Georges Corrieu

The Relative Effect of Milk Base, Starter, and Process on Yogurt Texture: A Review

Yogurt is a milk curd produced all over the world, obtained by a lactic fermentation of a milk base enriched with milk proteins, and sometimes sugars and thickeners. One of the most important sensorial attributes for yogurt is texture, which could be assessed by sensory or instrumental analysis. A lot of work has been published in studying the contribution of milk base, starter, and process on yogurt texture in order to develop new textures, or simply to reduce fat content, or the level of addition of protein and thickener in milk. However, these studies are limited to only a few factors. The topic of this review was to synthesize the data of literature, with the aim of extracting and classifying factors on the basis of their influence on yogurt texture. Three factors, milk base heating, starter, and yogurt shearing after fermentation, respectively, play a key role in the elaboration of texture. The control of these three parameters allows the improvement of the textural attributes of yogurts by 2 to 15 times.

Effect of Tween 80 and oleic acid on ligninase production by Phanerochaete chrysosporium INA-12

Ligninase production by Phanerochaete chrysosporium INA-12 was increased when culture medium was supplemented with sorbitan polyoxyethylene monooleate (Tween 80), oleic acid alone or emulsified with Tween 80. Results showed that fatty acids contained in Tween 80 were dissimilated by P. chrysosporium INA-12 in glycerol-containing cultures. Lipase activity (EC 3.1.1.3) was detected in the culture filtrate after one day of incubation. Ligninase production was markedly enhanced and fermentation time for maximum activity was reduced in presence of exogenous oleic acid emulsified with Tween 80 (0.04%, w/v). Ligninase activity in stationary and agitated conditions were 22.4 and 19.3 nkat ml−1respectively. Agitated cultures containing Tween 80 or emulsified oleic acid converted [ring-U-14C]lignin to14CO2at a rate in proportion to ligninase activity. The effect of additives as well as the development of suitable culture conditions for ligninase production by P. chrysosporium INA-12 using submerged agitated cultures is discussed.

Method of quantifying the loss of acidification activity of lactic acid starters during freezing and frozen storage.

We have developed a method to quantify the resistance to freezing and frozen storage of lactic acid starters, based on measuring the time necessary to reach the maximum acidification rate in milk (tm) using the Cinac system. Depending on the operating conditions, tm increased during the freezing step and storage. The loss of acidification activity during freezing was quantified by the difference (delta tm) between the tm values of the concentrated cell suspension before and after freezing. During storage at -20 degrees C, linear relationships between tm and the storage time were established. Their slope, k, allowed the quantitation of the decrease in acidification activity during 9-14 weeks of frozen storage. The method was applied to determine the resistance to freezing and frozen storage of four strains of lactic acid bacteria and to quantify the cryoprotective effect of glycerol.

Vanillin as a product of ferulic acid biotransformation by the white-rot fungus Pycnoporus cinnabarinus I-937: identification of metabolic pathways

Abstract Ferulic acid metabolism was studied in cultures of the white-rot fungus Pycnoporus cinnabarinus I-937. After 6 days of growth, during the secondary metabolism of the fungus, the concentration of vanillin in the culture medium reached a maximum of 64 mg l −1 , corresponding to a molar yield of 27.5%. During the biotransformation process, the propenoic side chain of ferulic acid was oxidatively cleaved to yield vanillic acid, further decarboxylated to 2-methoxyhydroquinone. In addition, two reductive routes were evidenced, involving the conversion of ferulic and vanillic acids into coniferyl and vanillyl alcohols, respectively. The biotransformation process was not easily controlled since the P. cinnabarinus species is known to release laccase into the growth medium. When produced, the enzyme countered the vanillin formation by promoting the polymerization of ferulic acid into lignin-like polymers.

Automatic method to quantify starter activity based on pH measurement

A new method for measuring the activity of mesophilic and thermophilic lactic acid bacteria is proposed based on measuring the pH of cultures at extremely short intervals (30–90 s) during growth and calculating several kinetic parameters, i.e. the maximum acidification rate (V m ), the time and pH at which V m occurred, the time or pH range during which the observed rates were greater than V m /2. These were easily calculated by coupling the pH meter to a microcomputer.

