The Influence of Roasting-Derived Polymeric Substances on the

Chapter 17

The Influence of Roasting-Derived Polymeric Substances on the Bitter Taste of Coffee Brew 1,2

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Downloaded by UNIV OF ARIZONA on July 26, 2012 | http://pubs.acs.org Publication Date: December 1, 2003 | doi: 10.1021/bk-2004-0871.ch017

G. P. Rizzi , L. J. Boekley , and A. Ekanayake 1

The Procter & Gamble Company, Miami Valley Laboratories, 11810 East Miami River Road, Cincinnati, OH 45252 Current address: 542 Blossomhill Lane, Cincinnati, OH 45224-1406 2

The isolation, chemical characterization and taste properties of bitter-tasting fractions of decaffeinated coffee brew is described. The flavor of brewed coffee consists of an alluring aroma coupled with a pleasant bitter/astringent taste making the foodstuff highly acceptable to consumers worldwide. In a given population the appeal of coffee flavor depends on an optimum level of bitterness. The bitter taste of coffee brew has been attributed to natural products in raw beans and novel compounds formed during roasting. Chemical and spectroscopic evidence suggest that chlorogenic acids and their roasting products may be a cause of the observed bitter taste. The bitterness of brewed coffee was reduced by admixing solvent-defatted spent coffee grounds with ordinary grounds prior to brewing.

© 2004 American Chemical Society In Nutraceutical Beverages; Shahidi, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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230 Because the flavor of coffee beverages is so widely accepted worldwide the chemical factors underlying the flavor are of great interest and have been thoroughly studied. Many aspects of coffee flavor are important for its acceptance including aroma, taste, texture and appearance, i.e., color. Coffee research has focussed mainly on aroma and modern analytical techniques have led to the identification of over 800 volatile compounds. In spite of such apparent chemical complexity careful organoleptic studies have recently shown that coffee aroma can be artificially reproduced by a mixture of only twentyseven key odorants (7). Less attention has been devoted to the non-volatile components of coffee flavor and the chemistry of coffee taste is only partially understood. Coffee brew contains about one percent dissolved solids that give rise to its sour, metallic, bitter and astringent taste impressions. The sour/metallic (salty) aspect of taste has been reasonably explained by the presence of numerous organic and mineral acids and their metallic salts (2). Coffee brew is in fact a complex buffer system whose sour/metallic taste ratio can be dramatically affected by small changes in pH (2). Superimposed on the sour/metallic taste are the effects of bitterness and astringency. The causes of bitterness perception in coffee have already been addressed by several researchers (5) and they were not pursued by us per se. Rather, it was our intent to devise a practical means for selectively controlling the bitterness component of coffee flavor during brewing. Toward this end we isolated a bitter fraction from coffee and used it as a model in adsorptive studies aimed at reducing bitterness. Adsorptive technology had previously been reported to benefit coffee flavor, but several shortcomings were apparent in efforts published prior to our study (4).

Experimental Isolation of Bitter-tasting Fractions From Coffee A cylindrical plastic vessel equipped with a drain plug and a coarse filter was charged with one pound of commercial (drip grind) decaffeinated roasted and ground (RG) coffee and 2.0 L of distilled water. After 16 h at 22°C the mixture was drained by gravity and the retained grounds were washed with fresh water. The damp grounds were re-brewed in ca. 100 g portions in a standardsized (12 cup) Proctor Silex coffee maker equipped with conical paper filters. Each 100 g portion was extracted with one pot of water and the volume of the combined extractions was reduced to ca. 500 mL by vacuum rotary evaporation. Finally the product was frozen andfreeze-driedto obtain ca. 17 g of a solid, ca. 4% of the original RG weight. The product was dialyzed by conventional static dialysis using cylindrical (32 mm d) cellulose tubing (SpectraPor No. 1, M W

In Nutraceutical Beverages; Shahidi, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

231 cutoff 6-8 kDa). Typically 1.4 g of re-brew solids suspended in 50 mL of water was dialyzed against water at 22°C for 3 h vs. 2 L and 16 h vs. 2L to obtain after vacuum concentration and freeze-drying 0.69 g of dialyzed re-brew solids (DRS) of MW < 8 kDa and 0.66 g of undialyzed re-brew solids (URS) of MW > 8 kDa. The bitter-taste properties of the dialysis fractions were assessed by tasting serial dilutions of the materials in water (Table I).


