Monier-Williams Distillation Research Articles

Characterization of the Biochemical Potential of Moroccan Onions (Allium cepa L.).

Allium cepa L. remains the most cultivated Allium species in Morocco and around the world. With the purpose of making the first biochemical characterization of Moroccan onions, several biochemical components have been measured in eleven onion ecotypes. Onions were collected as seeds from different geographical origins and cultivated in the same environment, to eliminate the influence of the environment on biochemical expression. Moisture, total phenols, flavonoids, antioxidant activity, total and reducing sugars, and sulfur dioxide were the biochemical properties of interest. Except for moisture, the eleven onion ecotypes revealed a highly significant variation in terms of the studied biochemical characters. The total phenol and flavonoid content ranged from 5.94 to 11.22 mg equivalent gallic acid/g dry weight and 0.67 to 1.52 mg equivalent quercetin/g dry weight, respectively. The antioxidant activity of the studied onions showed a strong correlation with the polyphenols (R2 = 0.7189), especially with the flavonoids (R2 = 0.8063). The sulfur dioxide content parted from 85.60 to 30.43 ppm when measured using the Monier-Williams distillation method. The current results show that there is no correlation between total sugars and reducing sugars. In conclusion, these findings present a clear biochemical profile of Moroccan onion ecotypes, as well as confirm, for the first time, the presence of a clear variation between the biochemical profiles of Moroccan onion ecotypes, which could be useful for future valorization programs.

Survey of sulfites in wine and various Turkish food and food products intended for export, 2007–2010

Surveys were carried out between 2007 and 2010 to determine the total levels of sulfites in 1245 samples of wines, dried apricots, dried vegetables, nuts, juices and purees, frozen foods and cereals containing dried fruit supplied by food inspectors and by food producers for testing or for export certification. Sulfite analysis of wine was carried out using the Ripper method with an LOQ of 5 mg l−1 and for dried and other foods the Monier-Williams distillation procedure was employed with an LOQ of 10 mg kg−1. In the survey all wines contained measurable sulfites, but with the exception of one sample of white wine they were otherwise below Turkish Food Codex limits of 160 mg kg−1 for red wine, 210 mg kg−1 to white wine and 235 mg kg−1 for sparkling wine. None of the cereal products, frozen foods, juices or purees contained sulfites above 10 mg kg−1. However, all dried apricot samples contained significant levels of sulfite with around 40% having levels exceeding the Turkish limit of 2000 mg kg−1. Significant levels of sulfite were found in other samples of dried fruit with even a fruit and nut bar containing 1395 mg kg−1 of sulfite, suggesting the dried fruit ingredients contained levels above regulatory limits.

Determination of Added Sulfites in Dried Garlic with a Modified Version of the Optimized Monier-Williams Method

The optimized Monier-Williams method is slightly modified so that it could be applied for determining sulfite content in dried garlic. Dried garlic sample is directly acidified in a reactor at a pH below 3. At this pH level, the alliinase enzyme activity is irreversibly blocked, and the sulfur-containing amino acids such as alliin (the most abundant) present in dried garlic cannot be transformed into corresponding thiosulfinates such as allicin, which is absent in dried garlic. This prevents allicin from reacting with added sulfites and being probably converted to S-allyl thiosulfate, which is not volatile and has no taste. It is found that at a pH below 2.4 and at boiling water temperature, allicin produces sulfur dioxide in adequate quantity to explain the false-positive results when utilizing the optimized Monier-Williams method with allicin suppression for unsulfited dried garlic samples. Finally, when garlic samples are stabilized in a phosphoric acid buffer at a final pH around 2.4, no sulfite is produced during the Monier-Williams distillation, which is further proof there are no naturally occurring sulfites in unsulfited dried garlic under these mild conditions.

Survey of stable sulfur isotope ratios (34S/32) of sulfite and sulfate in foods

A study was conducted to determine the natural abundance sulfur isotope ratios in foods containing sulphite preservatives. This involved determining (1) the accuracy and repeatability of sulfur isotope ratios measured using an elemental analyser (EA) coupled to an isotope ratio mass spectrometer (IRMS); and (2) the isotope ratios of 21 samples of commercially available S(IV) oxo-anion compounds, nine samples of S(VI) sulfate salts and the isotope ratios of the sulfate obtained by modified Monier-Williams distillation of SO2 from 33 retail foods containing sulfite preservatives. The sulfur isotope ratio data for SO2 recovered from foodstuffs showed a large spread of results, which suggested that the SO2 derived from sulfite preservatives does not have a distinctive sulfur signature ratio. The range of results (3.1–52.1) overlapped with that found for a range of commercially available sulfite and sulfate reagents commonly used to preserve food (sulfites 2.5–13.7, sulfates 10.0–16.9). Whilst the variability in isotope ratios originated from the food samples themselves, evidence from the analysis of SO2 gas suggested that isotope fractionation during dissolution, reaction and recovery was also a confounding factor.

The determination of the sulphite content of some foods and beverages by capillary electrophoresis

A method for the determination of the sulphite content in foods and beverages by capillary electrophoresis (CE) is described. The sulphite is converted to sulphur dioxide and finally to sulphate using a Monier-Williams distillation. The sulphate is then determined by CE using a 75 μm fused silica capillary column with a buffer consisting of 5 mm sodium chromate and 0.5 mM OFM Anion-BT reagent, pH 8.0, with indirect UV detection at 254 nm. Nitrate was used as the internal standard. The levels of sulphite in the products are in good agreement with those determined by titrimetry, except for one sample of fresh prawns, fresh garlic and some processed foods containing garlic and onion, where the levels determined by CE are lower. The instrument repeatability of the CE procedure is satisfactory and the level of detection is 5 mg/kg.

