Studies on the optical isomerization of dietary amino acids in vinegar and aqueous acetic acid

L -AAs in 6 M HCl at 100°C. The rate of isomerization on heating in vinegar and aqueous AcOH was aspartic acid (Asp)>serine (Ser)≫proline (Pro), whereas in 6 M HCl it was Asp>Pro≫Ser. Heating of L-Pro together with glucose and fructose in 5% AcOH at 100°C for 96 h led to the formation of 24% D-Pro relative to L-Pro. From the data it is concluded that high relative amounts of D-Pro detected in matured vinegars are not attributable to acid-catalyzed racemization. In balsamic vinegars high relative amounts of D-Pro are mainly attributed to the Maillard reaction. Received: 9 August 1999     

Evolution of fatty acid contents in Garnacha and Viura musts during fermentation and the aging of wine

 The evolution of fatty acids during the fermentation of Vitis vinífera var. Garnacha and var. Viura musts as well as during the aging of the rosé and white wines produced from the said musts was studied. In Garnacha must, practically all the fatty acids were consumed, with the exception of the medium-chain fatty acids, by the time that 50% of the sugar was used up. During the second half of fermentation 80.1% of the fatty acids were consumed, with 28.8% of the remaining fatty acids being used up during aging. In Viura must, the total fatty acid concentration declined 46.9% during the first half of fermentation (first 50% of sugar), most noteworthy was the high consumption of unsaturated large-chain fatty acids (72.3%); during the second half of fermentation, 77.2% of the fatty acids were used, with high consumption of the large-chain saturated and unsaturated acids. During the aging of wine, medium-chain fatty acids were excreted and a small amount of unsaturated acids was consumed. Received: 3 March 1997 / Revised version: 21 July 1997     

A rapid headspace gas chromatographic method for the determination of the butyric acid content in edible fats

 A rapid headspace gas chromatographic method for the precise and accurate quantitation of butyric acid in edible fats was developed and validated by means of a reference material with a certified content of butyric acid. Sample preparation consisted only of weighing a fat sample into a suitable container, and adding the transesterification reagent before crimp-sealing the vial. By using an automatic sampler the sample throughput was eight samples per hour. The calibration function relating peak area to butyric acid concentration was linear over the range 0.2 g to 4.5 g per 100 g fat. The relative standard deviation of repeated measurements, as an indicator of the repeatability of the method, was between 0.90 and 1.32%. Received: 15 October 1997 / Revised version: 5 December 1997     

A rapid headspace gas chromatographic method for the determination of the butyric acid content in edible fats

 A rapid headspace gas chromatographic method for the precise and accurate quantitation of butyric acid in edible fats was developed and validated by means of a reference material with a certified content of butyric acid. Sample preparation consisted only of weighing a fat sample into a suitable container, and adding the transesterification reagent before crimp-sealing the vial. By using an automatic sampler the sample throughput was eight samples per hour. The calibration function relating peak area to butyric acid concentration was linear over the range 0.2 g to 4.5 g per 100 g fat. The relative standard deviation of repeated measurements, as an indicator of the repeatability of the method, was between 0.90 and 1.32%. Received: 15 October 1997 / Revised version: 5 December 1997     

Use of manganese in ripe olive processing

 The use of manganese (Mn) salts in various steps of the processing of ripe olives of the Manzanilla cultivar can increase their oxidation rate and produce darker olives. The best results were obtained by adding a Mn concentration of 40 mg/l to the storage brine and allowing a period of contact of at least 15 days. Good results were also obtained when using a Mn concentration of 100–250 mg/l in a “desalting” step before darkening, as well as variable periods of contact and temperatures. When Mn was added to the washing waters after any of the lye treatments of the darkening step, especially the first one, the effect was also appreciable, although varying degrees of Mn residues in the flesh were found. Addition of Mn to the storage brine and application of the usual darkening process produced more uniform and darker ripe olives with Mn residue levels at below 30 mg/kg, which is approximately 15% of the recommended daily intake for this metal. Results suggest that this cation, recognized as GRAS in the USA, may be used as a catalyst in the darkening of ripe olives if authorized as a processing aid. Received: 25 July 1997 / Revised version: 16 October 1997     

Use of manganese in ripe olive processing

 The use of manganese (Mn) salts in various steps of the processing of ripe olives of the Manzanilla cultivar can increase their oxidation rate and produce darker olives. The best results were obtained by adding a Mn concentration of 40 mg/l to the storage brine and allowing a period of contact of at least 15 days. Good results were also obtained when using a Mn concentration of 100–250 mg/l in a “desalting” step before darkening, as well as variable periods of contact and temperatures. When Mn was added to the washing waters after any of the lye treatments of the darkening step, especially the first one, the effect was also appreciable, although varying degrees of Mn residues in the flesh were found. Addition of Mn to the storage brine and application of the usual darkening process produced more uniform and darker ripe olives with Mn residue levels at below 30 mg/kg, which is approximately 15% of the recommended daily intake for this metal. Results suggest that this cation, recognized as GRAS in the USA, may be used as a catalyst in the darkening of ripe olives if authorized as a processing aid. Received: 25 July 1997 / Revised version: 16 October 1997     

