generation of organic acids and monosaccharides by hydrolytic and oxidative transformation of food...

Bioresource Technology 96 (2005) 831–842

Generation of organic acids and monosaccharidesby hydrolytic and oxidative transformation

of food processing residues

Klaus Fischer a,*, Hans-Peter Bipp b

a Analytical and Ecological Chemistry Department, FB VI––Geography/Geosciences, University of Trier,

Universitatsring 15, D-54286 Trier, Germanyb Infineon Technologies AG, Rosenheimerstrasse 116, D-81669 Munich, Germany

Received in revised form 6 May 2004; accepted 8 July 2004

Available online 22 September 2004

Abstract

Carbohydrate-rich biomass residues, i.e. sugar beet molasses, whey powder, wine yeast, potato peel sludge, spent hops, malt dust

and apple marc, were tested as starting materials for the generation of marketable chemicals, e.g. aliphatic acids, sugar acids and

mono-/disaccharides. Residues were oxidized or hydrolyzed under acidic or alkaline conditions applying conventional laboratory

digestion methods and microwave assisted techniques. Yields and compositions of the oxidation products differed according to

the oxidizing agent used. Main products of oxidation by 30% HNO3 were acetic, glucaric, oxalic and glycolic acids. Applying

H2O2/CuO in alkaline solution, the organic acid yields were remarkably lower with formic, acetic and threonic acids as main prod-

ucts. Gluconic acid was formed instead of glucaric acid throughout. Reaction of a 10% H2O2 solution with sugar beet molasses gen-

erated formic and lactic acids mainly. Na2S2O8 solutions were very inefficient at oxidizing the residues.

Glucose, arabinose and galactose were formed during acidic hydrolysis of malt dust and apple marc. The glucose content reached

0.35g per gram of residue.

Important advantages of the microwave application were lower reaction times and reduced reagent demands.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Food processing residues; Hydrolysis; Microwave digestion; Aliphatic acids; Hydroxy carboxylic acids; Monosaccharides

1. Introduction

The development of strategies for the successfulreplacement of traditional chemical feedstocks, which

are based on fossil resources, by alternative feedstocks

based on the utilization of renewable resources is one

of the challenging tasks of ‘‘Green Chemistry’’. One area

of investigation focuses on the potential utilization of

carbohydrate-rich products derived from plants such

as coffee chicory (Chichorium intybus) and tobinambur

0960-8524/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biortech.2004.07.003

* Corresponding author. Tel./fax: +49 651 201 3617.

E-mail address: [email protected] (K. Fischer).

(Helianthus tuberosus) as sources of carbohydrates, such

as inulin (Ruhl and Bramm, 1996), which cannot be

processed from sugar beet (Beta vulgaris). The produc-tion of carbohydrates is also one goal of the so-called

‘‘Green Biorefinery’’, which is based on the utilization

of grass or green crops (Soyez et al., 1998; Kamm and

Kamm, 1999). Numerous products may be obtained

from processed carbohydrates including, amongst

others, biodegradable tensides and detergents, adhesives

and plastics (Baumann and Biermann, 1993; Jurges and

Turowski, 1996; Schmid, 1996; Hill et al., 1997).The chemical composition of agricultural byproducts

and of vegetable biomass residues from food engineering

mailto:[email protected]

832 K. Fischer, H.-P. Bipp / Bioresource Technology 96 (2005) 831–842

is often similar to that of cultivated crops. Hence these

residues bear the potential to be utilized as raw materials

additionally or alternatively to cultivated crops offering

the possibility of a sustainable exploitation of waste.

Typical carbohydrate containing high volume biomass

residues are molasses, potato peel sludge, whey powder,wine yeast, marc, spent hops, pomace, malt dust and

turnip shreds.

In the last decade various approaches were tried to

utilize these materials (Carlsson, 1993; Kiel, 1993;

Schulz and Annemuller, 1993). Several studies were di-

rected towards the utilization of food processing resi-

dues as fermentation media in the bioproduction of

ethanol (Paturau, 1989; Saddler, 1993; Quereshi andManderson, 1995; Andersen and Kiel, 1997). Some or-

ganic acids, e.g. lactic, citric, gluconic and itaconic acids

are prepared by the fermentation of sugar cane molas-

ses, sugar beet molasses, whey and others on a pilot-

plant or industrial scale (Tsao and Su, 1964; Paturau,

1989; Wang et al., 2000).

Studies on the production of sugars by acidic or alka-

line hydrolysis of high cellulose or hemicellulose materi-als are numerous and some of the methods developed

are already being used in practice (Paturau, 1989;

Brandt and Martin, 1996; Lavarack et al., 2002).

The formation of fatty acids and (poly)hydroxy carb-

oxylic acids during the alkaline degradation of cellulose

has been known for more than a hundred years (Kro-

chta and Hudson, 1985; Theander and Nelson, 1988;

Sjostrom, 1991; Knill and Kennedy, 2003). To increasethe yields of hydroxy carboxylic acids, the hydrolysis

is combined with an oxidation reaction using O2,

H2O2, HNO3 or other reagents with or without a cata-

lyst (Arts et al., 1997; Schutt et al., 2002). Recently, DD-

glucaric acid was processed from sugar beet molasses

by oxidation with mineral acids, applying vanadium

pentoxide as catalyst (Pamuk et al., 2001).

The initial impulse for the current investigationscame from the intention to transform carbohydrate bio-

mass residues into aqueous solutions of natural chelat-

ing agents, i.e. polyhydroxy carboxylic acids and

Table 1

Main components of several starting materials

Component Residue (wt.%)

Sugar beet molassesa

Water 18

Polysaccharides 63

Disaccharides 51(saccharose)

Protein n.d.P

Na, K, Ca �4.5

Ash n.d.