Determination of Sulfur Dioxide in Grapes: Comparison of the Monier-Williams Method and Two Ion Exclusion Chromatographic Methods

Results for determination of sulfur dioxide in grapes were compared by 3 methods: the modified Monier-Williams method, acid distillation/ion exclusion chromatography with electrochemical detection (AD/IEC-EC), and alkali extraction/ion exclusion chromatography with electrochemical detection (AE/IEC-EC). An unusual positive response was observed during the later stage of the Monier-Williams distillation of both control grapes and sulfited grapes. Development of volatile acidic compounds in parallel with this Monier-Williams response and darkening of sample was also observed by collection in an alkali trap and analysis using anion exclusion chromatography and photodiode array detection. No parallel increase in sulfite was observed by the more selective AD/IEC-EC method, which clearly demonstrated that the response observed during the later stage of the Monier-Williams method is a false positive, probably due to caramelization reaction products. Monier-Williams results for grapes containing ca 10 ppm sulfite were in reasonably good agreement with those by either the AD/IEC-EC or AE/IEC-EC methods, presumably because the false positive response in the Monier-Williams analysis compensated for the somewhat incomplete recovery of sulfite. The AE/IEC-EC method is recommended because it is rapid, sensitive, straightforward, and free from interference. Accurate results by Monier-Williams analysis could be obtained by limiting distillation to 60 min and correcting for recovery.

A Comparison of Selected Methods for Determining Sulfite in Prawns

As the standard technique for sulfite analysis, the Monier-Williams distillation, is time consuming and relatively insensitive, many alternative methods have been developed. Three of these methods were tested for their suitability for the routine determination of residual sulfite in prawns: i) a rapid steam distillation; ii) high performance liquid chromatography (HPLC), and iii) an enzyme-based procedure. Testing of the three selected methods involved analyses of standard sulfite solutions, prawns of known sulfite levels and prawns of unknown sulfite levels and comparison of results with those determined by the Monier-Williams procedure. Statistical analysis showed that the sulfite levels determined on standard solutions by rapid distillation were in close agreement with those obtained by the Monier-Williams distillation. However, a correction factor had to be introduced when analyzing prawns by the rapid distillation method to obtain comparable results with those produced by the Monier-Williams distillation. The same statistical analysis performed on levels determined by HPLC and the Monier-Williams distillation gave close agreement for both standard solutions and prawns. The enzyme test, after promising results for the standard solutions, proved to be unsatisfactory for analysis of sulfite in prawns.

Comparison of Three Liquid Chromatographic Methods with FDA Optimized Monier- Williams Method for Determination of Total Sulfite in Foods

Three liquid chromatographic (LC) methods employing amperometric detection were compared with the collaboratively studied FDA optimized Monier-Williams distillation method for the determination of total sulfite in 5 food types. The foods included lemon juice, white wine, instant mashed potatoes, golden raisins, and onion flakes. Two of the LC methods (one employing headspace sampling and the other direct injection) used ion-exchange chromatography with a basic mobile phase (pH about 10.8) and a glassy carbon electrode; the third (employing direct injection) used ion-exclusion chromatography with an acidic mobile phase (pH about 2) and a platinum electrode. All 4 methods produced similar results for the wine, lemon juice, and raisins. Results were different for instant mashed potatoes and onion flakes. The headspace-LC method and direct ion-exclusion LC method, both of which employed an alkaline sample extraction, yielded significantly higher values for sulfite in instant potatoes than did the other 2 methods. A large interfering peak with both direct LC methods prevented quantitation of sulfite in the onion flakes. All methods can detect sulfite as low as about 1 microgram/g in 4 of 5 food types examined.

Determination of Sulfur Dioxide in Foods by Modified Monier-Williams Distillation and Polarographic Detection

A rapid, sensitive polarographic method is presented for determining sulfiting agents in foods and beverages. The method is based on the modified Monier-Williams distillation followed by polarographic detection by differential pulse polarography or square wave voltammetry. A clearly defined wave is obtained by both techniques, with a current maximum at a potential (E) of about -600 mV vs an Ag/AgCl reference electrode. The reaction is based on the reduction of sulfur dioxide at a dropping mercury electrode. Peak current was linear over the range 0-20 micrograms/mL. Quantitation is done by linear regression analysis of standard addition data or by using a standard calibration graph. Screening levels of less than 1 ppm total SO2 were easily achieved in the foods analyzed, which had levels from less than 1 ppm (cereals) to thousands of parts per million (dried fruit). Recoveries from fortified samples ranged from 70 to 108% at fortification levels of 20, 100, and 1000 micrograms/g.

Reevaluation of Monier-Williams Method for Determining Sulfite in Food

The Monier-Williams distillation procedure has a long history of successful use for determining sulfite in fruit products and wine; however, a systematic evaluation of its accuracy and precision with other food matrices has not been undertaken. We found that the Monier-Williams distillation yielded greater than 90% recovery of sulfite added to foods such as table grapes, hominy, dried mangoes, and lemon juice. Less than 85% recovery was obtained with broccoli, soda crackers, cheese-peanut butter crackers, mushrooms, and potato chips. These results may, in fact, accurately reflect the residual levels of sulfite if a portion of the sulfite undergoes irreversible reaction with some food components. Analysis of commercial food products gave sulfite levels ranging from 1400 ppm in dried apple slices to 25 ppm in cream sherry.

A variation of the Monier-Williams distillation technique for the determination of sulphur dioxide in ginger ale.

A variation of the Monier-Williams distillation technique for the determination of sulphur dioxide in ginger ale.