Gas chromatographic triglyceride profiling of milk fat; comparison of packed and short metal capillary columns

 The quantitative aspects of triglyceride profiling of milk fat by capillary column gas chromatography (GC) were investigated. Fifty samples of pure Austrian milk fat were tested as described in Annex III of the European Community (EC) Commission regulation no. 454/95 by using a packed as well as a short metal capillary column. Capillary GC was equivalent to the packed column in terms of analytical precision (relative standard deviation of repeated injections). All samples tested fulfilled the purity criteria (S-values), irrespective of the chromatographic technique used. Nevertheless, the S-values obtained by capillary GC deviated to some extent from those of the packed column. The differences exceeded the value for reproducibility laid down in Annex III, EC regulation no. 454/95 in 18 of 50 cases. As a consequence, the equivalency of capillary GC for determining the purity of milk fat according to EC regulation no. 454/95 should be re-assessed. Received: 4 April 1997 / Revised version: 26 June 1997     

Endogenous formaldehyde level of foods and its biological significance

 Formaldehyde was measured in different food samples based on its reaction with hydralazine to form s-triazolo-(3,4-α)-phthalazine(Tri-P). Formaldehyde reacts with hydralazine under mild acidic conditions to form Tri-P. The reaction products were separated by reversed-phase, high-performance liquid chromatography. The method of sample preparation and chromatography are reviewed and the analytical method validated. Using the discussed methods we measured formaldehyde levels in 23 animals and 22 vegetable and fruit samples. Formaldehyde levels were one and a half times higher in vegetable samples compared to meat samples. Received: 23 December 1996 / Revised version: 17 March 1997     

Comparison between two commercial ELISAs and a culture procedure for the detection of Listeria spp.

 The efficiency of two commercial enzyme-linked immunosorbent assays (ELISAs), the Oxoid Listeria rapid test and the Tecra Listeria visual immunoassay for the detection of Listeria spp. was compared with that of a culture method. Of the 60 samples examined by ELISA and the culture procedure, Listeria was detected and confirmed by culture in 20, 34 and 44 samples by the Tecra, Oxoid and microbiological methods, respectively. The overall sensitivity of the Tecra and Oxoid tests was 50% and 93%, respectively. Both ELISAs had a specificity of 100%. The efficiency of the Oxoid test was 95%, while that of the Tecra assay was 67%. Differences in the results of the Oxoid test and those of the culture method were not statistically significant; however, the differences between the results of the Tecra assay and the culture procedure were significant. The Oxoid test was less labour intensive, less time consuming and easier to perform than the Tecra assay. It was concluded that the Oxoid test is a good alternative method to the culture procedures when screening for the presence of Listeria spp. in foods, especially when a large number of samples have to be analysed. Received: 24 June 1997     

Comparison between two commercial ELISAs and a culture procedure for the detection of Listeria spp.

 The efficiency of two commercial enzyme-linked immunosorbent assays (ELISAs), the Oxoid Listeria rapid test and the Tecra Listeria visual immunoassay for the detection of Listeria spp. was compared with that of a culture method. Of the 60 samples examined by ELISA and the culture procedure, Listeria was detected and confirmed by culture in 20, 34 and 44 samples by the Tecra, Oxoid and microbiological methods, respectively. The overall sensitivity of the Tecra and Oxoid tests was 50% and 93%, respectively. Both ELISAs had a specificity of 100%. The efficiency of the Oxoid test was 95%, while that of the Tecra assay was 67%. Differences in the results of the Oxoid test and those of the culture method were not statistically significant; however, the differences between the results of the Tecra assay and the culture procedure were significant. The Oxoid test was less labour intensive, less time consuming and easier to perform than the Tecra assay. It was concluded that the Oxoid test is a good alternative method to the culture procedures when screening for the presence of Listeria spp. in foods, especially when a large number of samples have to be analysed. Received: 24 June 1997     

Comparison of gas chromatographic methods for analysis of butyric acid in milk fat and fats containing milk fat