Aliphatic acids 5.3c

n.d.––no data.a Mean composition according to Vavrinecz (1974).b Company�s data.c Sum of formic (0.3), acetic (0.9), oxalic (0.1), succinic (0.1), glycolic (0.3

dicarboxylic acids, which can be used for the removal

of heavy metals from polluted soils and technical com-

bustion residues. After the workability of this approach

was proved (Bipp, 1996; Bipp et al., 1998; Fischer et al.,

1998) the scope was broadened to cover microwave as-

sisted conversion reactions conducted to yield monosac-charides from various biomass residues (Nuchter et al.,

1998).

The main goals of the present work were

(a) to compare qualitatively and quantitatively the

yields of carboxylic acids depending on the starting

material employed and on the oxidation agents

used,(b) to investigate practical benefits of the microwave

reaction technique for biomass conversion and

(c) to determine the yields of monosaccharides gener-

ated through the acidic hydrolysis of malt dust

and apple marc.

2. Experimental

2.1. Materials

Biomass residues: sugar beet molasses was received

from Sudzucker AG (Mannheim and Ochsenfurt, Ger-

many) and potato peel sludge was obtained from Pfanni

AG (Munich, Germany). Whey powder was obtained

from Meggle GmbH (Wasserburg, Germany) and wineyeast was received from the Obsthof Schroder distillery

(Ibesheim, Germany). Wheat spent hops were from the

Weihenstephan brewery (Freising, Germany), malt dust

was from the Bitburg brewery (Bitburg, Germany) and

apple marc was from the Matthias Mohn distillery (re-

gion of Trier, Germany). Most of the residues consisted

of dried powders or granules. Molasses was a thick

syrup containing about 18% of water. Apple marc wasa paste with a water content of 94.8%. The available

data on the composition of the crude residues were com-

piled in Table 1.

Potato peel sludgeb Whey powderb

n.d. 4

52 (starch) n.d.

61 42–45 (lactose)

16 20–23

n.d. 10.2

6.8 25

n.d. n.d.

), lactic (2.8), malic (0.4) and citric acids (0.4).

K. Fischer, H.-P. Bipp / Bioresource Technology 96 (2005) 831–842 833

All chemicals were of ‘‘per analysis’’ or of higher

quality. Aqueous solutions and dilutions were made

with ultrapure Milli-Q-Water (Millipore, Eschborn,

Germany). Calibration standards for liquid chromato-

graphy were prepared by mixing of aliquots of aqueous

stock solutions and subsequent dilution either with dis-tilled water or with the chromatographic eluent.

To determine the initial content of mono- and disac-

charides in malt dust and apple marc, these residues

were extracted with water at room temperature for 2h

in a horizontal shaker.

2.2. Biomass conversion methods

Different reaction conditions were applied to yield or-

ganic acids from the oxidative transformation of carbo-

hydrate containing biomass residues: (a) Oxidation by

nitric acid: between 2.5g and 5.0g of the residues were

added to 30% nitric acid at a solid: liquid ratio of 1:4

(w/v) in a one necked-flask, combined with a reflux con-

denser. A suction unit was mounted on top of the con-

denser. The mixture was then reacted under stirringfor 4h at 85 �C. Afterwards remaining solids were re-

moved by filtration. Alternatively a microwave-assisted

digestion procedure was conducted to transform molas-

ses into acidic reaction products. Applying a PC-con-

trolled high performance microwave unit (MEGA

1200; MLS GmbH Leutkirch/Allgau, Germany), 5.0g

of molasses and 20.0ml of 30% HNO3 were transferred

into high pressure PTFE reaction vessels having a vol-ume of 100ml. Up to six vessels were placed in a rotor

(HPR 1000/6, MLS Leutkirch), installed inside the

microwave apparatus. The digestion was performed

for 30min at a temperature of 85 �C (microwave power

200W, pressure limit 10bar).

(b) Acidic hydrolysis of the residues and subsequent

catalytic oxidation with H2O2 under alkaline conditions:

2.5g of the solid and 25ml of 2M HCl were filled in adigestion apparatus as described above and reacted un-

der reflux and stirring for 12h. After that the mixture

was filtered, neutralized with NaOH and diluted to

50ml with distilled water. To oxidize the monosaccha-

rides, 2.5ml of H2O2, 625mg of Na2CO3 and 62.5mg

of CuO were added and then the mixture was reacted

under agitation in a water bath at 60 �C for 4h. Finally,

the CuO particles were removed by filtration and thesolution was neutralized to pH 7 with HCl solution then

diluted to 100ml with distilled water.

(c) Microwave-assisted oxidation of molasses by

10% H2O2 and by aqueous Na2S2O8 solutions (1%,

3%, and 5%). The experiments were conducted in the

microwave unit described under (a). Using hydrogen

peroxide, 1.22g of molasses were added to 30ml of

10% H2O2. The digestion was performed for 10minat a temperature of 80 �C (microwave power 750W,

pressure limit 40bar). A control sample containing

molasses and water only was treated under the same

conditions.

Applying sodium peroxodisulfate, portions of 3g of

molasses were added to 30ml of solutions containing

1%, 3%, and 5% of Na2S2O8, respectively. The digestion

temperature was 110 �C; reaction time was 10min.To yield monosaccharides, the residues were conven-

tionally hydrolyzed under nonoxidizing conditions as

described under (b) or, alternatively, hydrolyzed with re-

duced concentrations of HCl under elevated pressure in

a microwave apparatus similar to (a) (MEGA 1600,

MLS Leutkirch). 0.2g of malt dust or 2.0g of apple

marc and 5.0ml of HCl (concentrations varied between

0.1 and 2.0M) were reacted in PTFE vessels. Thehydrolysis was performed at 100 �C under variation of

time (maximum reaction period: 2h).

