Use of immunoaffinity chromatography as a purification step for the determination of aflatoxin M1 in cheeses

Enzyme-assisted extraction for the HPLC determination of aflatoxin M1 in cheese

The extraction of aflatoxin M1 (AFM1) from cheese is generally carried out using chlorinated organic solvents. In this study, two innovative methods were developed, based on an enzyme-assisted (EA) extraction using proteolytic enzymes (pepsin or pepsin-pancreatin). After purification through an immunoaffinity column, AFM1 is determined by HPLC-FLD. A comparison between the new EA methods and an established chloroform (CH) method was carried out on 24 cheese samples. The results showed that the extraction efficiency of the EA methods was independent of ripening time of cheese, whereas the CH method was not able to fully recover AFM1 from ripened cheeses. The simpler (pepsin) of the two methods has been adopted by our laboratory for routine analysis of AFM1 in cheese. In comparison with the CH method, the pepsin-HCl (P-HCl) method is simpler, avoiding solvent evaporation, dissolution and partition in a separating funnel; moreover, it gives higher recoveries, comparable LOD and LOQ and more accurate results.

Method for determination of aflatoxin M₁ in cheese and butter by HPLC using an immunoaffinity column

A rapid, sensitive convenient method for determination of aflatoxin M₁ (AFM₁) in cheese and butter by HPLC was developed and validated. The method employs a safe extraction solution (mixture of acetonitrile, methanol and water) and an immunoaffinity column (IAC) for clean-up. Compared with the widely used method employing chloroform and a Florisil column, the IAC method has a short analytical time and there are no interference peaks. The limits of quantification (LOQ) of the IAC method were 0.12 and 0.14 µg/kg, while those of the Florisil column method were 0.47 and 0.23 µg/kg in cheese and buffer, respectively. The recovery and relative standard deviation (RSD) for cheese (spiked at 0.5 µg/kg) in the IAC method were 92% and 7%, respectively, while for the Florisil column method the corresponding values were 76% and 10%. The recovery and RSD for butter (spiked at 0.5 µg/kg) in the IAC method were 97% and 9%, and those in the Florisil method were 74% and 9%, respectively. In the IAC method, the values of in-house precision (n=2, day=5) of cheese and butter (spiked at 0.5 µg/kg) were 9% and 13%, respectively. The IAC method is superior to the Florisil column method in terms of safety, ease of handling, sensitivity and reliability. A survey of AFM₁ contamination in imported cheese and butter in Japan was conducted by the IAC method. AFM₁ was not detected in 60 samples of cheese and 30 samples of butter.

Development and application of solvent-free extraction for the detection of aflatoxin M1 in dairy products by enzyme immunoassay

The official methods for the quantification of aflatoxin M1 in dairy products (cheese and yogurt) include extraction into dichloromethane or chloroform, evaporation of the solvent, partitioning of the reconstituted residue with hexane, and subsequent analysis. To secure a rapid and inexpensive screen for aflatoxin M1 contamination, a sensitive competitive ELISA, using a rabbit polyclonal antibody, was developed for measuring aflatoxin M1 in milk and used in a comparative study for measuring the extraction efficiency of aflatoxin M1 in aqueous or organic solvent buffers using yogurt samples. An aqueous sodium citrate solution was found to be suitable for extracting aflatoxin M1, thus eliminating the need for organic solvents. The citrate extraction proved to be efficient (recovery ranged from 70 to 124%) in fortified samples of very different kinds of dairy products, including yogurt and six types of cheese. Fourteen yogurt and cheese samples were extracted with citrate solution and analyzed by ELISA. A good correlation was observed (y=0.95x-0.59, r2=0.98) when the data were compared with those obtained through the official method, across a wide range of aflatoxin M1 contaminations (10-200 ng/kg).

