Showing papers in "European Food Research and Technology in 1985"

Studies on curcumin and curcuminoids. VI. Kinetics of curcumin degradation in aqueous solution.

Abstract: The kinetics of the pH-dependant degradation of curcumin has been investigated. A plot of the rate constant against pH indicates the pKa values of the acid protons. The graph also indicates the complexity of the curcumin degradation.

Identification of the volatile flavour compounds of wheat bread crust — comparison with rye bread crust

Abstract: The volatile compounds from equal amounts of both wheat and rye bread crust were isolated and fractionated, and their aromagrams were compared. The flavour compounds that were sensorially detectable in the HRGC-effluents were identified by mass spectrometry. 2-Acetyl-1-pyrroline was found to be a character-impact compound in wheat bread crust. In comparison to rye crust, wheat contained only 16 of the 30 neutral/basic rye aroma compounds. Although acetylpyrazine was found in both crusts, other compounds: 2-acetylpyradine, 5-methyl-6,7-dihydro-5H-cyclopenta(b)pyrazine and 2-acetyl-1,4,5,6-tetrahydropyridine, all having crust-like odours, were only found in rye.

Resistant starch formation during baking -effect of baking time and temperature and variations in the recipe

Abstract: The dietary fibre content in white wheat bread, measured with an enzymatic, gravimetric method, was almost 20% higher than in the corresponding flour. The increment was largely explained by the formation of resistant starch, i.e. starch available to amyloglucosidase only after solubilization with 2m-KOH. The resistant starch was formed in the oven or upon cooling of the finished bread. The water content in the dough seemed to influence the extent of the resistant starch formation, whereas changes in the fat content had no effect. The results indicate that the resistant starch might be hard retrograded starch, possibly amylose.

Heat Stability of Anthocyanins

Abstract: The heat stability of anthocyanins has been studied spectrophotometrically in model solutions of the concentrate from elderberries (Sambucus nigra L.) and in the isolated anthocyanin preparation at temperatures of 50–100 °C. The influence of pH on the thermostability has been evaluated in the pH range 2.2–4.0. The complete decomposition of anthocyanins follows a rational exponent-order kinetics with order values ranging from 1.1 to 1.7. In the interval corresponding to the half-time of anthocyanin decomposition, degradation followed the first-order kinetics. The influence of pH has been significant only at temperatures below 70 °C, where the stability of anthocyanins increased with decreasing pH value. The heat stability of colorants in a concentrate and in an isolated preparation did not differ significantly, with the exception of temperatures of 90 and 100 °C, where a higher stability of anthocyanins in elderberry concentrate has been proved.

HPLC determination of caffeine and theophylline in Paullinia cupana Kunth (Guarana) and Cola spp. samples

Abstract: A reversed-phase high-performance liquid-chromatographic method for the determination of caffeine and theophylline in commercial guarana samples (drug obtained from the seeds of Paulinia cupana Kunth, Sapindaceae of the Amazon Region) and in Cola spp. samples is described and discussed. The methodology developed is simple and rapid with a minimum of samples preparation required. A comparison of five different techniques for the extraction of caffeine and theophylline is discussed. Furthermore the quantitative determination of caffeine and theophylline in five samples of Brasilian guarana, in two samples of dietetic products containing guarana, in two samples of Cola extract and in three of Cola seed powder are reported.

Über Veränderungen der Aromastoffe während der Flaschenlagerung von Weißweinen der Rebsorte Riesling