The reported data on the yields of the conversion

reactions are means from two batches.

2.3. Analytical methods

The dissolved organic carbon content (DOC) wasanalyzed either by a Rosemount–Dohrmann DC-180

TOC analyzer (combined UV-persulfate digestion) or

by a DC-190 TOC analyzer (catalytic oxygen

combustion).

Acidic hydrolysate components were determined by

means of ion exclusion chromatography (IEC). Two

IEC systems were applied. System A consisted of a

Gynkotek 600–200 dual piston high pressure pumpcombined with a Gynkotek 250-B ternary gradient for-

mer, a Rheodyne (Cotati, USA) 8125 injector fitted with

a 20ll sample loop and a Shimadzu (Duisburg, Ger-

many) dual beam UV/VIS-detector SPD-10 AV (detec-

tion wavelength 210nm, detection full scale 0.01

A.U.F.S.). Separations were carried out on a Merck

(Darmstadt, Germany) Polyspher OA-HY column,

packed with a sulfonated polystyrene–divinylbenzenecation exchange resin and combined with a guard col-

umn containing the same resin. The columns were

housed in a thermostat (Industrial Electronics, Langen-

zersdorf, Austria) at temperatures of 10 �C and 45 �C.5.0 and 50mM sulfuric acid at a flow rate of

0.5mlmin�1 served as eluent. Data collection and

processing was handled by the Gynkosoft (Gynkotek)

chromatography software. For further analytical detailssee Fischer et al. (1995).

System B was a DX-500 (Dionex, Idstein, Germany)

chromatographic unit. It comprised of a GP 40 gradient

pump, a He degassing/purge unit, an ASM autosampler,

a LC-20 chromatography module with a 25ll sample

loop and an ED-40 electrochemical detector operated

in the conductivity mode. Separations were performed

on an IonPac ICE-As6 column (Dionex), enclosed inthe same thermostat as described above, and operated

at temperatures between 10 �C and 60 �C. The column


was coupled on an AMMS-ICE micromembrane sup-

pressor (Dionex), which was continuously regenerated

with 5mM TBAOH. Perfluorobutyric acid (PFBA)

served as eluent in concentrations between 0.4 and

1.6mM at a flow rate of 1.0mlmin�1. Remote system

control, data acquisition, and processing was handledby the Peaknet Software (Dionex).

Mono- and disaccharides were analyzed with a DX-

500 unit also. Different from the instrumentation listed

above, separations were performed on a CarboPac PA

10 column and pre-column (Dionex), guarded by an

Aminotrap column (Dionex) additionally. Elution was

achieved with a pH-gradient at a flow rate of

1.0mlmin�1 and a column temperature of 25 �C (Cor-ban and Fischer, 2000). Injection volume was 100ll.The analytes were monitored using pulsed amperometric

detection (PAD) applying an Au working electrode and

an Ag/AgCl reference electrode.

All samples containing organic acids were analyzed

after neutralization and filtration through 0.1lm or

0.2lm cellulose nitrate membrane filters (Heraeus-Sepa-

tech, Osterode, Germany). Hydrolysates generatedunder nonoxidizing conditions were filtered through

0.45lm polyamide membranes (Sartorius). Prior to

membrane filtration, samples having higher particle con-

centrations were passed through folded filters or centri-

fuged. Lipophilic components and amino acids were

separated from carbohydrates by solid phase extraction

with RP-C18 cartridges (Baker, Gross-Gerau, Germany)

and OnGuard-H cartridges (Dionex). The analytes wereidentified by comparison of their chromatographic

retention times with those of reference compounds.

3. Results and discussion

3.1. Biomass conversion by nitric acid

The reaction of 30% nitric acid with sugar beet

molasses, whey powder, wine yeast, potato peel sludge,

and spent hops leads to an hydrolytic cleavage of poly-

and oligosaccharides, followed by the oxidation of the

formed monosaccharides. A significant portion of the

primarily formed C6 oxidation products undergoes fur-

ther degradation reactions, resulting in C4 and C2 com-

ponents mainly. Nine mono- and multifunctionalaliphatic carboxylic acids (oxalic, glucaric, tartaric, glu-

conic, threonic, glyceric, glycolic, lactic and acetic acid)

could be detected in the majority of hydrolysates,

whereas formic acid was found in the molasses hydroly-

sate only (Table 2).

The hydrolysates contained between 0.29g and 0.32g

DOC per gram of applied raw material. The DOC con-

tent of three out of five hydrolysates could be almostcompletely attributed to the investigated aliphatic acids.

In the case of molasses hydrolysate and of the spent

hops hydrolysate, about 75% of the DOC could be

traced back to these reaction products. The total acid

amounts dissolved or formed per gram residue ranged

from 630mg to 910mg. The amount of hydroxy and

dicarboxylic acids was lowest in the spent hops hydro-

lysate. The yields of multifunctional acids generated bythe transformation of the other residues were of the

same order of magnitude, if they were related to the

dry matter content of the starting materials.

A rough estimation of the reaction yield expressed as

degree of transformation of the initially sugar-bound

carbon into acid-bound carbon can be made for sugar

beet molasses. Following Vavrinecz (1974) and Pamuk

et al. (2001), the molasses should contain approximately51 weight percent of saccharose and about 3.0% of in-

vert sugar. The carbon content of the carbohydrates is

42% (saccharose) and 40% (invert sugar), respectively.

This amounts to 216.2mg of sugar-attributable organic

carbon per gram of residue. The total amount of organic

acid-attributable carbon found in the hydrolysate of 1g

of molasses was 226.2mg. If one considers that about

one third of the acetic acid content and the total amountof lactic acid may have originated from the untreated

raw material (Table 1), a more accurate value for the

oxidatively generated organic acid carbon is 210mg,

which corresponds to 97% of the theoretical yield.