Aflatoxin M1 determination in cheese by liquid chromatography–tandem mass spectrometry

A new method for determining aflatoxin M1 (AFM1) in cheese by liquid chromatography-tandem mass spectrometry has been developed. Two methodologies were compared for sample extraction. The first one involves sample extraction with dichloromethane for hard, aged cheese or acetone for fresh cheese and includes a preliminary matrix solid-phase dispersion-extraction step before solid-phase extraction (SPE) clean-up by a Carbograph-4 cartridge. The second method uses a water/methanol solution (90:10, v/v) extraction at 150 degrees C before clean-up. The average recoveries of AFM1 from samples spiked at levels of 0.25-0.45 microg/kg, were 81-92% and the precision (RSD) ranged from 3 to 7% with the first method, whilst the average recoveries were 79-84%, and RSD ranged from 7 to 15% for the second method. Due to different matrix effect, the quantification limits were 0.019-0.025 microg/kg in the first case and 0.048-0.143 microg/kg in the second one, depending on cheese typology.

Occurrence of aflatoxin M1 in UHT milk in Turkey

Aflatoxin M1 (AFM1) appears in milk as a direct result of the ingestion of food contaminated with aflatoxin B1 by cattle. The role of milk in human nutrition is well-known. The purpose of the study was to determine the levels of AFM1 in UHT milk samples in Central Anatolia, Turkey. The occurrence of AFM1 contamination in UHT milk samples was investigated by ELISA (Enzyme Linked Immunosorbent Assay) technique. A total of 129 samples of commercial UHT whole milk were analysed. The mean value was 108.17 ng/L. There was a high incidence rate of AFM1, with 75 (58.1%) milk samples being contaminated. Although 68 (53%) were below the limit, the remaining 61 (47%) were well above the limit permitted by the EU. Four of the samples exceeded the prescribed limit of US regulations. It can be concluded that AFM1 levels in the samples purchased in Central Anatolia Region, appear to be a serious public health problem at the moment. Dairy farmers must be educated by the government authorities on potential health consequences of aflatoxins.

Survey of the occurrence of Aflatoxin M1 in ovine milk by HPLC and its confirmation by MS

During the period of October-July 2000, 240 samples of dairy ewes milk, obtained from farms of Enna (Sicily, Italy), were checked for Aflatoxin M(1) (AFM(1)) by HPLC using a fluorimetric detector. The limit of detection and the limit of quantification were 250 ng/L for AFM(1). All the positive milk samples for AFM(1) were confirmed by LC-MS. AFM(1) was detected in 81% of milk samples, ranging from 2 to 108 ng/L. Three samples were over the legal limits (50 ng/L). Mean contamination of samples obtained from stabulated ewes was higher than that from grazing ewes (35.27 vs. 12.47 ng/L). Furthermore, samples collected in the period September-October showed higher contamination than samples collected during the other months (42.68 vs. 10.55 ng/L). Both differences are related to the administration of compound feed. Based on current toxicological knowledge we concluded that the AFM(1) contamination levels recorded in ewe milk did not present a serious human health hazard. However, as ewe milk is exclusively used to produce cheese due to its higher protein content, and also considering the preferential binding of AFM(1) to casein during coagulation of milk, a potentially high concentration effect could occur, thus the surveillance of contamination levels should be more continuous and widespread.

Distribution and stability of Aflatoxin M1during processing and ripening of traditional white pickled cheese

The distribution of aflatoxin M(1) (AFM(1)) has been studied between curd, whey, cheese and pickle samples of Turkish white pickled cheese produced according to traditional techniques and its stability studied during the ripening period. Cheeses were produced in three cheese-making trials using raw milk that was artificially contaminated with AFM(1) at the levels of 50, 250 and 750 ng/l and allowed to ripen for three months. AFM(1) determinations were carried out at intervals by LC with fluorescence detection after immunoaffinity column clean-up. During the syneresis of the cheese a proportionately high concentration of AFM(1) remained in curd and for each trial the level was 3.6, 3.8 and 4.0 times higher than levels in milk. At the end of the ripening, the distribution of AFM(1) for cheese/whey + brine samples was 0.9, 1.0 and 1.3 for first, second and third spiking respectively indicating that nearly half of the AFM(1) remained in cheese. It has been found that only 2-4% of the initial spiking of AFM(1) transferred into the brine solution. During the ripening period AFM(1) levels remained constant suggesting that AFM(1) was quite stable during manufacturing and ripening.

