Aspects of material science in food processing: changes in plant cell walls of fruits and vegetables

 Foods can be regarded as complex, dispersed systems which are normally metastable. Food processing causes state transitions (second-order transitions) when raw materials, food components or food systems are subjected to external stresses. The state transitions occurring during processing are detectable as changes in structure and properties of the investigated systems. The processing of fruits and vegetables is often connected with changes in cell walls. Cell wall materials in dispersed fruit and vegetable systems can be regarded as a model substrate of the dispersed phase. During processing, cell walls undergo modifications in terms of their physical state, macrostructure, microstructure, and composition, as well as structure-dependent changes in their functional and material properties. The interactions and connections (dependencies) between state transitions, and various changes in structure and properties, are very complex and multivariate and are not well understood as yet. For the evaluation of changing material properties during processing, examination of hydration, rheological (external mechanical stress) and thermal (external thermal stress) characteristics is important. The changes occurring during the processing of fruits and vegetables are determined by external factors (especially various mechanical and thermal stresses) and by internal factors. External stress in many cases causes solubilisation of the cell wall, loss of firmness and favours cell separation. Thermal processing increases pectin degradation by β-elimination. Internal factors such as pH and modified ionic strength, e.g. by applying soak solutions, can have an important influence on the changes in the cell wall during processing. So, calcium ions on the one hand can favour cell wall degradation by β-elimination and, on the other hand, after low temperature blanching and de-esterification of the pectin by activated pectin methyl esterase, can contribute to stabilisation of the texture by formation of a calcium-pectin complex. Knowledge about cell wall degradation mechanisms can be markedly improved by studies using model substrates such as pectin, or cell wall materials like the alcohol-insoluble residue and materials with cellular structure. This knowledge has been used to improve the technology used to process fruits and vegetables and to produce products with better properties. Moreover, testing and applying cell wall materials as ingredients for the production of textured foods and potential health-related foods is suggested. Received: 21 July 1998     

A new concept for the measurement of total volatile compounds of food

 The aim of our work was to develop a rapid and reliable method for the evaluation of the total volatile fraction of fruits (strawberries, raspberries, tomatoes and apples). Our method consists of trapping the volatile compounds of fruits on a solid-phase microextraction (SPME) fibre and determining the total amount of the adsorbed substances after desorption in a GC system, without performing any separation. The patterns obtained by using several types of SPME fibre permitted us to differentiate between the total volatile compounds present in the sample depending upon their chemical nature. Using strawberries as a model, we could show that our method: (1) leads to easily reproducible results; (2) allows differentiation between six varieties in a way which is consistent with an hedonic evaluation of these varieties; (3) shows the variation in total volatile compounds between individual fruits. The technique is rapid, practical, cheap, and promising for an objective evaluation of the volatile fraction of fruits. Received: 27 March 1998 / Revised version: 23 July 1998     

HPLC determination of stevioside in plant material and food samples

 An HPLC method for the determination of sweet-tasting stevioside (STS) in the leaves of the plant Stevia rebaudiana and in some beverages (e.g. tea, orange juice) was developed. The pre-separation procedure consisted of extraction of STS from the plant material using boiling water and a solid-phase extraction (SPE). Recovery rates of the SPE for the analysed matrices ranged from 92.8% to 97.8% (for concentrations of STS of 105, 210 and 300 μg/ml; Relative Standard Deviation (RSD)≤3.3%). The chromatographic separations were realized using a C₁₈ column, the mobile phase consisting of methanol and water, and with UV detection at 210 nm. The limits of determination of STS were 5 μg/ml for the leaf extracts and the tea sample and 8 μg/ml for the juice sample. Received: 7 April 1998     

HPLC determination of stevioside in plant material and food samples

 An HPLC method for the determination of sweet-tasting stevioside (STS) in the leaves of the plant Stevia rebaudiana and in some beverages (e.g. tea, orange juice) was developed. The pre-separation procedure consisted of extraction of STS from the plant material using boiling water and a solid-phase extraction (SPE). Recovery rates of the SPE for the analysed matrices ranged from 92.8% to 97.8% (for concentrations of STS of 105, 210 and 300 μg/ml; Relative Standard Deviation (RSD)≤3.3%). The chromatographic separations were realized using a C₁₈ column, the mobile phase consisting of methanol and water, and with UV detection at 210 nm. The limits of determination of STS were 5 μg/ml for the leaf extracts and the tea sample and 8 μg/ml for the juice sample. Received: 7 April 1998     