Acetic, glucaric, oxalic and glycolic acids were the

main organic constituents of the hydrolysates. Among

these acids, the contribution of acetic acid to the molar

sum of organic acids showed the greatest variability,ranging from 9.3% (molasses hydrolysate) to 52% (whey

powder hydrolysate). In contrast to this, the contents of

threonic, oxalic, and glucaric acids in the different resi-

due hydrolysates and their proportions to the DOC con-

tent of the reaction solutions were relatively uniform.

The exceptionally high amount of tartaric acid found

in the wine yeast hydrolysate gives rise to the assump-

tion that this component already was a constituent ofthe residue biochemically generated by the yeast.

The composition of the residue hydrolysates, espe-

cially of the molasses hydrolysate, which had a high sac-

charose content, is comparable to the product pattern

resulting from the oxidation of pure glucose and saccha-

rose by nitric acid (Hachihama and Fujita, 1935; Mehl-

tretter and Rist, 1953). For instance, the oxidation of

glucose proceeds along different pathways. One reactionscheme, which preserves the carbon frame of the mono-

saccharide, ends up with the formation of glucaric acid.

Mehltretter and Rist (1953) reported a DD-glucaric acid

yield of 41% by oxidizing dextrose with nitric acid. Re-

cently Pamuk et al. (2001) prepared DD-glucaric acid by

oxidation of sugar beet molasses in a packed bed reac-

tor, using a mixture of sodium nitrite, nitric and sulfuric

acids as oxidizing agents. The packed bed consisted ofvanadium pentoxide particles which served as oxidation

catalyst. At optimum conditions the conversion of

Table 2

Products of the oxidation of biomass residues by 30% nitric acid

Organic acids Physical

unit

Residue

Molassesa Whey

powder

Wine

yeast

Potato

peel

sludge

Spent hops Mean Coefficient

of variation

[%]

DOC mg 304 305 324 306 288

Oxalic mg 138 68 132 118 107 113 24.6

mM 1.52 0.76 1.44 1.30 1.18 1.24

% DOC 12.0 5.9 10.8 10.2 9.9 9.8 23.6

Glucaric mg 258 313 206 386 244 281 24.9

mM 1.22 1.48 0.98 1.83 1.16 1.33

% DOC 28.9 35.0 21.7 43.1 28.9 31.5 25.4

Tartaric mg 56 40 274 58 – 86 n.c.

mM 0.38 0.27 1.78 0.38 – 0.56

% DOC 5.9 4.2 27.0 6.0 – 8.6 n.c.

Gluconic mg 64 17 – – 35 n.c.

mM 0.33 0.09 – – 0.18

% DOC 7.9 2.3 – – 4.4 n.c.

Threonic mg 40 35 45 52 53 45 17.1

mM 0.29 0.25 0.33 0.38 0.39 0.33

% DOC 4.5 4.0 4.9 6.0 6.5 5.2 20.1

Glyceric mg 20 25 15 14 9 17 36.8

mM 0.18 0.24 0.14 0.13 0.09 0.16

% DOC 2.2 2.8 1.6 1.5 1.1 1.8 36.2

Glycolic mg 52 88 93 83 42 72 32.1

mM 0.69 1.16 1.22 1.09 0.55 0.94

% DOC 5.3 9.1 9.0 8.6 4.6 7.3 29.9

Lactic mg 22b 27 13 35 29 25 32.8

mM 0.25 0.30 0.15 0.38 0.32 0.28

% DOC 2.9 3.6 1.7 4.5 4.0 3.3 32.6

Acetic mg 30c 296 135 81 112 131 76.7

mM 0.51 4.92 2.26 1.34 1.86 2.18

% DOC 4.1 38.7 16.7 10.5 15.5 17.1 76.4

POrganic acids mg 684d 909 913 827 631 793 n.c.

mM 5.47e 9.47 8.30 6.83 5.73 7.16 n.c.

% DOC 74.4e 105.6 93.4 90.4 74.9 87.7 n.c.

Yields calculated for the conversion of 1g of residue.

n.c.––not calculated.a 82% dry matter content.b Initial content in the residue approximately 28mgg�1 (Vavrinecz, 1974).c Initial content in the residue approximately 9mgg�1 (Vavrinecz, 1974).d Including 4mg of formic acid.e Including 0.1mM of formic acid (0.7% DOC).


glucose to glucaric acid reached 63.9%. The side prod-

ucts formed were not analyzed.

Gluconic acid and glucuronic acid are intermediates

of this transformation process. Therefore it is not sur-

prising that the glucaric acid content of the hydrolysates

surpassed their gluconic acid content by far. Gluconic

acid was not detectable in two hydrolysates, whereas

glucuronic acid could not be found generally. Another

reason for the absence of glucuronic acid might be the

incomplete chromatographic resolution of glucaric and

glucuronic acid. The determination of glucuronic acid

was almost impossible at a high excess of glucaric acid.

Except for the wine yeast hydrolysate, the formation

of oxalic and tartaric acid can be traced back mainly to

the oxidative cleavage of glucose. The hydroxymono-

carboxylic acids threonic acid and glycolic acid are


intermediates which were not quantitatively trans-

formed into dicarboxylic acids. These findings are in

agreement with the classical studies of Kiliani (1921/

1922) and Dale and Rice (1933). Several older technical

procedures made use of this oxidation reaction for the

synthesis of both dicarboxylic acids (Brokes, 1944;Hales, 1947; Sanders, 1947).

