The concentrations of 68 heterocyclic compounds produced by heating a series of amino acid-ribose and lecithin-amino acid-ribose reaction mixtures at 140°C for 1 h were measured, after headspace collection on Tenax GC, by capillary gas chromatography-mass spectrometry. The amino acids studied were glycine, lysine and cysteine, and the classes of heterocyclic compounds quantified were pyrazines (14), pyridines (7), jkrans (14), thiophenes (26) and thiazoles (7). The studies showed that the addition of lecithin to the simple Maillard reaction mixtures led to a reduction in the concentrations of many of the heterocyclic compounds formed in these reactions. These findings support an earlier observation that phospholipids in foodstuffs, such as meat products, could exert a quenching effect on the amount of heterocyclic compounds formed in Maillard reactions during the cooking of food. A number of volatile compounds formed by the reaction of lipid degradation products with Maillard reaction intermediates were also identified.

The technique of direct probe mass spectrometry (DPMS) has been applied to the classification of 40 strains of lactic acid bacteria from refrigerated vacuum-packed beef, pork, lamb and bacon. Relationships between strains were examined by multi-variate statistical techniques using sets of ions selected for reproducibility and sample discrimination. Five groups were distinguished which corresponded closely to those detected in a previous numerical taxonomic study. Two groups contained all 12 representatives of a cluster of unidentifiable non-aciduric streptobacteria whose sub-division is supported by other taxonomic evidence. All twenty-one strains from a cluster of aciduric streptobacteria provisionally identified with Lactobacillus sake were contained in two further groups. The sub-division of these aciduric strains revealed by DPMS has not been verified by other techniques and requires further investigation. The fifth group contained Leuconostoc strains. The study demonstrates the value of DPMS in confirming and clarifying classification schemes obtained by conventional methods.

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Direct probe mass spectrometry of single colonies of different bacteria produced spectra which could be distinguished by calculations based on the intensities of 25 selected ions. Direct probe mas:; spectrometry has enormous potential as a rapid technique for characterizing micro-organisms. I N T R O D U C T I O NPyrolysis mass spectrometry (p.m.s.) (Meuzelaar & Kistemaker, 1973) is a proven method for discriminating micro-organisms, even at levels of relatedness below species (Gutteridge & Norris, 1979). 'The technique requires several permanent modifications to a standard mass spectrometer which has consequently led to the development of purpose-built instrumentation (Meuzelaar et al., 1976).This communication describes the use of an alternative technique, direct probe mass spectrometry (d.p.m.s.), in which samples are thermally degraded adjacent to the ion source using an accessory available on most mass spectrometers. Anhalt & Fenselau (1975) have already demonstrated the production of spectra from freeze-dried bacteria using d.p.m.s. but did not consider whether the quantitative aspects of the spectra could be used to differentiate micro-organisms in an equivalent manner to p.m.s. Risby & Yergey (1976, 1978) examined the temperature-dependent profiles of the decomposition products of bacteria using a direct probe technique which they called linear programmed thermal degradation mass spectrometry, but their method cannot be easily developed into a rapid and routine analysis. M E T H O D SOrganisms and growth conditions. Pseudomonas ueruginosa NCIB 8295, P. acidovorans NCIB 9681 , P. chlororaphis NCIB 9392, P. aureofaciens NCIB 9030 and P. maltophilia NCIB 9203 were cultured on plate count agar (Oxoid) for 24 h at 25 "C.Preparation ofsamples for mass spectrometry. Less than 50 pg (wet wt) of whole cells from a single colony was taken from the surface of a culture plate using a platinum wire and placed inside a clean quartz-glass tube attached to the stainless steel probe.Mass spectrometry. An LKB 9000 magnetic sector instrument was used with an electron energy of 70 eV, an accelerating voltage of 3.5 kV and a source temperature of 270 "C. The probe was inserted into the high vacuum of the instrument and temperature programmed at 34 "C min-l from ambient to an instrument reading of 120 "C, although the actual temperature of thermal degradation is thought to be in the region of 200 to 300 "C. Spectra were recorded on ultraviolet sensitive paper (Oscilloscript 'D ', Agfa-Gevaert Ltd) t

The technique of direct probe mass spectrometry is described together with its application to the analysis of 50 strains of Gram-negative bacteria representing seven genera. Thirty-six of these strains were analysed in duplicate, and the 72 spectra held in a computer library. The intensities of 63 ions from each of the spectra were analysed by discriminant analysis techniques and all seven groups could be distinguished by as few as six ions. A set of 29 strains, 15 of which already in the data base, were used as unknowns to challenge the library on two separate occasions. The success rate of these challenges was 97% and 90% using the full spectra, but only 72% and 62% using the selection of six ions. Possible explanations for this are discussed as well as the scope and limitations of the method as a means of characterizing micro-organisms.