Impact of the Inoculation Method of Geotrichum candidum, Used as Biocontrol Agent, on T-2 Toxin Produced by Fusarium sporotrichioides and F. langsethiae during the Malting Process

In malt production, steeping and germination steps offer favorable environmental conditions for fungal proliferation when barley is already contaminated by Fusarium species, T-2 toxin producers. However, the use of G. candidum as a biocontrol agent can prevent this proliferation. Indeed, in previous work, a correlation between phenyllactic acid (PLA) production by G. candidum and the reduction in Fusarium sporotrichioides and F. langsethiae growth and T-2 toxin concentration was demonstrated. In the present study, to improve the efficiency of G. candidum, the effects of the inoculum concentration and the inoculation method of G. candidum on PLA and T-2 toxin concentrations were evaluated. First, co-culture experiments with Fusarium species and G. candidum were conducted in a liquid synthetic medium. The results showed that inoculation of G. candidum in the freeze-dried form at 0.4 g/L allowed the production of PLA from the second day of incubation associated with a reduction in T-2 toxin concentration of 82% and 69% produced by F. sporotrichioides and F. langsethiae, respectively. Moreover, the activated form of G. candidum at 0.4 g/L enhanced PLA concentration leading to better T-2 toxin reduction. Second, experiments were conducted on artificially infected barley kernels with both Fusarium species under conditions mimicking the malting step. As for co-culture experiments, the use of the activated form of G. candidum was established as the best condition for T-2 toxin concentration reduction for a 3 day malting period.

Metabolomics analyses of the combined effects of lactic acid bacteria and Penicillium camemberti on the generation of volatile compounds in model mold-surface-ripened cheeses

The flavor of white mold cheese is attributed to the formation of aroma compounds associated with complex effects of bacteria and fungi, resulting in difficulties in flavor design for new cheeses. This study aimed to identify the microbial basis of flavor by identifying the combined effects of LD type lactic acid bacteria (LAB) starters and Penicillium camemberti on the generation of metabolites during the ripening process. Metabolomics analyses were performed on three model cheeses: normal cheese, no-mold cheese with only LAB, and no-LAB cheese with only white mold. Aroma compounds and their potential precursors were analyzed using headspace solid-phase microextraction-gas chromatography/mass spectrometry (GC/MS) and solvent extraction-GC/MS, respectively. Measurements during ripening and multivariate analyses on the data revealed the relationship between the microorganisms and metabolic activities, which were classified into four groups: metabolites generated by LAB and degraded by P. camemberti; metabolites generated by P. camemberti and degraded or inhibited by LAB; metabolites generated by P. camemberti and enhanced by LAB; and metabolites exhibiting no interaction between P. camemberti and LAB. The characteristic compounds in LAB and white mold cheeses were mainly products of sugar and protein metabolism, respectively. The involvement of fatty acids, methyl ketones, and secondary alcohol metabolic pathways in the late-ripening stage was confirmed, and the profiles of volatile metabolites contributing to the characteristic aroma of the white mold cheese in the fermentation process were also confirmed.

Solid-state fermentation of palm kernels by Yarrowia lipolytica modulates the aroma of palm kernel oil

Solid-state fermentation with Yarrowia lipolytica was applied to palm kernels (PK) with the aim to modulate the aroma of palm kernel oil (PKO) obtained after kernel roasting. The results showed that, the metabolic activities of Y. lipolityca brought about significant changes to the volatile profile of obtained PKO either by providing thermal reaction reactants or by directly contributing aroma compounds. After fermentation, a decreased content in glucose (60%) while an elevated amount (7-fold) in free amino acids was found in PK, which further impacted the formation of volatile compounds by influencing the Maillard reaction and Strecker degradation during roasting. More Strecker aldehydes and N-heterocyclic compounds were formed in PKO derived from fermented PK especially after intensified roasting. In addition, the catabolism of Y. lipolytica imparted some distinct volatile compounds such as 2-phenylethanol to the obtained PKO. However, the lipase excreted by Y. lipolytica hydrolysed PK lipids and released 5-fold more free fatty acids in fermented PKO, relative to the blank and control PKO, which likely contributed to the off-flavor. On the basis of all volatile categories, principal component analysis (PCA) clearly separated the fermented PKO from the blank and control PKO, with light roasted, fermented PKO being correlated with acids, alcohols and aliphatic aldehydes; medium and dark roasted, fermented PKO tending to be dominated by pyrroles, pyrazines and furanones, which is in correspondence with sensory changes of PKO. This study demonstrated that combining fermentation with roasting could provide a novel way to modulate the volatile composition and aroma of PKO.