The oxidation of fructose by nitric acid leads to the

formation of erythronic, oxalic, glycolic and formic

acids. The cleavage of fructose into C2- and C4-products

proceeds either directly or via the formation of 2-keto-

gluconic acid (Militzer and Angier, 1946). Since the sac-

charose content of molasses was essentially higher than

that of the other residues, the oxalic acid content of themolasses hydrolysate was highest. Furthermore, the

detection of formic acid was restricted to this

hydrolysate.

Due to various side reactions, lactic acid, glyceric

acid and acetic acid are formed additionally. Since the

transformation of DD-glucose from lactose into the corre-

sponding C6 sugar acids was almost complete in the

whey powder hydrolysate, the high acetic acid contentof this hydrolysate must have originated from the con-

version of the DD-galactose moiety mainly. A specific

reaction pathway must have favoured the formation of

C2 monocarboxylic acids (acetic and glycolic acids).

These two acids account for about 64.2% of the mole

sum of organic acids in the whey powder hydrolysate.

This portion varied between 43.6% (spent hops) and

21.9% (molasses) in the other hydrolysates. Alsoremarkable is the high excess of C2 monocarboxylic

acids over oxalic acid in the whey powder hydrolysate

(molar ratio 8:1). Except for the molasses hydrolysate

this ratio is approximately 2:1. At least in the case of

molasses hydrolysate a significant portion of lactic acid

may have originated from its initial content in the un-

treated residue, which amounts to about 2.8 weight per-

cent according to Vavrinecz (1974).The oxidative treatment of the lactose-containing

whey powder was expected to yield higher amounts of

DD-galactaric acid. In fact this component was not de-

tected in the hydrolysate. This might have been caused

by the low solubility of its calcium salt and of the free

acid itself at neutral pH. An indication for the presence

of this compound was the formation of a colorless pre-

cipitate during the neutralization of the acidic reactionsolution. Additionally the chromatographic determina-

tion of DD-galactaric acid was impeded by its poor sepa-

ration from glucaric acid. Therefore, the possibility that

a residual amount of DD-galactaric acid was erroneously

identified and quantified as glucaric acid cannot be ex-

cluded. The overdetermination of the DOC content of

the whey powder hydrolysate might be an indication

of that.Regarding the relation between the residue composi-

tion and the product pattern of the oxidation reaction

and with respect to a further enrichment and utilization

of the reaction products, the following conclusions can

be drawn. Due to the high starch content of potato peel

sludge, the hydrolysate offered the highest glucaric acid

content. Presumably caused by a specific transformation

of the DD-galactose moiety, the whey powder hydrolysatecontained the highest amounts of C2 monocarboxylic

acids, especially of acetic acid. The wine yeast hydroly-

sate was the best source of tartaric acid, but contained

the lowest amounts of C6 sugar acids. The DOC content

and the overall yield of organic acids were lowest in the

spent hops hydrolysate and none of the acids was

formed preferentially.

3.2. Biomass conversion by H2O2/CuO in alkaline solution

As Table 3 reveals, the amount of organic carbon dis-

solved in the hydrolysates generated by the oxidation of

the biomass residues with 10% H2O2/CuO under alka-

line conditions was considerably lower than in the nitric

acid digests. The DOC contents of the hydrolysates of

wine yeast, potato peel sludge and spent hops reachedonly about 50% of the amounts found in the corre-

sponding acid digests. On average 42% of the DOC

could be chromatographically identified as organic acids

by comparison of their retention times with those of

about 30 relevant reference compounds, whereas be-

tween 75% and approximately 100% of the DOC could

be characterized in the HNO3 digests. Since several un-

known signals were recorded in the IEC chromatogramsof the H2O2/CuO reaction products, at least a portion of

the unidentified DOC must have consisted of organic

acids too. The difference in the degree of DOC identifi-

cation (and therefore in the DOC composition) between

the two reaction products was highest for the potato

peel sludge.

The overall low degree of DOC identification ex-

presses far-reaching differences in the composition ofconversion products formed under the attack of the var-

ious oxidizing agents, which are presumably the result of

fundamental differences in the degradation mechanisms.

The narrower ratios between the total masses and mole

sums of organic acids generated in the H2O2/CuO

hydrolysates illustrate that under these conditions low

molecular weight components were produced

preferentially.Generally, the composition of the H2O2/CuO hydro-

lysates was marked by relatively high formic acid con-

tents (highest amounts of all identified components,

calculated on a molar scale) and lower concentrations

of most of the other acids (with the exception of glu-

conic acid). Except for the whey powder hydrolysate,

the sum of formic and acetic acids accounted for 60%

or more of the mole sum of the dissolved organic acids.Another feature of the H2O2/CuO hydrolysates was the

remarkably low content of dicarboxylic acids. Glucaric

Table 3

Oxidative conversion of biomass residues by H2O2/CuO in alkaline solution

Organic acids Physical unit Residue

Molassesa Whey

powder

Wine

yeast

Potato

peel sludge

Spent

hops

Mean Coefficient

of variation [%]

DOC mg 268 204 176 152 141

Oxalic mg 15.2 9.2 16.4 5.8 5.4 10.4 49.7

lM 171 104 182 64 59 116

% DOC 1.5 1.2 2.5 1.1 1.1 1.5 40.1

Tartaric mg – 14.8 54.4 – – n.c.

lM – 98 362 – – n.c.

% DOC – 2.4 10.0 – – n.c.

Gluconic mg 47.6 74.0 – 17.0 26.5 33.0 n.c.

lM 243 378 – 87 135 169

% DOC 6.6 13.3 – 4.2 6.8 6.2 n.c.

