Joseph-Pierre Guiraud

High-pressure homogenisation of raw bovine milk. Effects on fat globule size distribution and microbial inactivation

Whole raw milk was processed using a 15 L h−1 homogeniser with a high-pressure (HP) valve immediately followed by a cooling heat exchanger. The influence of homogenisation pressure (100–300 MPa) and milk inlet temperature Tin (4°C, 14°C or 24°C) on milk temperature T2 at the HP valve outlet, on fat globule size distribution and on the reduction of the endogenous flora were investigated. The Tin values of 4–24°C led to milk temperatures of 14–33°C before the HP valve, mainly because of compression heating. High Tin and/or homogenisation pressure decreased the fat globule size. At 200 MPa, the d4.3 diameter of fat globules decreased from 3.8±0.2 (control milk) to 0.80±0.08 μm, 0.65±0.10 or 0.37±0.07 μm at Tin=4, 14°C or 24°C, respectively. A second homogenisation pass at 200 MPa (Tin=4°C, 14°C or 24°C) further decreased d4.3 diameters to about 0.2 μm and narrowed the size distribution. At all Tin tested, an homogenisation pressure of 300 MPa induced clusters of fat globules, easily dissociated with SDS, and probably formed by sharing protein constituents adsorbed at the fat globule surface. The total endogenous flora of raw milk was reduced by more than 1 log cycle, provided homogenisation pressure was ⩾200 MPa at Tin=24°C (T2∼60°C), 250 MPa at Tin=14°C (T2∼62°C), or 300 MPa at Tin=4°C (T2∼65°C). At all Tin tested, a second pass through the HP valve (200 MPa) doubled the inactivation ratio of the total flora. Microbial patterns of raw milk were also affected; Gram-negative bacteria were less resistant than Gram-positive bacteria.

Enterotoxin production by staphylococci isolated from foods in France

Two hundred and thirteen Staphylococcus aureus and 51 other staphylococcal strains were isolated from 121 foodstuffs of current consumption and two cutaneous samples. Their ability to produce staphylococcal enterotoxins was tested and S. aureus strains were biotyped. The S. aureus strains (30.5%) produced at least one of the five known staphylococcal enterotoxins whereas coagulase negative staphylococci did not produce any of them. The raw milk cheeses analysed were primarily contaminated by strains belonging to animal or unspecified biovars. Only 15.9% of the S. aureus strains isolated from these products produced enterotoxins whereas 43% were found to be enterotoxigenic amongst the S. aureus strains isolated from the other foodstuffs. The scheme of biotyping used seems to be reliable, allowing the classification of 73.7% of the strains. S. aureus strains of human biovar origin were most often enterotoxigenic and enterotoxin C was the predominant type identified. It was produced by 66% of the enterotoxigenic strains, singly or in combination with other enterotoxins. Approximately 77% of the human strains also produced enterotoxin C, which is an amazing epidemiological distinctive feature of the strains studied. Moreover ELISA tests used in this work exhibit problems of specificity.

Microbiological and biochemical study of coffee fermentation

The coffee fermentation microflora were rich and mainly constituted of aerobic Gram-negative bacilli, with Erwinia and Klebsiella genuses at the highest frequencies. The best population increase was observed with lactic acid bacteria and yeasts, whereas those microorganisms that counted on a pectin medium remained constant during the fermentation step. Qualitatively, lactic acid bacteria belonged mainly to Leuconostoc mesenteroides species but the others microflora were relatively heterogeneous. The microorganisms isolated on pectin medium were Enterobacteriaceae, identified as Erwinia herbicola and Klebsiella pneumoniae, not reported as strong pectolytic strains. Throughout coffee fermentation, 60% of the simple sugars were degraded by the total microflora and not specifically by pectolytic microorganisms.

Study of the phenotypic and genotypic biodiversity of potentially ochratoxigenic black aspergilli isolated from grapes.