The effect of raw milk microbial flora on the sensory characteristics of Salers-type cheeses

The sensory characteristics of Salers Protected Denomination of Origin raw-milk cheeses are linked to the biochemical composition of the raw material (milk) and to the resultant microbial community. To evaluate the influence of the microbial community on sensory characteristics, Salers-type cheeses were manufactured with the same pasteurized milk, reinoculated with 3 different microbial communities from 3 different filtrates from microfiltered milks. Each cheese was subjected to microbial counts (on selective media), biochemical tests, and volatile and sensory component analyses at different times of ripening. Adding different microbial communities to specimens of the same (biochemically identical) pasteurized milk lead to different sensory characteristics of the cheeses. Cheeses with fresh cream, hazelnut, and caramel attributes were opposed to those with fermented cream, chemical, and garlic flavors. The aromatic compounds identified (esters, acids, alcohols, and aldehydes) in these cheeses were quite similar. Nevertheless, one milk was distinguished by a higher content of acetoin, and lower 2-butanone and 3-methylpentanone concentrations. Over the production period of 1 mo, the different cheeses were characterized by the same balance of the microbial population assessed by microbial counts on different media. This was associated with the stability of some sensory attributes describing these cheeses. Nevertheless, there was no linear correlation between microbial flora data and sensory characteristics as measured in this study.

Interests in Geotrichum candidum for cheese technology

The wide genotypic and phenotypic diversity of Geotrichum candidum strains does not facilitate its classification as yeast or a yeast-like fungus that is still a matter of debate. Whatever its classification, G. candidum possesses many different metabolic pathways that are of particular interest to the dairy industry. G. candidum is of importance in the maturation of cheese, and much is known about its direct contribution to cheese ripening and flavour formation. Its diverse metabolic potential means that G. candidum can play an important role in the ripening of many soft and semi-hard cheeses and make a positive contribution to the development of taste and aroma. It may also influence the growth of other microorganisms, both valuable and detrimental. The significance of the presence of G. candidum in cheese depends on the particular type of production and on the presence of biotypes featuring specific types of metabolism. However, in situ metabolic pathways involved in cheese ripening and their regulations are mainly unknown. The information available provides a good understanding of the potential of G. candidum strains that are used in cheese manufacture, and permits a better choice of strain depending on the characteristics required. The biochemical activities of G. candidum and its application in the dairy industry are presented in this review.

An iterative sensory procedure to select odor-active associations in complex consortia of microorganisms: application to the construction of a cheese model

The aim of this study was to develop and validate an iterative procedure based on odor assessment to select odor-active associations of microorganisms from a starting association of 82 strains (G1), which were chosen to be representative of Livarot cheese biodiversity. A 3-step dichotomous procedure was applied to reduce the starting association G1. At each step, 3 methods were used to evaluate the odor proximity between mother (n strains) and daughter (n/2 strains) associations: a direct assessment of odor dissimilarity using an original bidimensional scale system and 2 indirect methods based on comparisons of odor profile or hedonic scores. Odor dissimilarity ratings and odor profile gave reliable and sometimes complementary criteria to select G3 and G4 at the first iteration, G31 and G42 at the second iteration, and G312 and G421 at the final iteration. Principal component analysis of odor profile data permitted the interpretation at least in part, of the 2D multidimensional scaling representation of the similarity data. The second part of the study was dedicated to 1) validating the choice of the dichotomous procedure made at each iteration, and 2) evaluating together the magnitude of odor differences that may exist between G1 and its subsequent simplified associations. The strategy consisted of assessing odor similarity between the 13 cheese models by comparing the contents of their odor-active compounds. By using a purge-and-trap gas chromatography-olfactory/mass spectrometry device, 50 potent odorants were identified in models G312, G421, and in a typical Protected Denomination of Origin Livarot cheese. Their contributions to the odor profile of both selected model cheeses are discussed. These compounds were quantified by purge and trap-gas chromatography-mass spectrometry in the 13 products and the normalized data matrix was transformed to a between-product distance matrix. This instrumental assessment of odor similarities allowed validation of the choice of G312 as the best 10-strain ecosystem.