Threonic mg 54.0 80.8 16.7 8.4 15.0 35.0 89.3

lM 397 594 123 61 110 257

% DOC 7.2 13.9 3.4 1.8 3.7 6.0 80.6

Glyceric mg 32.8 26.8 16.3 7.8 11.8 19.1 54.7

lM 309 254 153 73 111 180

% DOC 3.7 3.9 2.7 1.6 2.6 2.9 32.1

Glycolic mg – 22.8 17.8 9.8 11.8 12.4 n.c.

lM – 301 234 128 156 164

% DOC – 3.5 3.2 2.1 2.6 2.3 n.c.

Lactic mg 27.6b 18.4 5.8 12.8 6.8 14.3 63.1

lM 306 205 64 142 75 158

% DOC 4.2 3.5 1.4 3.4 2.0 2.9 40.0

Formic mg 29.2 34.0 81.7 33.0 23.7 40.3 58.2

lM 638 742 1776 724 516 879

% DOC 2.8 4.3 12.0 5.8 4.3 5.8 61.7

Acetic mg 131.2c 48.8 21.6 19.4 26.7 49.5 95.1

lM 2185 813 359 323 444 825

% DOC 19.7 9.6 5.0 5.3 7.7 9.5 63.7

POrganic acids mg 338 398d 231 114 128 242 n.c.

mM 4.25 3.90e 3.25 1.60 1.61 2.92 n.c.

% DOC 45.7 67.8e 40.2 25.3 30.8 42.0 n.c.

Yields calculated for the conversion of 1g of residue.

n.c.––not calculated.a 82% dry matter content.b Initial content in the residue approximately 28mgg�1 (Vavrinecz, 1974).c Initial content in the residue approximately 9mgg�1 (Vavrinecz, 1974).d Including 68.8mg of ribonic acid.e Including 414lM of ribonic acid (12.2% DOC).


acid was completely absent and the mean oxalic acid

concentration amounted to 10% of the HNO3 hydroly-

sates. Tartaric acid was found in two hydrolysates only

and even in the wine yeast hydrolysate, the concentra-

tion of this compound was 20% of that in the corre-

sponding HNO3 hydrolysate.

As the higher coefficients of variation of the acid con-tents indicate (Table 3), the residue specific differences in

the qualitative and quantitative composition of the

hydrolysates were greater than the HNO3 digests. The

whey powder hydrolysate was notable for the highest

gluconic and threonic acid concentrations. Acetic acid

contributed to the molasses hydrolysate to a high extent

and ribonic acid was exclusively found in this solution.

Formic acid was the main constituent of the wine yeast

hydrolysate. These findings indicate complex degrada-tion mechanisms and the occurrence of various side

reactions.


The ability of H2O2 to react simultaneously under

given conditions via different pathways, e.g. ionic and

radical mechanisms, is the main reason for the complex

degradation process. The initial degradation step is

accompanied or followed by various rearrangement,

elimination, isomerization and cleavage reactions,depending on the structure of the attacked carbohydrate

and the stability of the intermediates generated. The OH

radicals formed by the catalytic decomposition of H2O2

in the presence of copper ions react almost nonselec-

tively. A certain selectivity is introduced into the reac-

tion by the ability of the Cu(II) ions to stabilize

intermediate reaction products such as endiols through

complexation (Marshall and Waters, 1960; Singh,1961). Final products of the oxidation of monosaccha-

rides such as glucose are formic, acetic, and glycolic

acids (Isbell et al., 1973).

When the C(1) atom of the hexose unit is protected as

in the case of maltodextrins and starch, base catalyzed

rearrangement of the radical formed at C(2) results in

the cleavage of the glycosidic bond (Arts et al., 1997).

As shown by Paruovi et al. (1995), the incomplete oxida-tion of starch by H2O2/Cu(II) yields a product mixture

having a carbonyl to carboxyl unit ratio of 4.7:1. This

might provide an explanation for the low organic acid

yield from potato peel sludge, which consists of starch

mainly.

As investigated by Isbell and Sniegoski (1964), Isbell

et al. (1973), and Isbell and Frush (1973), the ionic deg-

radation pathway starts with the nucleophilic additionof a perhydroxyl anion to the carbonyl group of aldoses,

followed by a heterolytic a-hydroxy-hydroperoxide

Table 4

Composition of molasses hydrolysates produced by various oxidizing digest

Organic acids Oxidizing agent concentration (mMg�1 molasses)a

HNO3 30% Na2S2O8 1%

Method Ac Ad Be Bf

Glucaric 1.22 0.64 0.47 0.01

Gluconic 0.33 n.d. n.d. 0.01

Tartaric 0.38 0.13 0.04 n.d.

Threonic 0.29 0.08 0.08 n.d.

Glyceric 0.18 0.23 0.16 0.02

Glycolic 0.67 0.35 0.16 0.01

Lactic 0.25 0.25 0.29 0.22

Formic 0.10 0.27 0.29 0.03

Acetic 0.51 0.18 0.17 0.06P

Organic acids 3.93 2.13 1.71 0.36

A: Conventional digest.

B: Microwave-assisted digest.

n.d.––not detectable.a 82% dry matter content.b The typical acid contents of sugar beet molasses are listed in Table 1.c Molasses batch 1, oxalic acid detectable in the hydrolysate.d Molasses batch 2.e Molasses batch 2, solid:liquid ratio 1:4,30min. 85�C (200W).f Molasses batch 2, solid:liquid ratio 1:10,10min. 80�C (750W).g Molasses batch 2, solid:liquid ratio 1:25,10min. 80�C (750W).

cleavage of the adduct, leading to the next lower aldose.

Repetition of the process results in stepwise degradation

of the aldose to formic acid. Ketoses are degraded in the

same manner to one mole of glycolic acid and four mo-

les of formic acid per mole of hexose.

