous phase (bottom layer) was acidified with hydrochloric acid (18 % v/v) to p H 1.5 to 2 and the acidic volatiles were extracted with diethyl ether three times, dried over anhydrous N a S 0 , and concentrated under a nitrogen gas stream. 5

3

2


2

4

4

Gas Chromatography-Olfactometry ( G C O ) The G C O system consisted of a HP5890 series I I G C (Hewlett-Packard Co., Palo Alto, C A ) equipped with a flame ionization detector (FID), a sniffing port, and on-column injector. Each extract (2 μ!) was injected into a polar capillary column ( D B - W A X or D B - F F A P 30 m length χ 0.25 mm i.d. χ 0.25 μτη film thickness (d )) and a nonpolar column (DB-5ms 30 m length χ 0.32 mm i.d. χ 0.25 μιη d ; J & W Scientific, Foison, C A ) . Column eluate was split 1:1 between the FID and sniffing port using deactivated fused silica capillaries (1 m length χ 0.25 mm i.d.). The G C oven temperature was programmed from 35 to 200 °C at a rate of 10 °C/min with initial and final hold times of 5 and 30 min, respectively. The FID and sniffing port were maintained at a temperature of 250 °C. The sniffing port was supplied with humidified air at 30 mL/min. Two experienced panelists conducted G C O analysis. The extracts containing the neutral/basic and acidic volatiles were diluted stepwise with diethyl ether at a ratio of 1:3 (v/v). The dilution procedure was performed until no odorants were detected by G C O . The highest dilution was defined as flavor dilution (FD) factor (11). f

f

Gas Chromatography-Mass Spectrometry (GC-MS) The system consisted of an HP5890 Series Π G C / 5972 mass selective detector (MSD, Hewlett-Packard, Co.). Separations were performed on fused silica capillary columns ( D B - W A X 60 m length χ 0.25 mm i.d. χ 0.25 μηι d J & W Scientific). The carrier gas was helium at a constant flow of 0.96 mL/min. Oven temperature was programmed from 35 to 200 °C at a rate of 3 °C/min with initial and final hold times of 5 and 45 min, respectively. The M S D conditions were as follows: capillary direct interface temperature, 280 °C; ionization f>


112 energy, 70 eV; mass range, 33 to 350 a.m.u; E M voltage (Atune+200 V ) ; scan rate, 2.2 scans/s. Each extract ( 2 μ ί ) was injected in the on-column mode.


Identification of Odorants Positive identifications were made by comparing retention indices (RI), mass spectra, and odor properties of unknowns with those of authentic standard compounds analyzed under identical conditions. Tentative identifications were based on comparing mass spectra of unknown compounds with those in the Wiley 138 mass spectral database (John Wiley & Sons, Inc., 1990) and a database generated from authentic standards in the Department of Food Science and Technology-Flavor Laboratory (Mississippi State University, Mississippi State, M S ) or on matching the RI values and odor properties of unknowns against those of authentic standards. Retention indices were calculated by using an n-alkane series (12). To identify methional, selected ion monitoring mode (SIM) of Mass Spectrometry was used. The selected ions for methional were m/z 104 and 76.

Quantification of Selected Compounds Reconstituted milk, after deodorization by high vacuum distillation, or water was used as a matrix to prepare standard solutions. Calibration was accomplished by addition of 0 (blank), 2 μL, 5 \iL or 20 μL from a stock solution. Each mixture was also spiked with 10 μL of internal standard solution. The same procedure for preparation of extracts from samples was followed to prepare the standard solutions. The standard stock solution contained 54.6 μg of no. 1, 15.48 mg of no.2, 11.6 mg of no. 3, 48.4 μg of no. 4, 1.22 mg of no. 5, 12.68 mg of no. 12,12.94 mg of no. 14, 58.4 μg of no. 18, 51 μg of no. 21, 2.34 mg of no. 26, 22.2 μg of no.31, 56.6 μg of no. 34, 10.84 mg of no. 38, 34.6 μg of no. 40, 110.6 μg of no. 42, 3.9 mg of no. 44, 119.2 μg of no. 45, 46.4 μg of no. 51 per ml methanol. For quantification of these compounds, a D B - F F A P column was used.

Sensory Evaluation Sample Preparation and Descriptive Sensory Analysis For flavor evaluation, 10 g of N D M was suspended in 100 m L of odor-free water at 40 °C and mixed by electric mixer (Biospec Products, Inc.; Bartlesville, O K ) at the lowest speed for 2 min. Samples were evaluated at 20 ± 2 °C.