Comparison of Volatile Compounds Produced in Model Cheese Medium Deacidified by Debaryomyces hansenii or Kluyveromyces marxianus

The aroma of a deacidified cheese medium is the result of the overall perception of a large number of molecules belonging to different classes. The volatile compound composition of (60%) cheese medium (pH 5.8) deacidified by Debaryomyces hansenii (DCM(Dh)) was compared with the one deacidified by Kluyveromyces marxianus (DCM(Km)). It was determined by dynamic headspace extraction, followed by gas chromatography separation and quantification as well as by mass spectrometry identification. Whatever the media tested, a first class of volatile compounds can be represented by the ones not produced by any of the yeasts, but some of them are affected by K. marxianus or by D. hansenii. A second class of volatile compounds can be represented by the ones produced by K. marxianus, which were essentially esters. Their concentrations were generally higher than their thresholds, explaining the DCM(Km) global fruity odor. A third class can be represented by the ones generated by D. hansenii, which were essentially methyl ketones with fruity, floral (rose), moldy, cheesy, or wine odor plus 2-phenylethanol with a faded-rose odor. The impact of methyl ketones on the DCMDh global flavor was lower than the impact of 2-phenylethanol and even negligible. Therefore, the global faded-rose odor of D. hansenii DCM can be explained by a high concentration of 2-phenylethanol.

Aroma compound production in cheese curd by coculturing with selected yeast and bacteria

The microorganisms involved in cheese ripening produce various volatile compounds and induce typical flavors that contribute to cheese variety. To investigate aroma compound generation of cheese microflora, we used a dynamic headspace-gas chromatography-mass spectrometry analysis. To obtain good sensitivity and repeatability of quantification, dynamic headspace conditions and sample preparation were first optimized and led to an extraction set up in which samples were heated at 60 degrees C and diluted with water without pH adjustment. Then three different yeasts and three Geotrichum candidum commonly used in mold surface ripened cheeses were studied in pure culture in a cheese model medium. Thirty-nine cocultures of these three yeasts, the three G. candidum, and five bacteria were studied in the same medium to assess the interaction between microorganisms on aroma compound production. Twenty-four volatile compounds belonging to different chemical classes (alcohols, aldehydes, esters, sulfides, terpenes) were identified and quantified. Yeasts and especially Kluyveromyces lactis produced large amounts of alcohols, aldehydes, esters, and terpenes when cultured alone or in association. Geotrichum candidum and especially G. candidum strain G3 generated the largest amount of sulfides when cultured alone or in association. Finally, bacteria also produced aroma compounds but, except for Brevibacterium linens strain B5, which produced dimethyl trisulfide and ketones, no specific trend in the production of particular aroma compounds could be evidenced.

Interactions between yeasts and bacteria in the smear surface-ripened cheeses

In the initial phase of ripening, the microflora of bacterial smear surface-ripened cheeses such as Limburger, Taleggio, Brick, Münster and Saint-Paulin and that of surface mould-ripened cheeses such as Camembert and Brie may be similar, but at the end of the ripening, bacteria such as Brevibacterium spp., Arthrobacter spp., Micrococcus spp., Corynebacterium spp. and moulds such as Penicillium camemberti are, respectively, the dominant microorganisms. Yeasts such as Candida spp., Cryptococcus spp., Debaryomyces spp., Geotrichum candidum, Pichia spp., Rhodotorula spp., Saccharomyces spp. and Yarrowia lipolytica are often and variably isolated from the smear surface-ripened cheeses. Although not dominant within the microorganisms of the smear surface-ripened cheeses, yeasts establish significant interactions with moulds and especially bacteria, including surface bacteria and lactic acid bacteria. Some aspects of the interactions between yeasts and bacteria in such type of cheeses are considered in this paper.

The growth, properties and interactions of yeasts and bacteria associated with the maturation of Camembert and blue-veined cheeses

The growth of yeasts and bacteria were monitored during the maturation of Camembert and blue-veined cheese produced in Australia. Yeasts were prominent throughout maturation, growing to 10(5)-10(9)/g, depending on the manufacturer. Debaryomyces hansenii predominated, but there were lesser, inconsistent contributions from Yarrowia lipolytica. Of the non-lactic acid bacteria, Acinetobacter species were significant during the maturation of Camembert but not blue-veined cheeses, and grew to 10(6)-10(8) cfu/g. Staphylococcus and Micrococcus species were consistently isolated from the cheeses with Staphylococcus xylosus growing to 10(5)-10(9) cfu/g, depending on the product. Lactic acid bacteria (10(7)-10(9) cfu/g) were present throughout maturation but were not identified. Interactions between the various yeasts and bacterial isolates were examined. Several strains of D. hansenii exhibited killer activity but not against Y. lipolytica. None of the yeasts were antagonistic towards the bacteria but some strains of D. hansenii enhanced the growth of Y. lipolytica and S. xylosus. The yeast and bacterial isolates exhibited various degrees of extracellular proteolytic and lipolytic activities.