Determination of aflatoxin M1 in milk and milk powder using high-flow solid-phase extraction and liquid chromatography-tandem mass spectrometry

Animal feeds occasionally have some degree of contamination by Aspergillus spp. Even pasteurized milk at times contains the toxic liver carcinogen aflatoxin M1 (AFM1). Confirmation of its presence is now done with solid-phase extraction (SPE) and high-performance liquid chromatography (HPLC)-fluorescence, using a small enough sample that SPE time is reasonable. In this study 200 mL of milk was extracted using a C18 disk at a flow rate of approximately 100 mL/min and AFM1 quantified by HPLC-tandem mass spectrometry with negative electrospray ionization. The effectiveness and cleanup efficacy of immunoaffinity columns (IAC) was compared with that of Mycosep multifunctional cleanup columns (MFC). Average recovery and detection limits of whole milk and low-fat milk cleaned up by IAC were significantly superior to those obtained with the MFC (78-87% and 0.59-0.66 ng/L, respectively). The new procedure improves extraction speed, sensitivity, and specificity.

A six year survey (1999-2004) of the ocurrence of aflatoxin M1 in daily products produced in Portugal

Aflatoxin M1, a mutagenic and carcinogenic metabolite of aflatoxin B1, occurs in milk from animals fed on food contaminated with some species ofAspergillus. Aiming to investigate the occurrence of AFM in dairy products produced in Portugal. 598 samples of raw milk were analysed during six years (1999-2004). 25 samples of powder milk and 42 traditional fresh cheeses were also analysed. The toxin was extracted using an immunoaffinity column method and quantified by HPLC. AFM1 was detected in 394 (65.8%) of the raw milk samples. Along the analysed period AFM: was detected at a low level (0.005-0.05μg/l) in 54.8% of the samples and at a level ranging from 0.041-0.05 in 2.8% of the samples. From 2001 to 2004, 49 samples (8.2%) were contaminated with levels above the maximum permitted level (>0.051 to 0.08). None of the samples of powder milk and traditional fresh cheese revealed to be contaminated with AFM1.

High-performance liquid chromatography with post-column derivatisation and fluorescence detection for sensitive determination of aflatoxin M1 in milk and cheese

A new HPLC method with fluorescence detection using pyridinium hydrobromide perbromide as a post-column derivatising agent has been developed to determine aflatoxin M1 in milk and cheese. The detection limits were 1 ng/kg for milk and 5 ng/kg for cheese. The calibration curve was linear from 0.001 to 0.1 ng injected. The method includes a preliminary C18-SPE clean-up and the average recoveries of Aflatoxin M1 from milk and cheese, spiked at levels of 25-75 ng/kg and 100-300 ng/kg, respectively, were 90 and 76%; the precision (RSDr) ranged from 1.7 to 2.6% for milk and from 3.5 to 6.5% for cheese. The method is rapid, easily automatable and therefore useful for accurate and precise screening of aflatoxin M1 in milk and cheese.

Occurrence of aflatoxin M1in randomly selected North African milk and cheese samples

Forty-nine samples of raw cow's milk and 20 samples of fresh white soft cheese were collected directly from 20 local dairy factories in the north-west of Libya and analysed for the presence of aflatoxin M1 (AFM1). The samples were analysed using a high-performance liquid chromatography technique for toxin detection and quantification. Thirty-five of the 49 milk samples (71.4%) showed AFM1 levels between 0.03 and 3.13 ng ml(-1) milk. Multiple analyses of five milk samples free of AFM1 artificially contaminated with concentrations of AFM1 at 0.01, 0.05, 0.1, 1.0 and 3.0 ng ml(-1) showed average recoveries of 66.85, 72.41, 83.29, 97.94 and 98.25%, with coefficients of variations of 3.77, 4.11, 1.57, 1.29 and 0.54%, respectively. Fifteen of 20 white soft cheese samples (75.0%) showed the presence of AFM1 in concentrations between 0. 11 and 0.52 ng g(-1) of cheese. Multiple assays of five cheese samples free of AFM1 spiked with different concentration of AFM1 (0.1, 0.5, 1.0 and 3.0 ng g(-1)) showed average recoveries of 63.23, 78.14,83.29 and 88.68%, with coefficients of variation of 1.53, 9.90, 4.87 and 3.79%, respectively. The concentrations of AFM1 were lower in the cheese products than in the raw milk samples.