Table I. Taste Properties of Coffee Extracts Material % in Water Taste DRS (MW 8 kDa) 5-CQA (K-salt)

C 49.05 51.49 48.9

Η 5.00 6.09 4.33

Κ Να DRS (MW < 8 kDa) 11.7 0.191 DRS (MW > 8 kDa) 0.89 0.069 5-CQA (K-salt) 10.0 Percent oxygen determined by difference

Ν 1.60 3.39 0

Ο 31.63 36.69 36.7

Ash 12.72 2.90 10.0

Ca 0.044 0.31

Mg 0.033 0.224

Total 11.97 1.49 10.0

Analysis of DRS and URS Fractions Combustion analysis showed that C, Η, Κ, Ν and probably Ο were major elements in both dialysis products (Table II). UV spectra were obtained in dilute aqueous solution and FT-IR spectra were obtained on solid samples using


232 standard attentuated total reflectance (ATR) methodology. The more bittertasting DRS fraction was further analyzed by ion-exchange (IE) and high performance liquid chromatography (HPLC) (Table ΙΠ). Table III. Ion-exchange and HPLC of Coffee Extract DRS Initial DRS Fraction —> Relative Amount —> bitter Taste @ 0.05% ->


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Pyridine 14% si. Bitter

Basic 0% none

Neutral acid 75% bitter

HPLC Data % % 3-CQA 4.00 3.08 4-CQA 3.56 5.16 5-CQA 8.47 6.26 3-FQA 0.52 2.55 5-FQA 1.18 1.05 3,4-diCQA 0.14 2.34 3,5-diCQA NF 1.75 4,5-diCQA NF 2.65 PPA-1 0.28 0.47 PPA-2 NF 0.09 PPA-4 0.47 NF PPA-5 0.34 4.18 PPA-6 3.38 0.59 Total identified 17.9 34.6 CQA = caffeoylquinic acid, FQA = feruoylquinic acid, PPA = polyphenols acid of unknown structure. Tasting done after pH was adjusted to 5.4 with KOH. NF = none found —

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For ion-exchange separation an aqueous solution containing DRS was allowed to flow through a column of sulfonated polystyrene resin (100 mesh, H+ form) and vacuum evaporated to obtain a neutral + acidic fraction. Further elution with 0.1 Ν ammonium hydroxide and pyridine yieldedfractionscontaining basic and strongly adsorbed organic materials, respectively. Some fractions were analyzed by HPLC using previously published procedures (5).

Adsorbant Screening Method Aqueous solutions containing 1.0 mg/mL of DRS (25 mL) were treated with 1, 10 or 100 mg of insoluble adsorbant candidates and stirred magnetically at


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22°C. At time intervals samples were withdrawn and analyzed by UV (320 nm) to assess residual DRS content. For quantitation a calibration curve was prepared with DRS and used with LOTUS 123 software to produce timedependant adsorption data in graphical form. Representative data points were derived from the graphs to make practical comparisons between various adsorbants (Table IV).


Table IV. Adsorption of DRS by Solid Adsorbants

Solid Adsorbant Activated charcoal Alumina Hydroxyapatite β-Cyclodextrin polymer Chitosan Alginic acid Polyvinylpyrrolidone (cross-linked) Nylon-6 Lactalbumin Corn zein Keratin Defatted spent coffee grounds

% DRS Removed From 0.1% DRS Solution in2h@ 22° C 99 66 15 50 28 23 39 29 32 20 19 14

Initial adsorbant to substrate weight ratio =10:1

Preparation of Defatted Spent Coffee Grounds (DFG) A detailed description of DFG preparation and use is published elsewhere (4). In brief, damp extracted coffee grounds were stirred at 50-60°C in a mixture of ethanol, ethyl acetate and water, filtered and desolventized under vacuum to obtain afinebrown powder.

Results and Discussion Isolation and Characterization of a Bitter-tasting Fraction From Coffee Brew A bitter-tastingfractionwas isolated from decaffeinated RG coffee by a two step extraction procedure. In the first step the coffee was extracted with water at room temperature (ca. 22°C) for 16 h to remove mainly non-bitter solubles.