Abstract: Mit Hilfe der gaschromatographisch-massenspektrometrischen Analysentechnik wurden die Veranderungen in den Gehalten der Aromastoffe des Weines bei der Flaschenlagerung untersucht. Die Untersuchungen wurden an Weinen der Rebsorte Riesling von 10 verschiedenen Jahrgangen (1964,1973–1975,1977–1982) aus dem gleichen Weingut durchgefuhrt. Ferner wurde die Aromastoffzusammensetzung eines „eingefrorenen” Rieslingweines mit der normalgelagerten („kellergelagerten”) Variante aus dem gleichen Gargebinde verglichen. Infolge der chemischen Gleichgewichtseinstellung in der wasrig-ethanolischen Losung nehmen die Gehalte der Essigsaureester (Acetate) ab, die Gehalte der Di- und Monoethylester von Dicarbonsauren zu. Ferner werden die Bildung bzw. die Gehaltszunahme von Substanzen aus dem Carotinoid-Abbau (Vitispirane, 1,1,6-Trimethyl-1,2-dihydro-naphtalin) und dem Kohlenhydrat-Abbau (Dehydratisierungsreaktionen in saurer Losung) unter Bildung u. a. von 2-Furfural, 2-Hydroxymethyl-5-furfural (HMF) beobachtet. Die Monoterpene erfahren wahrend der Alterung des Weines z. T. betrachtliche Veranderungen. Die Gehalte der Monoterpenalkohole Linalool, Geraniol und Citronellol nehmen ab, die Gehalte der vier isomeren Linalooloxide sowie von Neroloxid, Hotrienol, Hydroxylinalool und Hydroxycitronellol nehmen zu. Ferner wird die Bildung von 2,6,6-Trimethyl-2-vinyl-tetrahydropyran, von 2,2-Dimethyl-5-(1-methylpropenyl)-tetrahydrofuran, von cis-1,8-Terpin sowie

[Lipoamide dehydrogenase, citrate synthase and beta-hydroxyacyl-CoA-dehydrogenase in skeletal muscle. VIII. The influence of temperature and rate of freezing of bovine muscle on the activity and subcellular distribution of the enzymes in the thawed tissue].

Abstract: Samples of bovine muscle (post rigor) were frozen at different temperatures between -5 degrees and -196 degrees C at different freezing rates, and thawed at room temperature. The activities of the mitochondrial enzymes lipoamide dehydrogenase, citrate synthase and beta-hydroxyacyl-CoA-dehydrogenase were determined in the supernatant of the tissue homogenates in phosphate buffer (total enzyme activity), as well as in the press juice of the intact tissue (enzyme activity in the sarcoplasma). Neither the temperature nor the rate of freezing (varying from 25.5 to 0.01 min/degrees C) showed a significant influence on the total enzyme activities. Freezing at -5 degrees and -10 degrees C (at different rates but without intracellular freezing) and thawing did not result in an appreciable release of enzymes. Below -10 degrees C the release of the three enzymes from their binding to the inner membrane of the mitochondrion into the sarcoplasmic fluid increased upon rapid freezing with decreasing temperature i.e. with increasing intracellular ice formation, whereas at slow freezing (with extracellular ice formation only) freezing below -20 degrees C did not cause further enzyme release. At freezing temperatures below -20 degrees C rapid freezing resulted in a significantly stronger release of the three enzymes than slow freezing. From these results it was concluded that the damage to mitochondrial membranes upon fast freezing is primarily a result of intracellular (and perhaps also intramitochondrial) ice formation, whereas the membrane damage during slow freezing is primarily due to dehydration caused by the migration of water from the muscle fibers into the extracellular space as a result of osmotic effects. Ion concentration in the nonfreezing fraction of tissue water seems to be only of minor importance for the disintegration of mitochondrial membranes.

Die Mikroflora des Sauerteiges. XXI. Mitteilung: Die in Sauerteigen schwedischer Bäckereien vorkommenden Lactobacillen

Abstract: Es wurden Untersuchungen angestellt uber die Bakterienflora von 9 Sauerteigen, die in schwedischen Backereien bei der Bereitung von Roggen/Weizen-Mischbroten und Weizenbroten Anwendung finden. Bei diesen Sauerteigen fanden als Starter Sauerteige Anwendung, die z. T. seit mehr als 10 Jahren fortgefuhrt werden. Es wurden insgesamt 238 Isolate gewonnen und als Vertreter der GattungLactobacillus identifiziert. Von den Isolaten waren 31 der UntergattungThermobacterium (Lactobacillus delbruekkii,L. delbrueckii ssp.bulgaricus,L. acidophilus) zuzuordnen, 40 Isolate der UntergattungStreptobacterium (Lactobacillus plantarum,L. casei rhamnosus,L. farciminis) und 112 Isolate der UntergattungBetabacterium (Lactobacillus fermentum,L. brevis ssp.lindneri,L. viridescens,L. brevis). Weitere 55 Isolate homo- und heterofermentativer „Sauerteigbakterien” waren nicht eindeutig einer Art des GenusLactobacillus zuzuordnen. Aufgrund der Verbreitung, der Haufigkeit des Auftretens und des Verhaltens im Sauerungsversuch sind fur die untersuchten SauerteigeLactobacillus fermentum sowie die „nicht identifizierten Sauerteigbakterien” der Gruppierung „h” als typisch anzusehen.