Occurrence of 3-chloro-propane-1,2-diol (3-MCPD) and related compounds in foods: a review

A critical review of the occurrence of 3-chloro-propane-1,2-diol (3-MCPD) in foods not known to contain hydrolysed vegetable proteins is presented. The review covers the properties and chemistry of 3-MCPD and the current methods of analysis in foodstuffs. The results of UK surveys of 3-MCPD occurrence in both retail foods and commercial food ingredients are discussed with particular reference to cereal, meat and dairy products. The possible mechanisms for the formation and decay of 3-MCPD in foods are suggested. The review does not cover the detailed toxicology of 3-MCPD and its occurrence in hydrolysed vegetable proteins, which have been considered elsewhere, nor possible issues such as in-vivo formation.     

Quantitative and qualitative evaluation of nine different extraction methods for nucleic acids on soya bean food samples

 Deoxyribonucleic acids (DNA) and ribonucleic acids (RNA) as normal compounds of many foodstuffs have so far been of minor interest with respect to human and animal nutrition. Through the approval of various genetically modified crops in the United States and Europe in recent years, these nucleic acids (NA) have become an important tool in food analysis. The reason for this is that in most cases the discrimination between a genetically modified foodstuff and an unmodified one can best be achieved directly at the DNA level by means of proving the presence or absence of the introduced gene(s). So far, the various methods that can be used to purify DNA for such types of analysis have rarely been compared in a comprehensive manner. In this work the effects of nine different extraction methods on yield and quality of previously purified high-molecular-weight DNA from soya beans and of total NA from tofu, soya flour and lecithin have been tested. Quantification was accomplished using UV absorption at 260 nm and quality assessment of the DNA was done by polymerase chain reaction (PCR) with the soya bean-specific lectin 1 system. Using this approach we assessed the limits of DNA amplification for each food sample extracted using different extraction protocols. It was demonstrated that extraction methods using NA-binding resins like Wizard or DNeasy resulted in comparatively low yields. However, the DNA extracted with these methods is of high quality and suitable for downstream processing like PCR. Simpler and faster methods resulted in relatively high yields of NA but of relatively poor quality. Inhibition of PCR was observed due to one of the buffers used during NA extraction. Finally, the different extraction methods used in this study were compared with respect to the costs and the time needed to perform the extractions. Received: 21 January 1998 / Revised version: 17 February 1998     

Determination of low levels of aziridine in official EU food simulants by capillary gas chromatography

 An analytical method for the determination of the specific migration of aziridine (ethyleneimine) into food simulants at trace levels is described. The method comprises a two-phase (aqueous-organic solvent) derivatization procedure with 4-fluorobenzoyl chloride, using propyleneimine as an internal standard. The derivatization reaction is accomplished very quickly and the organic layer containing the derivatized imine is analysed by capillary gas chromatography using selective nitrogen detection (nitrogen-phosphorus detector). The detection limits of the method were lower than 5 μg/kg in the food simulant. Received: 5 June 1996     

The inherent genotoxic potency of food mutagens and other heterocyclic and carbocyclic aromatic amines and corresponding azides

 Relationships between the chemical structure of aromatic amines (including heterocyclic food mutagens) and genotoxic potency were originally established on the basis of Salmonella mutagenicity data. These relationships are reviewed. We report here that also quite different genotoxic effects, namely the binding to deoxyguanosine-3′-phosphate (dGp), hypoxanthine phosphoribosyl-transferase (HPRT) mutations, and sister chromatid exchange in Chinese hamster cells follow essentially the same structure-activity relationships. The heterocyclic amines of the aminoimidazoquinoline, aminoimidazoquinoxaline and aminoimidazopyiridine types unite a number of structural characteristics which endow these compounds, or rather their reactive species, presumed to be nitrenium ions, with an extremely high inherent genotoxic potency. This conclusion is supported by experimental work and by calculations of electronic properties of these compounds and their nitrenium ions. Received: 23 April 1998     