To prove the composition of reaction products pro-posed for the transformation of pure carbohydrates,

fructose, glucose, saccharose, and lactose were digested

under given reaction conditions additionally (results

not shown in detail, see Bipp, 1996). The high yields

of formic acid (between 70 and 85mol%) and the consid-

erable amounts of glycolic acid formed confirmed the

theoretical expectations. The DOC yields of the pure

carbohydrates were more than twice as high as mostof the residues. Except for the glucose hydrolysate,

90% or more of the DOC content of the carbohydrate

hydrolysates were identified by IEC analysis.

The variable compositions of the residue hydrolysates

give rise to the assumption that minor components, such

as alkaline and alkaline earth ions, alter the reaction

mechanism and reduce the catalytic activity of the

Cu(II) ions (De Belder et al., 1963).

3.3. Oxidative conversion of sugar beet molasses

Various oxidizing agents were tested for their ability

to transform sugar beet molasses into mixtures of carb-

oxylic acids, applying a conventional digestion method

and a microwave-assisted technique. The results summa-

rized in Table 4 reveal far-reaching differences in thehydrolysate composition, depending on the oxidizing

agent used.

ion methods

Na2S2O8 3% Na2S2O8 5% H2O2 10% (H2O)b

Bf Bf Bg Bg

0.01 0.01 n.d. n.d.

0.02 0.02 0.16 0.08

n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d.

0.02 0.02 0.04 0.02

0.02 0.03 n.d. 0.01

0.21 0.21 0.85 0.31

0.05 0.09 3.09 n.d.

0.08 0.10 0.13 0.12

0.41 0.48 4.27 0.54


Almost independently from the concentrations used,

the solutions of sodium peroxodisulfate were very ineffi-

cient at generating organic acids applying microwave

irradiation. The resulting concentrations of several

hydroxy acids were of the same order as in the micro-

wave irradiated aqueous suspension of the residue,reflecting the initial organic acid content of the starting

material (Table 1). The organic acid yield was highest in

the hydrogen peroxide digest (pH 5.33), containing for-

mic acid (72mol%) mainly. Lactic acid contributed

about 20% to the mole sum of acids formed. The con-

centrations of the other acids did not differ from the

background values (water extract) remarkably except

for gluconic acid, whose concentration was found tobe slightly elevated. Compared with the alkaline H2O2/

CuO digest, the reverse formic acid:acetic acid ratio

indicates that the saccharose oxidation proceeded via

an ionic reaction mechanism primarily. The increased

lactic acid content and the absence of threonic acid are

also noteworthy.

The oxidation with nitric acid produced the highest

amounts of hydroxy acids. About 30% of the molesum of organic acids was apportioned to glucaric acid,

which was the main conversion product generally. The

juxtaposition of the two conventionally generated nitric

acid digests indicates that batch-to-batch variations of

the residue may alter the organic acid yield considerably.

The qualitative composition of the products generated

either conventionally or by the microwave assisted reac-

tion was more or less the same. The overall lower reac-tion yield of the microwave assisted reaction might have

been caused by nonoptimized process conditions. Nev-

ertheless the increased velocity of the residue transfor-

mation and the energy saving dissipative heat transfer

are fundamental advantages of the microwave technique

justifying intense efforts to define adequate application

conditions.

Table 5

Contents of monosaccharides and maltose in water extracts and HCl hydro

Residue Carbohydrates mgg�1 residue (dry matter content)

Arabinose Fructose Galactose

Malt dust

WEa + 19 n.d.

Conv. Hb 42 n.d. +

lW-H.b 42 + +

Apple marc

WE n.d. 10 n.d.

Conv. H. 73 + 31

lW-H. 85 n.d. 40

WE: water extract, Conv. H: conventional hydrolysis, lW-H.: microwave a

n.d.––not detectable.a Saccharose detectable.b Presumably xylose containing.

3.4. Acidic hydrolysis of malt dust and apple marc

The HCl hydrolysis of malt dust and of apple marc

under conventional conditions and with microwave

assistance led to the formation of considerable amounts

of monosaccharides, especially glucose (Table 5). Thesugar yield was almost independent from the hydrolysis

technique used. The hydrolysis of malt dust elevated the

amount of glucose by a factor of 8. The arabinose con-

tent increased slightly. The complete hydrolytic cleavage

of the disaccharide maltose into glucose demonstrates

the efficiency of the procedure. Since only 20% of the

glucose increase is attributable to the degradation of

maltose, the hydrolysis of polysaccharides, e.g. starchand cellulose, must have been the primary source for this

sugar. Presumably side reactions which transform fruc-

tose into 5-hydroxymethyl furfural were responsible

for the loss of this ketohexose.

The primary product of the hydrolysis of apple marc

was arabinose. Minor compounds were galactose and

glucose. Arabinose is a main constituent of high mole-

cular weight hemicelluloses. Its formation indicates anat least partial degradation of this structural plant com-

pound. The sugar yield of the hydrolysis of apple marc,

related to the sugar content of the corresponding water

extract, was essentially higher than that of the malt dust

conversion. Nevertheless, the total amount of sugar ob-

tained from the treatment of apple marc was less than

half the amount received from malt dust.

Some efforts were made to increase the efficiency ofthe microwave assisted hydrolysis of malt dust through

a reduction of the reaction time (Table 6 (Panel A))

and of the HCl concentration (Table 6 (Panel B)). As

the time dependent evolution of the glucose and maltose

concentrations illustrates, the reaction was almost com-

pleted after 30min. Doubling the reaction time led to an

increase in the glucose yield of only 14%.

lysates of malt dust and apple marc

Glucose Maltose Sum

44 58 121

350 n.d. 392

333 n.d. 375

n.d. n.d. 10

41 n.d. 145

46 n.d. 171

ssisted hydrolysis, standard conditions (2h, 1M HCl), +: detectable,

Table 6

Microwave assisted HCl hydrolysis of malt dust: variation of reaction conditions

Carbohydrate Yield (% weighed sample, dry matter content), time [min]

5 30 60

Panel A: Variation of reaction time

Arabinose 4.3 4.1 4.5

Galactose 0.95 1.2 1.4

Glucose 8.9 38.6 44.1

Fructose 0.8 0.8 0.7

Maltosea 6.7 0.4 n.d.