[High performance liquid chromatographic determination of organic acids, sugars, glycerin and alcohol in wine on a cation exchange resin].

Abstract: Mit der HPLC ist es moglich, die wichtigsten Bestandteile von Wein (Ethanol, Glycerin, Glucose, Fructose, Tartrat, Malat, Lactat, Succinat, Acetat und Citrat) in einem Lauf innerhalb von 25 min zu trennen und quantitativ zu bestimmen. Als stationare Phase dient der Kationenaustauscher HPX 87 H und als mobile Phase verdunnte Schwefelsaure. Die Untersuchungsergebnisse an einigen Weinen werden dargestellt. Es besteht eine gute Ubereinstimmung zwischen den Analysenergebnissen der HPLC und herkommlichen Methoden.[/p]

[Quantitative analysis of auto-oxidation products of cholesterol in food of animal origin].

Abstract: Nach Extraktion mit Methylenchlorid und Isolierung der unverseifbaren Lipid-Fraktion wurden die Oxycholesterole uber zwei saulenchromatographische Schritte angereichert und dann als Trimethylsilylether gaschromatographisch an Dunnfilmcapillaren getrennt und massenspektrometrisch identifiziert. Als Hauptprodukte wurden Cholest-5-en-3β,7α-(I) und 7β-diol(II) sowie das 5,6-Epoxy-cholestan-3β-ol(III) und in einigen Proben Spuren von Cholestan-3β,5α,6β-triol und Cholest-5-en-3β,25-diol identifiziert. Quantitative Analysen wurden nach Zusatz von Cholest-5-en-3β,19-diol als interner Standard durchgefuhrt. Spruhgetrocknete Eipulver enthielten die hochsten Konzentrationen von I-III (insgesamt 15-60 μg/g). Im Parmesankase, Butterschmalz und bei Wurst proben lagen die Konzentrationen von I-III wesentlich niedriger (insgesamt 0,1-2,6 gg/g). Wurde Butterschmalz oder Talg unter Luftzutritt auf 170 °C langere Zeit erhitzt, so nahmen I-III stark zu.[/p]

Trace element solubility from food following enzymolysis

Abstract: Food items were treated with enzymes simulating gastric and intestinal digestive juices and the amounts of soluble lead, cadmium, zinc, iron, and copper, were determined. Enzyme treatment was conducted in two stages involving (I) pepsin at pH 2,5 followed by (II) pancreatin and amylase at neutral pH. Solubility was determined after each stage and additionally after post enzymolysis acidification. The foods examined comprised wholemeal bread, spinach, canned tomato, ox liver, pig kidney, canned crabmeat, beefburger, and canned corned beef, the last sampled from within the bulk and adjacent to the side seam of the can. Analyte solubility varied with (I) different food items, (II) processing or preparation of foods of similar origin, (III) action of pepsin or pancreatic enzymes and (IV) pH. Reasons for the variations are discussed.

Metal analysis in difficult materials with platform furnace Zeeman-atomic absorption spectroscopy. 2. Direct determination of cadmium and lead in milk.

Abstract: A procedure is described for the direct determination of cadmium and lead in whole milk, skim milk, condensed milk, and human milk. Using a Perkin-Elmer 5000 Z instrument with HGA 500 and L'vov platform and by application of oxygen ashing at approx. 600 °C, determination limits of approx. 0.02 μg/l and 0.7 μg/l for cadmium and lead, respectively, are attainable. Day-to-day precision is 10% for 0.1 μg/l of cadmium and 2 μg/l of lead. Accuracy control at least at higher levels was possible with DPASV after wet digestion. The contents found with this procedure in cows milk are at the lowest limit of very recent literature data, i. e. on average at 0.05 μg/l for cadmium and 2 μg/l for lead for samples from nonpolluted regions. The results indicate that milk does not contribute significantly to heavy metal exposure of man.

Accumulation of nitrite and sulfite in yeast cells and synergistic depletion of the intracellular ATP content.