Influence of extraction on the determination of vitamin B6 in food by HPLC

 The effect of the extraction procedure on the total vitamin B₆ content in different food matrices is presented. The use of hydrochloric acid, sulphuric acid and trichloroacetic acid, combined with enzymatic hydrolysis using several commercial enzyme preparations, was tested. The results of two liquid chromatographic methods (isocratic and gradient elution) and those of microbiological assay were compared. Reproducible results were achieved using a mineral acid followed by β-glucosidase/acid phosphatase hydrolysis for extraction of the same sample and only a few unknown compounds caused interference in the reversed-phase chromatogram. However, the conversion efficiency of the enzyme preparation should always be checked. The liquid chromatographic method indicated that there was a higher vitamin content compared with the microbiological assay. This is probably due to a lower growth response of Saccharomyces with pyridoxamine and pyridoxal rather than pyridoxine. Received: 5 March 1996/Revised version: 22 July 1996     

Influence of extraction on the determination of vitamin B6 in food by HPLC

 The effect of the extraction procedure on the total vitamin B₆ content in different food matrices is presented. The use of hydrochloric acid, sulphuric acid and trichloroacetic acid, combined with enzymatic hydrolysis using several commercial enzyme preparations, was tested. The results of two liquid chromatographic methods (isocratic and gradient elution) and those of microbiological assay were compared. Reproducible results were achieved using a mineral acid followed by β-glucosidase/acid phosphatase hydrolysis for extraction of the same sample and only a few unknown compounds caused interference in the reversed-phase chromatogram. However, the conversion efficiency of the enzyme preparation should always be checked. The liquid chromatographic method indicated that there was a higher vitamin content compared with the microbiological assay. This is probably due to a lower growth response of Saccharomyces with pyridoxamine and pyridoxal rather than pyridoxine. Received: 5 March 1996/Revised version: 22 July 1996     

HPLC determination of trimethoprim in meat and milk with an SPE preseparation procedure

 An HPLC method in combination with solid-phase extraction and dual wavelength detection has been developed in order to monitor trimethoprim residues in meat and milk. The extraction recoveries for the maximal residual limit concentration in food (50 ng/g) were 86±5% for meat and 73±6 % for milk samples. The limits of determination of trimethoprim were 5 ng/g at 229 nm and about 15 ng/g at 280 nm. The method has been developed in response to specific needs of food control laboratories. This publication introduces the assay for milk and meat samples. Received: 27 March 1996/Revised version: 23 July 1996     

Superchilling of food: A review

Food preservation is very important for the safety and the reliability of the product. Superchilling as used for preserving foods, has been defined as a process by which the temperature of a food product is lowered to 1-2 °C below the initial freezing point. Fresh and high quality food products are in great demand worldwide. Temperature is a major factor determining the shelf life and quality of food products. Fish and meat are perishable food commodities, where better and more advanced preservation technology is needed. Deterioration of these foods mainly occurs as a result of chemical, enzymatic and bacteriological activities leading to loss of quality and subsequent spoilage. Storing food at superchilling temperature has three distinct advantages: maintaining food freshness, retaining high food quality and suppressing growth of harmful microbes. It can reduce the use of freezing/thawing for production and thereby increase yield, reduce energy, labour and transport costs. The study on the growth mechanism of ice crystals, modelling and computer simulation of foods during superchilling and superchilling storage is needed.     

Utilization of nitrogen compounds in garnacha must inoculated with killer strains of Saccharomyces cerevisiae

 The utilization of nitrogen compounds in garnacha must inoculated with two Saccharomyces cerevisiae killer strains (ICV D47, ICV K1M) was studied. These samples were compared with control must fermented with wild yeast. The results show that in the samples where the inoculated killer yeasts predominated, excretion of amino acids took place from an early stage in the alcoholic fermentation (25% of sugars consumed). This did not occur in the control sample. At the end of the fermentation process, the excretion of amino acids in the samples inoculated with the killer strains was high, so these wines would be considered more unstable from the microbiological point of view. Received: 29 March 1999     