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Liquid samples for flavor evaluation were served in Styrofoam cups equipped with plastic lids. Descriptive analysis of flavor was conducted on skim milk powders (13). Panelists (n=13) were asked to identify and define flavor terms for the milk powders. Terms identified and selected by the panelists are listed in Table ΙΠ. Panelists marked responses on 10-point numerical intensity scales anchored on the left with "none" and on the right with "extreme". Powders were evaluated in duplicate in a randomized block design (14).

Table I. Preparation of Reference Materials for Descriptive Sensory Evaluation of Nonfat Dry Milk Descriptor Reference Preparation Cooked/sulfurous

- Heated milk

Caramelized/ Burnt sugar

- Autoclaved milk - Caramel syrup

Sweet aromatic/ Cake mix Cereal/grass-like

-Pillsbury-White cake mix

Brothy/potato-like

- Kroger-Canned white potato slices - Knox-unflavored gelatin

Animal/gelatin-like/ Wet dog

- breakfast cereals (corn flakes, oat and wheaties)

Papery/cardboard

• cardboard paper

Sweet taste Astringent

- sucrose - tea

- heat pasteurized skim m i l k t o 8 5 ° C f o r 4 5 min - autoclave whole milk at 121 °C for 30 min. - dilute small amount of caramel syrup in skim milk

- soak one cup cereal into three cups milk for 30 min and filter to remove cereals - remove the sliced potatoes from the broth - dissolve one bag of gelatin (28 g) in two cups of distilled water soak pieces of cardboard paper in skim milk overnight - 5% sucrose solution - soak 6 tea bags in water for 10 min



11 12 13 14 15 16 17

10

2 3 4 5 6 7 8 9

1

No.

A

A

A

A

A

B

8

A

A

B

B

A

A

A

A

2,5-Dimethyl-4-hydroxy 3(2H)furanone (Furaneol®) Butanoic acid* Methional o-Aminoacetophenone 6-Decalactone Unknown (E)-4,5-Epoxy-(E)-2-decenal Pentanoic acid 4,5-Dimethyl 3-hydroxy-2(5H)furanone (sotolon) Vanillin (3-Methoxy-4-hydroxybenzaldehyde) 2-Acetyl-1 -pyrroline Hexanoic acid Phenylacetic acid Octanoic acid Nonanal l-Octen-3-one 2-Acetyl-2-thiazoline

Odorant

N/B A A A N/B N/B N/B

A

A N/B, A N/B N/B N/B N/B A A

A

Fraction "

3

1331 1834 2568 2048 1384 1294 1762

2540

1606 1443 2218 2183 2202 2000 1756 2204

DBWAX 2027

RI

919 1008 1265 1289 1099 977 1107

1401

802 904 1308 1502 1560 1392 902 1118

1057

DBS

b

Popcorn Sour, vinegar, cheesy Rosy Waxy, soapy, sweaty Fatty, stale, soapy Mushroom Popcorn

Vanilla, pudding

Caramelized, burnt sugar Rancid, cheesy Boiled potato Grape, foxy Burnt, sweet, fatty Cilantro Metallic, green Cheesy, sweaty Curry, butterscotch

\jaOV

4 4 3 5 4 3 4

4

5 5 5 5 5 3 4 3

L 5

3

5 4 4 3 3 3 5

5

5 5 5 6 6 5 3 4

M 5

4 4 5 4 5 5 3

5

6 6 6 5 5 6 5 5

H 6

Log FD Factor" By Heat Treatment

Table II. Aroma-Active Compounds (Log FD Factor > 2) of Low (L), Medium (M) and High (H) Heat-Treated Fresh Nonfat Dry Milks Detected During Aroma Extract Dilution Analysis



A

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

A

c

Β

A

A

A

A

B

B

Λ

A

A

A

A

B

a

3

4 4 4 Cilantro, sweet 2398 1656 N/B j*-Dodecalactone 1532 1162 Hay, cucumber 4 3 4 N/B (£)-2-Nonenal Waxy, green 4 4 3 1367 1722 N/B (£)-2-Undecenal 1807 1320 Fried fatty 3 4 3 N/B (£,E)-2,4-Decadienal Fresh fishy 3 3 4 2130 1582 N/B Unknown Fresh air, milky 4 3 3 981 1361 N/B Unknown Sour, rose-like 3 4 4 1356 A >2600 3-Phenylpropionic acid N/B 2108 1345 Mint, green 2 4 4 Unknown Cotton candy 3 3 4 1978 1088 A Maltol Bug, Swiss cheese 3 2 3 1556 959 A Isobutyric a c i d Ν 1822 1370 Apple sauce 1 2 1 β-Damascenone Metallic, waxy 3 3 1 1199 A Unknown 1146 Fatty, waxy 3 1 2 N/B Unknown 1218 Stale, fatty, soapy 2

Aroma Characterization of Fresh and Stored-Nonfat Dry Milk - ACS

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