Abstract: When nitrite or sulfite are applied to yeast cells below pH 50, an enormous intracellular accumulation occurs It is assumed that nitrite and sulfite penetrate the cell membrane in their undissociated forms as nitrous acid (pK = 33) or sulfurous acid (pK =18), respectively Due to the neutral intracellular pH they are trapped inside the cell in their anionic forms, which are impermeable to the cell membrane It has previously been shown that sulfite causes a rapid depletion of the ATP content of yeast cells [Schimz, K L and Holzer, H (1979) resp Hinze et al as above] Similarly, millimolar concentrations of nitrite decrease the ATP level to less than 10% of the initial value Nitrite and sulfite in combination deplete the ATP content of yeast cells much stronger than expected for the sum of the separate effects of these compounds (“synergistic effect”)

Effect of sulfite or nitrite on the ATP content and the carbohydrate metabolism in yeast

Abstract: Low concentrations of sulfite or nitrite (about 0.5 mmol) when applied at pH 3.6, caused a rapid and drastic decrease of the concentration of ATP in yeast cells. Under these conditions, alcoholic fermentation was inhibited by sulfite and to a lesser extent by nitrite. Ethanol consumption under aerobic conditions was shown to be more sensitive to nitrite than to sulfite. This indicates a higher sensitivity of respiratory processes to nitrite than to sulfite. Among 15 enzyme activities assayed in extracts from yeast cells after incubation with sulfite or nitrite, glyceraldehyde-3-phosphate dehydrogenase was shown to be the most sensitive. Analysis of the steady-state concentrations of intermediates of alcoholic fermentation in intact yeast cells also implies inhibition by sulfite or nitrite of the glyceraldehyde-3-phosphate dehydrogenase step of fermentation. In contrast to nitrite, sulfite had an additional effect by accumulating the intracellular steady state concentration of glyceraldehyde-3-phosphate 10 to 100-fold over the concentration in the absence of sulfite.In vitro studies on the equilibrium catalyzed by triosephosphate isomerase or aldolase confirmed the postulated shift of equilibrium concentrations by a formation of complex of glyceraldehyde-3-phosphate with sulfite.

Pectinbestimmung mit meta-Phenylphenol

Abstract: Die zur Erfassung von Pectinstoffen in Fruchtsaften angewendete Bestimmung mit Carbazol wird mit der m-Phenylphenol-Methode verglichen. Es ergibt sich, das die zuletzt genannte colorimetrische Bestimmung bei vergleichbarem Arbeitsund Zeitaufwand genauere Werte liefert, da Neutralzucker nicht storen. Dies gilt insbesondere fur die zur Erkennung von Verfalschungen wichtige wasserlosliche Pectinfraktion. Es wird empfohlen, der m-Phenylphenol-Methode bei der Bestimmung der Pectingehalte von Fruchtsaften den Vorzug gegenuber der Carbazol-Methode zu geben.

Caviare: Proximate composition, amino acid content and identification of fish species

Abstract: Different types of caviare were analyzed for proximate composition (crude protein, total lipids, moisture and salt) and amino acid contents. For the unbiassed identification of caviare the following biochemical method was developed: 1. Salt and lipids were removed from the samples. 2. Protein was extracted using urea/2-mercaptoethanol. 3. The extracts were analyzed by ultrathin-layer isoelectric focusing with polyacrylamide gels containing urea.

Extraktion von Thymian mit flüssigem CO2 im Labormaßstab

Abstract: Der Aufbau eines Hochdruck-CO2-Extraktors wird beschrieben, der die Gewinnung etherischer Ole im Labormasstab ermoglicht. Aus Thymian (Thymus vulgaris L.) wird das etherische Ol mittels CO2 extrahiert und die Inhaltsstoffe mit denen eines Wasserdampfdestillats und eines handelsublichen Thymianols verglichen. Bei der Analyse des etherischen Ols wurden gaschromatographisch und massenspektrometrisch etwa 50 Komponenten identifiziert.

Influence of germination on the nutritional quality of lentil seeds.

Abstract: The effect of germination on the nutritional quality of two varieties of lentil seeds was studied. It was found that germination improved the nutritive value by reducing haemagglutinins, trypsin inhibitor activities, tannins, and pentosans, and by increasing the in vitro protein digestibility, nitrogen solubility and FAAN. Except for Fe, ash and other minerals were not affected. The changes in these properties were more pronounced in Giza (9) variety than Syrian type.