Presence of cork-taint responsible compounds in wines and their cork stoppers

 Chloroanisoles [2,4-dichloroanisole, 2,4,6-trichloroanisole (TCA), 2,3,4,6-tetrachloroanisole and pentachloroanisole], chlorophenols [2,4,6-trichlorophenol, 2,3,4,6-tetrachlorophenol and pentachlorophenol (PCP)] and guaiacol were detected in red and white cork-tainted bottled wines. These compounds were also found in the cork stoppers from those bottles. A significant correlation was found between TCA in wines and TCA in cork stoppers, and between TCA in wine and intensity of cork taint. At low levels of TCA, the presence of guaiacol or PCP were also found to influence in cork taint. Received: 29 November 1999     

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     

Development and application of a heterologous internal standard for polymerase-chain-reaction-based detection of Campylobacter jejuni and Campylobacter coli in foods

 A heterologous internal standard, termed "mimic", was developed for the polymerase chain reaction (PCR)-based detection of Campylobacter jejuni and Campylobacter coli in food. Mimic was designed to contain a heterologous DNA fragment of plasmid pUC18, flanked by a primer binding site, identical to the bacterial target DNA. Application of mimic in the PCR permitted its co-amplification together with the bacterial DNA with similar efficiency. As the length of the amplified products differed, they were easily detectable by agarose gel electrophoresis. The presence or absence of the mimic PCR product was indicative of the efficacy of the PCR. The use of approximately 60 mimic molecules per reaction was optimal for determining the reliability of the diagnostic PCR assays without decreasing the detection limit. This system for the detection of the two species of Campylobacter was successfully applied in routine food surveillance. Received: 4 January 1999     

Physicochemical characteristics of weaning food formulated from different blends of cereal and soybean

 The physicochemical characteristics of porridges of adult and infant weaning food prepared from fermented blends of cereals and soybean are reported in this study. The pH of the formulated flour blends was less than 5.0 within 12 h of fermentation with F15B and F410B, attaining a pH of 3.4. Titratable acidity, which was not detected in unfermented samples, increased as the fermentation progressed. An increase in the gross energy content was noted with F25B, yielding a value of 439.0 kcal/100 g. All the fermented flour blends reconstituted well in boiling water and the water-holding capacity also increased. There was a slight difference in the bulk densities of the loose and packed flour. The index of gelatinization of unfermented formulated flour ranged from 82 (B. U.) in F15A to 334 (B. U.) in F310A. However, there was an increase in the values of the index of gelatinization of the blends at the end of the fermentation. All the blends had relatively low viscosities except F310B. Based on the parameters considered in this study, F15B is recommended as a potential infant and adult food. Received: 10 June 1996 / Revised version: 17 September 1997     

Amyl alcohols as compounds indicative of raw cocoa bean quality

 Amyl alcohols were isolated from raw cocoa beans by steam distillation and quantified by means of gas chromatography. Concentration ratios between primary and secondary amyl alcohols and between alcohols, aldehydes and acetates were calculated. These ratios indicate the fermentation degree and possible staling of cocoa beans. Hence, flavour quality of raw cocoa beans may be measured and the method used for industrial quality control. Received: 27 October 1995/Revised version: 25 March 1996     

Evolution of capillary zone electrophoresis profiles of methanol soluble compounds during fish chilling: relation to freshness

 A methodology for assessing fish freshness based on the capillary zone electrophoresis analysis of methanol-soluble compounds extracted from white muscle is presented. Two fatty fish species, mackerel (Scomber scombrus) and horse mackerel (Trachurus trachurus), and two lean species, hake (Merluccius merluccius) and blue whiting (Micromesistius poutassou), were studied. Peaks were characterized by their retention times and intensities relative to imidazole, and the four species showed similar electrophoretic profiles. The intensity of peaks with short retention times decreased during chilling storage, and that of peaks with longer retention times increased during storage. The variability and calibration of the instrumental method were established by correlation analysis with the time of chilling and sensory measurements. Received: 22 April 1999 / Revised version: 9 July 1999     

Amyl alcohols as compounds indicative of raw cocoa bean quality

 Amyl alcohols were isolated from raw cocoa beans by steam distillation and quantified by means of gas chromatography. Concentration ratios between primary and secondary amyl alcohols and between alcohols, aldehydes and acetates were calculated. These ratios indicate the fermentation degree and possible staling of cocoa beans. Hence, flavour quality of raw cocoa beans may be measured and the method used for industrial quality control. Received: 27 October 1995/Revised version: 25 March 1996