Panel B: Variation of HCl concentration

Yield (% weighed sample, dry matter content), HCl conc. [M]

0.1 0.4 1.0 2.0

Arabinose 4.4 3.8 4.2 4.6

Glucose 32.4 34.9 33.3 38.5

Panel A: HCl conc. 1M, 100�C.Panel B: Some other carbohydrates are detectable, concentrations below determination limit. Reaction time 2h, 100�C.

a Disaccharide.


The hydrolysis experiments conducted under varia-

tion of the HCl concentration demonstrate that the

sugar yields of the reactions applying 0.1 or 1.0M HCl

were almost identical. The carbohydrate pattern of the

malt dust hydrolysates could be almost completely iden-

tified by means of high performance anion exchange

chromatography. The use of a 2.0M HCl solution re-

sulted in a 17% increase of the glucose yield. Due tothe limited number of parallel tests and of acid concen-

trations selected, this result might not be significant. De-

spite this restriction it seems to be justified to assume

that the microwave technique offers the advantage of

performing acidic hydrolysis reactions with compara-

tively low amounts of acid.

4. Conclusions

Carbohydrate-rich biomass residues from agricul-

ture and food industry proved to be a useful source

for the generation of aliphatic acids, carbohydrate der-

ivates, and of monosaccharides. The qualitative and

quantitative composition of the reaction products ob-

tained by the oxidative transformation of the residueswas ruled by the applied oxidizing agent and by the

composition of the starting materials, thus offering

the selection of specific product patterns. Highest

yields of organic acids, especially of dicarboxylic acids,

were obtainable by the use of nitric acid as oxidizing

agent. Potato peel sludge was recognized as a source

for glucaric acid and wine yeast offered advantages

for the production of tartaric acid. Acetic acid wasproduced in large amounts by the oxidation of whey

powder. A large portion of the DOC of the hydroly-

sates could be attributed to aliphatic carboxylic acids

by IEC.

Except for the fact that H2O2 is a more environmen-

tally benign reagent than nitric acid, the application of

this reagent in combination with CuO and high hydrox-

ide ion concentration offers no advantages. If a high for-

mic acid yield is desired, the oxidation of sugar beet

molasses by 10% H2O2 is recommended. Sodium per-

oxodisulfate was very inefficient at generating organic

acids.Applying HCl hydrolysis in combination with a

microwave digestion technique, up to 45% of the dry

matter content of malt dust could be converted into

monosaccharides with glucose as main product.

Composition and yield of products formed by the

microwave assisted residue digestion were almost identi-

cal with those generated through conventional methods.

Since the microwave digestion technique facilitates aconsiderable reduction of the reaction time and of the

reagent demand, further efforts to optimize its applica-

tion in the field of residue conversion are recommended.

A reduction of the nitric acid concentration seems to

be a prerequisite for a further scaling up of the process.

The use of a hot 30% nitric acid solution creates harsh

process conditions which require special equipment

materials. Furthermore, the base consumption for aneutralization of the excess acid content and the result-

ing salt concentration are high, if a method for recycling

the remaining nitric acid is not found.

Another task is to adapt methods to separate the

reaction products on a preparative scale, because appli-

cations of the product mixtures are scarce. In the case of

the residue conversion by HNO3 the problem might be

mitigated by the selection of lower reagent concentra-tions, probably leading to a higher reaction selectivity

and to the formation of fewer products. On the other

hand, various separation techniques such as precipita-

tion with alkaline or alkaline earth ions (Paturau,


1989; Pamuk et al., 2001), extraction with organic sol-

vents (Kertes and King, 1986; Paturau, 1989; Malmary

et al., 1995, 2000), separation through liquid membranes

(Schafer and Hossain, 1996; Di Luccio et al., 2000;

McMurray and Griffin, 2002), and chromatographic

separation (Wang et al., 1994, 2000), have been appliedsuccessfully in recent years to enrich target compounds

from reaction solutions and fermentation broths.

The residues and chemical transformation methods

utilized represent only a small segment of the available

materials and useful chemical processes. Nevertheless,

the results achieved seem to warrant the suggestion that

they have a certain potential to act as starting points for

further developments towards the aim to produce mar-ketable chemicals from agricultural residues.

On the basis of these results which demonstrate that a

combined feedstock mix/product mix is the practical

outcome of the conversion of different biomass residues

by the same oxidation agent, it should be possible to

apply advanced treatment technologies for substantial

utilization of products in the product line of the chemi-

cal industry.In addition to classical chemical oxidation agents,

new chemo-enzymatic oxidation agents could be used.

Acknowledgements

A part of the work was conducted at the GSF––Na-

tional Research Centre for Environment and Health,Institute of Ecological Chemistry (Director: Prof. Dr.

A. Kettrup), and integrated in the Bavarian Research

Association for Waste Research and Reuse of Residues

(BayFORREST). This part was fully funded by the

Bavarian Stateministeries ‘‘fur Landesentwicklung und

Umweltfragen’’ and ‘‘fur Unterricht, Kultur, Wissensc-

haft und Kunst’’. Some of the microwave assisted resi-

due digests were carried out in cooperation with Dr.U. Nuchter, University of Leipzig, and with Dr. M.

Nuchter and Prof. Dr. B. Ondruschka, University of

Jena.

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