 The analysis of butyric acid (C4) is of importance for the determination of the proportion of milk fat in mixed fats. Three gas chromatographic methods were compared with regard to their precision for the measurement of C4, i.e. analysis of butyric acid methyl ester after trans-esterification of fat by sodium methylate (method A) or trimethyl sulphonium hydroxide (method B), as well as analysis of free butyric acid (method C), using an internal standard with each method. The examination of 30 milk fats which varied greatly in terms of their C4 content, using methods A, B and C, resulted in mean values of C4 of 3.42 g/100 g fat, 3.71 g/100 g fat and 3.06 g/100 g fat, respectively. The value determined using method B seemed too high, and this may have been due to the presence of co-eluting artefacts, whereas the value determined using method C was clearly too low, and can probably be attributed to losses during sample preparation. The standard deviation (SD) of 0.015 obtained from repeated analyses using method A was quite good. Results obtained using methods B and C had SDs of 0.029 and 0.074, respectively. Different levels of free fatty acids did not affect the results obtained using method A. When method A was checked by analysis of the reference fat, CRM 164, the C4 level determined was found to deviate from the certified C4 content of 3.49 (± 0.06) g/100 g fat by only 0.05 g C4/fat 100 g. Thus method A proved the most suitable for the determination of the proportion of milk fat in mixed fats by analysis of butyric acid. Received: 1 October 1997     

Microwave-assisted extraction of free amino acids from foods

 A microwave-assisted solvent extraction technique was developed for the extraction of free amino acids from foods. Four different samples were subjected to microwave extraction with varying amounts of extractant at several temperature values, for varying times to determine optimum extraction conditions. The extractions were performed with or without shaking the vessels in order to investigate the effects of mixing on the extraction efficiency and reproducibility. A comparison was made between this method and the traditional 1 h shaking extraction. The experiments indicated that the microwave-assisted extraction is an effective technique for the extraction of free amino acids from samples of plant or animal origin. The extraction yields obtained by the microwave method were about 10% better than by the conventional shaking technique. The times needed for the complete sample preparations were reduced by 66% with the application of microwaves. Received: 10 October 1997 / Revised version: 23 January 1998     

Comparison of gas chromatographic methods for analysis of butyric acid in milk fat and fats containing milk fat

 The analysis of butyric acid (C4) is of importance for the determination of the proportion of milk fat in mixed fats. Three gas chromatographic methods were compared with regard to their precision for the measurement of C4, i.e. analysis of butyric acid methyl ester after trans-esterification of fat by sodium methylate (method A) or trimethyl sulphonium hydroxide (method B), as well as analysis of free butyric acid (method C), using an internal standard with each method. The examination of 30 milk fats which varied greatly in terms of their C4 content, using methods A, B and C, resulted in mean values of C4 of 3.42 g/100 g fat, 3.71 g/100 g fat and 3.06 g/100 g fat, respectively. The value determined using method B seemed too high, and this may have been due to the presence of co-eluting artefacts, whereas the value determined using method C was clearly too low, and can probably be attributed to losses during sample preparation. The standard deviation (SD) of 0.015 obtained from repeated analyses using method A was quite good. Results obtained using methods B and C had SDs of 0.029 and 0.074, respectively. Different levels of free fatty acids did not affect the results obtained using method A. When method A was checked by analysis of the reference fat, CRM 164, the C4 level determined was found to deviate from the certified C4 content of 3.49 (± 0.06) g/100 g fat by only 0.05 g C4/fat 100 g. Thus method A proved the most suitable for the determination of the proportion of milk fat in mixed fats by analysis of butyric acid. Received: 1 October 1997     

Changes of lactose, lactic acid, and acetic acid contents in Serra cheese during ripening

 Changes in the quantities of lactose, lactic acid and acetic acid in Serra cheese were monitored using a triplicate two-way factorial design over a ripening period of 35 days (sampling at 0, 7, 21 and 35 days) throughout the cheesemaking season (sampling in November, February and June). The amount of lactose in total solids of cheese (TS) decreased slowly from 6.17% to 0.21% (w/w_TS) as ripening time elapsed. As a result of sugar metabolism, the lactic acid content increased from 0.07% at day 0 to 2.10% (w/w_TS) by 35 days, whereas the acetic acid content increased from 0.00% to 0.24% (w/w_TS) during the first week. The lactose content was statistically correlated with the lactic acid content but not with the acetic acid content. Received: 18 November 1996     

Characterisation of pseudocereal and cereal proteins by protein and amino acid analyses

Maize, wheat, amaranth, rice and soybean were screened for protein content. Alcohol‐soluble (A1 and A2) and glutelin (G1 and G2) fractions were isolated and compared in terms of their amino acid and protein compositions. The average proportions of nitrogen content between total alcohol‐soluble proteins (TASP) and total glutelins (TGlu) in the pseudocereals amaranth and soybean were about 1.8:26.9 and 14.9:12.3 respectively. In the cereals maize and wheat these proportions were 47.8:33.2 and 44.7:31.2 respectively. The sum of essential amino acids was 47.6 and 60.3 g per 100 g protein in amaranth and soybean respectively. The highest contents of methionine, lysine and arginine were found in the pseudocereals. The relatively high content of essential amino acids shows that pseudocereals could be used as a nutrient substitute for cereals. © 2002 Society of Chemical Industry     