Ochratoxin A (OTA) is a mycotoxin with nephrotoxic, carcinogenic, teratogenic and immunotoxic effects, naturally found in agricultural products including grapes and wine. Black Aspergillus species (Section Nigri) are mainly responsible for OTA accumulation in wine grapes and in particular Aspergillus carbonarius and Aspergillus niger aggregate. The biodiversity of potentially ochratoxigenic strains of black aspergilli from different French vineyards in the southern Mediterranean region of Languedoc-Roussillon was studied. One hundred and eighty nine black strains were isolated from grapes and studied according to harvest year, production zone, grape variety and pre-harvest treatment of grapevines. The strains were identified and classified in two groups according to macroscopic and microscopic characters; these were called the A. carbonarius representative group and the A. niger aggregate representative group. Members of each group were classified in subgroups based on macroscopic morphological colony characters. Strain biodiversity was studied according to phenotypic and genotypic characterization and to the OTA production of selected strains on PDA medium. After identification was confirmed by specific PCR using primer pair ITS1/CAR and ITS1/NIG, 24 potential ochratoxigenic strains belonging to A. carbonarius and A. niger aggregate were discriminated by RAPD-PCR using 8 different OPC primers. The use of specific primers supported the identification based on phenotypic and morphological characters. RAPD-PCR patterns demonstrated a considerable diversity among the strains. Clustering among A. niger aggregate strains was associated with production zone and harvest year, but not grape variety or pre-harvest treatment. Clustering among A. carbonarius strains was not associated with any of the above parameters. OTA production of strains on culture medium seemed to correlate better with morphological characters than with genotypic profiles. No clear relation could be established between phenotypic and genotypic characters of the studied black aspergilli.

Enzymatic browning and biochemical alterations in black spots of pineapple [Ananas comosus (L.) Merr.]

Penicillium funiculosum Thom. was consistently isolated from pineapple-infected fruitlet (black spots). Polyphenol oxidase, peroxidase, and laccase activities were determined in extracts from contiguous and infected fruitlets. Healthy fruitlets showed a rather high level of polyphenol oxidase (optimum pH 7.0), and this activity was tremendously increased (×10) in contiguous infected fruitlets. Furthermore, infected fruitlets also exhibited laccase activity (optimum pH 4.0), while peroxidase was rather constant in both fruitlets. Browning reactions were attributed to qualitative and quantitative modifications of the enzymatic equipment (polyphenol oxidase and laccase) (p<0.0001). In infected fruiltets, sucrose and L-malic acid were present at significantly lower amounts than in healthy ones, likely owing to fungal metabolism (p<0.0001), whereas cell wall material was three times higher, which could be viewed as a defense mechanism to limit expansion of the mycelium. RID=”” ID=”” Correspondence to: S. Avallone; email: avallone@siarc.cnearc.fr

Influence of Yeast Flocculation on the Rate of Jerusalem Artichoke Extract Fermentation

Variations in residual sugar composition have been observed during Jerusalem artichoke extract fermentations by using Saccharomyces diastaticus NCYC 625, a flocculating yeast strain. In batch cultures, these differences were due to the inulin polymer size distribution of the extracts: measurements of enzymatic activities on different polymerized substrates have shown that the hydrolysis and fermentation yield decreased when the fructose/glucose ratio of the extract increased. Inulin hydrolysis appeared to be the limiting factor of the fermentation rate. A comparison of continuous and batch cultures with the same extract showed that fermentability differences were related to the structure and size of the yeast flocs. This led to an hydrolysis selectivity of the inulin polymers according to their size: the chemostat culture in which the floc average size was larger gave longer chained residual sugars.

Fructose syrups and ethanol production by selective fermentation of inulin

Jerusalem artichoke is a favorable substrate for inulin or fructose syrup production. The sugar content and the fructose ratio of inulin depend on various factors, particularly on the date of harvest. Incomplete fermentation of extracts by selected yeasts allows the production of inulin with increased fructose content. The yeast strains (Saccharomyces cerevisiae, S. diastaticus...) are chosen for their ability to ferment sucrose and inulin small polymers, but not easily inulin large polymers. A good increase in the fructose ratio and a good yield in residual sugars can be obtained with the better strains. After fermentation and acid or enzymatic hydrolysis, extracts from “early” and “late” harvested tubers lead to syrups of good quality containing up to 95% and 90% of fructose respectively. This fermentative enrichment process is competitive with others (for example, chromatographic enrichment), is appropriate to raw extracts, simplifies the purification steps, and also permits the simultaneous benefit of production of by-products in the form of ethanol and yeast (in addition to the pulps). Unhydrolyzed inulin polymers with high fructose content can be recovered by this selective fermentation.