234 Typically, 15% of the coffee is solubilized under these conditions. The damp grounds from room temperature brewing were reextracted with hot water (ca. 85°C) to afford a bitter-tasting fraction designated as rebrew solids (RS). After freeze-drying the RS was obtained as a fluffy grey matter amounting to ca. 5% of original RG weight. A 5% aqueous solution of RS was colored black and had a pronounced bitter-dirty-ashy taste. Equilibrium dialysis of RS separated a bittertasting fraction (< 8 kDa) designated dialyzable RS (DRS) from a more burntashy tasting part (> 8 kDa) termed undialyzable RS (URS). Taste evaluations (Table I) indicated a bitter taste threshhold for DRS at ca. 0.01% in water or approximately one-tenth the bitterness of quinine hydrochloride. The more bitter-tasting DRS material was subjected to further study including proximate chemical analyses. Combustion analysis of DRS (Table II) indicated a carbohydrate-like material similar in composition to the chlorogenic acids well known to occur in coffee (2). The high ash content (mainly potassium) is explained if acidic materials, i.e., phenolic acids are partially present as metallic salts. The low level of nitrogen (1.6%) and a weak ninhydrin test indicated the possibility of a small amount of soluble proteins or Maillard reaction products. Aqueous solutions of DRS absorbed strongly in the ultraviolet producing a spectrum closely resembling the spectrum of 5-caffeoylquinic acid (5-CQA) with a UV max at 320 nm. However, the absorbance of DRS at its U V max was only 50% that of pure 5-CQA. Consistent with phenol chemistry the peak at 320 nm disappeared in 0.1 Ν sodium hydroxide solution giving rise to a weaker phenolate ion absorption centered at 370 nm. The IR spectrum of DRS qualitatively resembled the IR spectrum of 5-CQA, however, a reversal in absorption intensities at 1600 and 1700 cm" was observed that was probably due to partial salt formation in the coffee isolate. Also, DRS has unique bands at 1780 and 760 cm" which are not found in the spectrum of 5-CQA. The DRS wasfractionatedinto neutral-acidic and basic components via an ion-exchange procedure in order to locate the chemical origin of its bitter taste (Table III). Neutral-acidic material comprised 75% of the DRS and retained most of the bitter taste. Continued elution of the resin with dilute ammonia solution and pyridine gave 14% of basic and/or highly adsorbed materials which had less bitter taste and were not investigated further. The balance of the DRS weight (ca. 11%) was presumed due to metallic ions which were not eluted from the resin. HPLC analysis of DRS identified fourteen phenolic acids comprising 35% of the sample weight. Of the fourteen compounds eight were identified as mono or dicinnamoylquinic acids based on comparison with reference standards and six were tentatively identified as polyphenols acids (PPAs) and assigned phenolic acid status based solely on UV spectral data. Similar analysis of the bitter-tasting neutral-acidic IE fraction indicated that it contained less total phenolic acids compared with the parent DRS (18% vs 35%) and that the difference resulted from less dicinnamoylquinic acids and PPAs in the IE fraction. Since DRS and the derived neutral-acidic IE fraction had comparable 1

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235 bitter taste at 0.05% it seemed unlikely therefore that the dicinnamoylquinic acids or the PPAs are the cause of the bitter taste. In spite of this result phenolic acids (of undetermined structure) may yet be responsible for bitter taste since total phenolics in DRS assessed by UV absorption (50%) minus 35% identified by HPLC still leaves 15% unidentified phenolic acid materials with unknown organoleptic properties.


Use of Solid Adsorbants to Reduce the Bitter Taste of Brewed Coffee Based on the foregoing experiments we hypothesized that polyphenols compounds (of unidentified structure) might be responsible for the excessive bitter taste in brewed coffee and that these materials could be reduced or removed by adding an insoluble adsorptive material to RG coffee prior to brewing. To expedite the selection of adsorptive materials we devised a screening procedure in which adsorbants were exposed to dilute aqueous solutions of DRS. The loss of DRS from solution was interpreted as a measure of adsorbant effectiveness (Table IV). The effectiveness of proteinacious materials was of special interest to us in view of safety concerns surrounding all new food additives. It occurred to us that spent (pre-extracted) coffee grounds might also function as an adsorbant since they are known to contain about 14% protein. Spent coffee grounds are a common by-product of instant coffee manufacturing with little current value apartfromuse as a fuel or as landfill. As described previously (4) spent grounds are solvent-extracted and dried to remove excess lipids and water and pulverized to obtain a finely divided, odorless, tasteless adsorbant material. The defatted grounds (DFG) exhibited relatively weak adsorbancy (14%) in our DRS screening assay at 22°C, but the material showed promise for flavor modification in a practical test in which 5.7% DFG was added to RG coffee prior to brewing (Table V). Addition of DFG to RG coffee under conventional brewing conditions produced a brew that was Table V. Flavor Effects of Defatted Coffee Grounds (DFG) in Brewed Coffee

Flavor attribute Aroma (overall) Burnt aroma Taste intensity (overall) Bitter taste Burnt taste Ashy taste Color

Averaged panelists ' scores Control Control + 5.7% DFG 28.1 26.3 12.3 6.5 33.3 30.6 26.8 21.1 21.0 15.9 16.0 13.1 32.5 32.3

Scale = 0-60 panel units; number of panelists = 11


236 perceived by expert taste panelists to have significantly reduced bitter taste, burnt taste and burnt aroma. The actual cause of bitterness perception in brewed coffee was not determined in this work and it remains as an area for future research.


References

1. Czerny, M . ; Mayer, F.; Grosch, W. J. Agric. Food Chem. 1999,47, 695-699. 2. Clarke, R. J.; Macrae, R., Eds. Coffee, Volume 1: Chemistry, Elsevier Applied Science Publishers, New York, NY, 1985. 3. McCamey, D.A.; Thorpe, T.M.; McCarthy, J.P. In Dev. in Food Science 25, Bitterness in Foods andBeverages;Rouseff, R.L., Ed.; Elsevier, New York, 1990, pp 169-181. 4. Rizzi, G.P.; Gutwein, R.W. U.S. Patent 5,328,708, 1994. 5. Purdon, M.P.; McCamey, D.A. J. FoodSci.1987, 52, 1680-1683.


The Influence of Roasting-Derived Polymeric Substances on the

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