Mineral contents of some southern Italian wines. II. Determination of Li, Na, Mg, K, Ca, Co, As, Rb, Sr, Ag, Sb, Ba.

Abstract: Following our study of the mineral contents of some Southern Italian wines, this work reports on the detection of 12 elemental components. The elements were determined using ICP-AES, flame atomic emission or absorption spectroscopy.

The effect of heating conditions on losses and regeneration of betacyanins

Abstract: The losses of Betacyanins during heating at 90 °C and their regeneration after heating were investigated in red beet juice and solutions of pure betanin in different buffers. It was found that the regeneration of pigments during storage of heated samples is greater at 5 °C than at 20 °C. The time, in which the pigments reach the maximum concentration is different for juice and pure betanin solutions. The influence of the type of buffer and its concentration on heating in the absence of oxygen is insignificant, on the other hand the influence is distinctly marked when the betanin heated in the presence of oxygen. When heated in the presence of air, the betanin losses are influenced by the rate of diffusion of oxygen from the air.

Bestimmung von Vitamin B6 in Lebensmitteln mit Hilfe der Hochdruckflüssig-Chromatographie (HPLC)

Abstract: A method for the quantitative determination of vitamin B6 in food by using high-pressure liquid chromatography and a fluorescence detector is described. The analytical method measures any vitamin B6 compound present in food (pyridoxamine, pyridoxal, pyridoxine and their phosphoric acid esters). Pyridoxamine (PM), pyridoxal (PL) and pyridoxine (PN) are released from the food after 30 min hydrolysis with 0.2 N-sulfuric acid at 120 degrees C. The filtered extract is applied directly to a reversed phase Spherisorb ODS column. PM, PL and PN are separated as mobile phase by using 0.08 N-sulfuric acid and measured by means of the fluorescence detector (excitation: 290 nm, emission: 395 nm). Recovery rates of PM, PL and PN added to the samples were between 94 and 102% depending on the kind of food. The detection limit is 5 micrograms PM and 10 micrograms PL or PN, resp. per 100 g food. The analytical procedure is simple, specific and has many applications; the results are well reproducible.

Acids in coffee. XI. The proportion of individual acids in the total titratable acid

Abstract: 22 acids in ground roast coffees and instant coffees were determined by GLC of their silyl derivatives (after preseparation by gel electrophoresis) or isotachophoresis. The contribution to the total acidity (which was estimated by titration to pH 8 after cation exchange of the coffee solutions) was calculated for each individual acid. The mentioned acids contribute with 67% (roast coffee) and 72% (instant coffee) to the total acidity. In the first place citric acid (12.2% in roast coffee/10.7% in instant coffee), acetic acid (11.2%/8.8%) and the high molecular weight acids (8%/9%) contribute to the total acidity. Also to be mentioned are the shares of chlorogenic acids (9%/4.8%), formic acid (5.3%/4.6%), quinic acid (4.7%/5.9%), malic acid (3.9%/3%) and phosphoric acid (2.5%/5.2%). A notable difference in the contribution to total acidity between roast and instant coffee was found for phosphoric acid and pyrrolidonecarboxylic acid (0.7%/1.9%). It can be concluded that those two acids are formed or released from e.g. their esters in higher amounts than other acids during the production of instant coffee.

Volatile flavour components of marula juice

Abstract: The volatile aromatics of marula fruit picked in the Republic of South Africa were enriched by means of an established liquid-liquid extraction technique. GC and GC-MS analyses were employed to identify 60 components with Kovats Indices lower than 2,000 (CW20M).

Analytik von Proteinen in Lebensmitteln mit elektrophoretischen und chromatographischen Verfahren