Amounts of conjugated linoleic acid (CLA) in German foods and evaluation of daily intake

 The quantities of the biologically active isomer of conjugated linoleic acid (CLA) – C18:2 c9t11 – in 139 German foods were analysed by capillary gas chromatography (results are given as a % of all identified fatty acid methyl esters). The CLA content ranged from 0.40% (Gouda) to 1.70% (Jurassic cheese, Old Emmentaler) in dairy products, from 0.11% (rabbit) to 1.20% (lamb) in meat, and from 0.01% (pike-perch) to 0.09% (carp) in fish. CLA could be detected in neither vegetable fats or oils nor in margarines (CLA <0.01%). Crisps, chocolates, cakes and pastries, and other foods have only a negligible CLA content. The average estimated CLA intake in Germany was calculated to be 0.35g CLA/day for women and 0.43g CLA/day for men. Received: 22 July 1997     

Amounts of conjugated linoleic acid (CLA) in German foods and evaluation of daily intake

 The quantities of the biologically active isomer of conjugated linoleic acid (CLA) – C18:2 c9t11 – in 139 German foods were analysed by capillary gas chromatography (results are given as a % of all identified fatty acid methyl esters). The CLA content ranged from 0.40% (Gouda) to 1.70% (Jurassic cheese, Old Emmentaler) in dairy products, from 0.11% (rabbit) to 1.20% (lamb) in meat, and from 0.01% (pike-perch) to 0.09% (carp) in fish. CLA could be detected in neither vegetable fats or oils nor in margarines (CLA <0.01%). Crisps, chocolates, cakes and pastries, and other foods have only a negligible CLA content. The average estimated CLA intake in Germany was calculated to be 0.35g CLA/day for women and 0.43g CLA/day for men. Received: 22 July 1997     

Determination of aflatoxins in food using LC/MS/MS

 A liquid chromatography/atmospheric pressure chemical ionization tandem mass spectrometric method is described for the determination of aflatoxins B₁, B₂, G₁ and G₂ in food with the use of aflatoxin M₁ as an internal standard. The method works well with matrices such as those of figs and peanuts, but there are problems with spices, due to limitations of the clean-up method used. Received: 15 October 1997     

Isolation and characterization of a major allergen in kiwi fruit

 The isolation of an important allergen in kiwi fruit (Actinidia chinensis) by ion-exchange chromatography (IEC) and micropreparative SDS-PAGE followed by electroelution is reported. The purity of the allergen was analysed by SDS-PAGE and immunoblotting with sera from patients who have an allergy to kiwi. The allergen was shown to have a molecular weight of 43 kDa by SDS-PAGE/immunoblotting and an isoelectric point of approximately 6.9 as estimated by IEC. In accordance with World Health Organization nomenclature, this allergen is called Act c 2. By immunoblot inhibition it was shown that epitopes from different allergens in kiwi fruit are also located on Act c 2. N-terminal amino acid sequencing of 17 amino acid residues did not reveal homology with the major allergens in birch pollen (Bet v 1), apple (Mal d 1) or with other proteins of allergenic plant foods. In addition, the isoelectric point of a 67-kDa allergen in kiwi fruit was estimated to be 7.4 by IEC, but micropreparative isolation of this allergen failed because of its very low content in the fruit. Received: 14 April 1997     

Isolation and characterization of a major allergen in kiwi fruit

 The isolation of an important allergen in kiwi fruit (Actinidia chinensis) by ion-exchange chromatography (IEC) and micropreparative SDS-PAGE followed by electroelution is reported. The purity of the allergen was analysed by SDS-PAGE and immunoblotting with sera from patients who have an allergy to kiwi. The allergen was shown to have a molecular weight of 43 kDa by SDS-PAGE/immunoblotting and an isoelectric point of approximately 6.9 as estimated by IEC. In accordance with World Health Organization nomenclature, this allergen is called Act c 2. By immunoblot inhibition it was shown that epitopes from different allergens in kiwi fruit are also located on Act c 2. N-terminal amino acid sequencing of 17 amino acid residues did not reveal homology with the major allergens in birch pollen (Bet v 1), apple (Mal d 1) or with other proteins of allergenic plant foods. In addition, the isoelectric point of a 67-kDa allergen in kiwi fruit was estimated to be 7.4 by IEC, but micropreparative isolation of this allergen failed because of its very low content in the fruit. Received: 14 April 1997