Aflatoxin M1 contamination in commercial samples of milk and dairy products in Kuwait

As part of the programme on monitoring of environmental contaminants in food stuff in Kuwait, 54 samples of fresh full cream and skimmed milk, powdered milk, yoghurt, and infant formula were analysed for aflatoxin M1 (AFM1) by HPLC following sample clean up using immuno-affinity columns. Of samples, 28% were contaminated with AFM1 with 6% being above the maximum permissible limit of 0.2 microgl(-1). Three fresh cow milk samples collected from a private local producer showed the highest level of 0.21 microg l(-1) AFM1. There was no contamination with AFM1 in powdered milk and infant formulas. These results show the necessity of a survey involving a larger number of milk and its products and suggest that presently the contamination of milk and milk products with AFM1 does not appear to be a serious health problem in Kuwait. Nevertheless, a continuous surveillance programme may be warranted to monitor regularly the occurrence of aflatoxins in the animal feeds responsible for current limited contamination and to note rapidly and worsening in the situation that may depend on market changes or on unfavourable climatic developments.

Distribution and stability of aflatoxin M1during processing, ripening and storage of Telemes cheese

Telemes cheeses were produced using milk that was artificially-contaminated with aflatoxin M1 at the levels of 0.050 and 0.100 microg/l. The cheeses produced in the two cheese-making trials were allowed to ripen for 2 months and stored for an additional 4 months to simulate commercial production of Telemes cheese. Concentrations of aflatoxin M1 in whey, curd, brine, and the produced cheeses were determined at intervals by liquid chromatography and fluorometric detection coupled with immunoaffinity column extraction. Concentrations of aflatoxin M1 in the produced curds were found to be 3.9 and 4.4 times higher than those in milk, whereas concentrations in whey were lower than those in curd and milk. Aflatoxin M1 was present in cheese at higher concentrations at the beginning than at the end of the ripening/storage period, and it declined to concentrations 2.7 and 3.4 times higher than those initially present in milk by the end of the sixth month of storage. Concentrations of aflatoxin M1 in brine started low and increased by the end of the ripening/storage period but only a portion of the amounts of aflatoxin M1 lost from cheese was found in the brine. Results showed that Telemes cheeses produced from milk containing aflatoxin M1 at a concentration close to either the maximum acceptable level of 0.05 microg/l set by the European union (EU) or at double this value, will contain the toxin at a level that is much lower or slightly higher, respectively, than the maximum acceptable level of 0.250 microg of aflatoxin M1/kg cheese set by some countries.

Application of the assay of aflatoxins by liquid chromatography with fluorescence detection in food analysis

HPLC using fluorescence detection has already become the most accepted method for the determination of aflatoxins due to its several advantages over other analytical methods. Both normal- and reversed-phase HPLC can be used. However the reversed-phase HPLC methods are more popular. Liquid chromatographic determination of aflatoxins using fluorescence detection and its application in food analysis is reviewed in this article.

Occurrence of aflatoxin M1 in commercial pasteurized milk determined with ELISA and HPLC

Eighty-one samples of commercial pasteurized milk from Athens market were analysed for the presence of aflatoxin M1 (AFM1). A combination of a commercial ELISA kit and a modified HPLC method was applied for the rapid and reliable determination of AFM1. AFM1 concentrations in milk extracts were initially estimated by ELISA. Samples found to contain more than 5 ng/l were further quantitated by HPLC. Determination was performed after derivatization of AFM1 to its hydroxylated product AFM2a. The recovery of the HPLC method used was found to be close to 100%. Thirty-two samples contained aflatoxin M1 at levels of 2.5-5 ng/l, none contained more than 5 ng/l, while 31 contained only traces of aflatoxin (0.5-1 ng/l). In nine samples no AFM1 was detected. There was no seasonal influence on the aflatoxin content of the milk samples analysed.