Abstract: Die Leistungsfahigkeit der elektrophoretischen Methoden (Gelelektrophoresen, isoelektrische Focussierung, zweidimensionale Techniken) und chromatographischen Verfahren (Ausschlus-, Ionenaustausch-, Umkehrphasen-Hochdruckflussigkeitschromatographie) zur Analytik von Proteinen in Lebensmitteln wird an Hand von umfangreicher Literatur sowie von ausgewahlten Anwendungsbeispielen beschrieben. Die durch die Vielfalt der Proteinmuster gegebenen Unterscheidungsmoglichkeiten lassen sich bei Verwendung charakteristischer Proteinkomponenten („Indikatorproteine”) infolge der Anpassungsfahigkeit und des hohen Auflosungsvermogens der elektrophoretischen und chromatographischen Methoden heute schon weitgehend ausnutzen. Auf diese Weise konnen auch eng verwandte, sehr ahnliche Lebensmittel tierischer oder pflanzlicher Herkunft anhand ihrer Proteinmuster eindeutig differenziert und in Mischungen meist auch quantitativ bestimmt werden. Durch gute Reproduzierbarkeit, Zuverlassigkeit und schnelle Ausfuhrung eignen sich die genannten Methoden fur einen Einsatz im Routinebetrieb. In der Literatur liegen Untersuchungen vor uber Fisch, Fleisch und abgeleitete Produkte, Fremdeiweis in Fleischprodukten, Milch, Kase, Cerealien und daraus hergestellte Lebensmittel, Leguminosen- und Olsaatsamen, Fruchte und Gemuse.

Beitrag zur Sortencharakterisierung der Rebsorte Weißer Riesling

Abstract: Alle untersuchten Weine der Rebsorte Riesling (Weiser Riesling) besitzen ein ahnliches Terpenmuster („Terpenprofil”), das die Monoterpengehalte mit den naturlichen Schwankungsbreiten (u. a. bedingt durch verschiedene Reife, Alterung) in einer fur den Weisen Riesling sortencharakteristischen Zusammensetzung zeigt.

Trace metals in muscle tissue of fish taken from the southern Baltic

Abstract: The determination of Fe, Mn, Ni, Pb, Cu, Co, Zn and Cd was investigated on 123 samples of muscle tissue of cod, herring, sprat and some other species of fish caught in 1981 in the southern Baltic Sea. The ranges and mean levels of metals recorded in this paper are compared with values reported previously by other authors for cod, herring and sprat from different regions of the Baltic Sea. The correlation coefficients between the metal concentrations in cod and herring muscles were calculated. The species-dependent changes of some metals in species analysed were observed.

Flüchtige Säuren in Tropenfrüchten: Cherimoya (Annona cherimolia, Mill.), Guava (Psidium guajava, L.), Mango (Mangifera indica, L., var. Alphonso), Papaya (Carica papaya, L.)

Abstract: Die extraktiv (Pentan/Dichlormethan, 2 + 1) erfasbaren fluchtigen Sauren in Fruchtpulpen von Tropenfruchten sind mittels Capillargaschromatographie (HRGC) und Capillargaschromatographie-Massenspektrometrie (HRGC-EI/CIMS) identifiziert und bestimmt worden. In Cherimoya (A. cherimolia, Mill.)-Fruchtpulpe konnten 47 Sauren charakterisiert werden; Hauptkomponenten waren Hexan- (3 mg/kg) und Octansaure (1 mg/kg). In Guava (P. guajava, L.) sind 51, in Mango (M. indica, L., var. Alphonso) 54 und in Papaya (C. papaya, L.) 56 Sauren identifiziert worden. Als Hauptkomponenten wurden (E)-Zimtsaure (0,4 mg/kg) und (Z)-3-Hexensaure (0,2 mg/kg) in Guava, 5-Hydroxy-(Z)-7-decensaure (2 mg/kg) und 3-Hydroxyoctansaure (1,1 mg/kg) in Mango sowie Butansaure (1,2 mg/kg) in Papaya bestimmt.

Determination of wheat gliadins in food with ELISA

Abstract: Two Sandwich-ELISA methods suitable for quantitative determination of wheat alpha-gliadin and whole gliadin (i.g. alpha-, beta-, and gamma-gliadin) in unheated gluten-containing and gluten-free foods are described. An optimal method for extraction of gliadin from foods was developed. Foods are extracted with 70% ethanol except those containing cocoa, where 3 M-urea and 10% casein were added prior to extraction. Wheat flours are extracted at room temperature or at 40 degrees C, gluten-free foods at 40 degrees C. The recovery of added gliadin (0.1% wheat flour) is at least 80% in foods without cocoa, at least 70% in cocoa-containing foods (1% wheat flour added). Both methods are highly specific for wheat, cross-reactions e.g. with rye are insignificant. The sensitivity assay of the assay in buffer is 0.5 ng/ml alpha-gliadin an 10 ng/ml whole gliadin and it can be decreased by prolonged incubation with sample. The unspecific effect of food components is discussed; it can be eliminated to a large extent.