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Previous studies have shown that isolated β-(1,3 and 1,6)-d-glucans and related alkali-extracted fractions from the cell wall of Saccharomyces cerevisiaeare able to complex with zearalenone in vitro (affinity up to 50%) and thus may reduce the bioavailability of toxins in the digestive tract. The complexation mechanisms involve cooperative interaction between the two chemical entities that can be computed by Hill's model. Various linear or branched soluble or insoluble β-d-glucans were evaluated to elucidate their roles in the adsorption mechanisms under three pH conditions (3.0, 6.0, and 8.0) found in the digestive tract. A constant quantity of each β-d-glucans (1 mg/ml) was mixed at 39°C with increasing amounts of zearalenone (2 to 100 μg/ml), and the amount of bound toxin was measured. Acidic and neutral conditions gave the highest affinity rates (64 to 77%) by β-(1,3)-d-glucans, whereas alkaline conditions decreased adsorption except when β-(1,6)-d-glucan side chains were branched on β-(1,3)-d-glucans. Alkaline conditions appear to impede the active three dimensional conformation of β-d-glucans and favor single helix and/or random coil structures. Study of the equilibrium between β-d-glucan–bound and free toxins revealed that two types of chemical interactions occur during toxin complexation with β-d-glucans, identified as weak chemical linkages such as hydrogen and van der Waals bonds.

Cell walls of yeasts and bacteria are able to complex with mycotoxins and limit their bioavailability in the digestive tract when these yeasts and bacteria are given as feed additives to animals. To identify the component(s) of the yeast cell wall and the chemical interaction(s) involved in complex formation with zearalenone, four strains of Saccharomyces cerevisiaediffering in their cell wall glucan and mannan content were tested. Laboratory strains wt292, fks1, and mnn9 were compared with industrial S. cerevisiaestrain sc1026. The complex-forming capacity of the yeast cell walls was determined in vitro by modelling the plots of amount of toxin bound versus amount of toxin added using Hill's model. A cooperative relationship between toxin and adsorbent was shown, and a correlation between the amount of β-d-glucans in cell walls and complex-forming efficacy was revealed (R2= 0.889). Cell walls of strains wt292 and mnn9, which have higher levels of β-d-glucans, were able to complex larger amounts of zearalenone, with higher association constants and higher affinity rates than those of the fks1 and sc1026 strains. The high chitin content in strains mnn9 and fks1 increased the alkali insolubility of β-d-glucans from isolated cell walls and decreased the flexibility of these cell walls, which restricted access of zearalenone to the chemical sites of the β-d-glucans involved in complex formation. The strains with high chitin content thus had a lower complex-forming capacity than expected based on their β-d-glucans content. Cooperativity and the three-dimensional structure of β-d-glucans indicate that weak noncovalent bonds are involved in the complex-forming mechanisms associated with zearalenone. The chemical interactions between β-d-glucans and zearalenone are therefore more of an adsorption type than a binding type.

The determination ofkLaby a gas balance method coupled with sulphite oxidation is compared for three kinds of processes (stirred tank, bubble column and fixed-bed column reactors) with a gassing-in and with a classical chemical sulphite oxidation method. The mathematical relations required for the determination of thekLavalue are detailed. In coalescing gas–liquid conditions, the values calculated by the three methods are shown to be comparable. The gas balance method is more rapid than either the steady-state gassing-in or the chemical sulphite reaction rate measurement methods. It is also well adapted for three-phase systems (gas–liquid–solid) in which the non-coalescing effects of sulphite solution are reduced by solid interferences.The determination ofkLaby a gas balance method coupled with sulphite oxidation is compared for three kinds of processes (stirred tank, bubble column and fixed-bed column reactors) with a gassing-in and with a classical chemical sulphite oxidation method. The mathematical relations required for the determination of thekLavalue are detailed. In coalescing gas–liquid conditions, the values calculated by the three methods are shown to be comparable. The gas balance method is more rapid than either the steady-state gassing-in or the chemical sulphite reaction rate measurement methods. It is also well adapted for three-phase systems (gas–liquid–solid) in which the non-coalescing effects of sulphite solution are reduced by solid interferences.