Controlled production of Camembert-type cheeses. Part I: Microbiological and physicochemical evolutions

A holistic approach of a mould cheese ripening is presented. The objective was to establish relationships between the different microbiological and biochemical changes during cheese ripening. Model cheeses were prepared from pasteurized milk inoculated with Kluyveromyces lactis, Geotrichum candidum, Penicillium camemberti and Brevibacterium linens under aseptic conditions. Two cheese-making trials with efficient control of environmental parameters were carried out and showed similar ripening characteristics. K. lactis grew rapidly between days 1 and 6 (generation time around 48 h). G. candidum grew exponentially between days 4 and 10 (generation time around 4.6 d). Brevi. linens also grew exponentially but after day 6 when Pen. camemberti mycelium began developing and the pH of the rind was close to 7. Its exponential growth presented 3 phases in relation to carbon and nitrogen substrate availability. Concentrations of Pen. camemberti mycelium were not followed by viable cell count but they were evaluated visually. The viable microorganism concentrations were well correlated with the carbon substrate concentrations in the core and in the rind. The lactose concentrations were negligible after 10 d ripening, and changes in lactate quantities were correlated with fungi flora. The pH of the inner part depended on NH3. Surface pH was significantly related to NH3 concentration and to fungi growth. The acid-soluble nitrogen (ASN) and non-protein nitrogen (NPN) indexes and NH3 concentrations of the rind were low until day 6, and then increased rapidly to follow the fungi concentrations until day 45. The ASN and NPN indexes and NH3 concentrations in the core were lower than in the rind and they showed the same evolution. G. candidum and Pen. camemberti populations have a major effect on proteolysis; nevertheless, K. lactis and Brevi. linens cell lysis also had an impact on proteolysis. Viable cell counts of K. lactis, G. candidum, Pen. camemberti and Brevi. linens were correlated with the environmental conditions, with proteolytic products and with carbon substrate assimilation. NH3 diffusion from surface to the cheese core during ripening was highly suspected. Interaction phenomena between microorganisms are discussed.

Alcoholic fermentation in winemaking: On-line measurement of density and carbon dioxide evolution

Abstract Linear correlations on the laboratory scale (15 litres) have been made between the concentrations of ethanol and sugars, and the volume of CO 2 released and the density of the fermentation medium. The on-line measurement of these variables in a 700-litre winemaking tank by corresponding sensors (flowmeter and pressure transducers) and application of resulting correlations have led to the real-time determination of the concentrations of ethanol and sugars. The proposed methods were validated by off-line measurements. The method using the volume of CO 2 released is easier to use. It is an accurate means for following alcoholic fermentation in winemaking.

Phospholipid and fatty acid enrichment of Phanerochaete chrysosporium INA-12 in relation to ligninase production

SummaryLigninase activity of Phanerochaete chrysosporium INA-12 was increased when vegetable oils emulsified with sorbitan polyoxyethylene monooleate (Tween 80) were added to growth medium. Maximal enzyme yield was 22.0 nkat·ml-1 in olive oil cultures after 4 days incubation. P. chrysosporium INA-12 was also able to utilize tall oil fatty acids for ligninase synthesis. An extracellular lipase activity was detected during the primary phase of growth in culture containing vegetable oils. On the other hand, ligninase production was 1.5-fold enhanced when olive oil cultures were supplemented with soybean asolectin as a phospholipid source. In cultures supplied with olive oil plus asolectin, P. chrysosporium INA-12 mycelium exhibited a preferential enrichment of oleic acid (C18:1), phosphatidylcholine (PC) and lysophosphatidylcholine (LPC) as compared to lipid-free medium. PC and LPC enrichment was associated with an increased ratio of saturated versus unsaturated fatty acids of phospholipids.

Growth of Debaryomyces hansenii on a bacterial surface-ripened soft cheese

Experimental cheeses were prepared in triplicate from pasteurized milk inoculated with Debaryomyces hansenii under aseptic conditions. Three cheesemaking replicates, with efficient control of environmental parameters (temperature, relative humidity, atmospheric composition) showed similar ripening characteristics. Deb. hansenii grew only on the cheese surface, where its oxygen demand was satisfied, especially during the first 24 h (mean generation time, 5·8 h). Salting in a sterile saturated brine solution reduced its growth and decreased viability. Growth was slower after 48 h because of the decrease in ripening temperature (mean generation time, 94 h). The total count of Deb. hansenii was maximum (≈3×10 7 yeasts/mm 2 ) after 6 d ripening and its viable cell concentration was ≈2×10 6 cfu/mm 2 . This difference was due to the ‘non-viability’ of part of the population. The viable Deb. hansenii concentration was highly correlated ( r 2 >0·95) with the lactate concentration in the inner part and with the surface and inner lactose concentrations, up to day 10 of ripening. This emphasized the importance of the diffusion of carbon substrate from the inner part to the surface of the cheese during ripening. The pH of the inner part depended significantly on the lactate and lactose concentrations. Surface pH was significantly related to inner lactate concentration, temperature and relative humidity. This also demonstrated the controlling role of carbon source diffusion.

Influence of controlled pH and temperature on the growth and acidification of pure cultures of Streptococcus thermophilus 404 and Lactobacillus bulgaricus 398

SummaryThe effect of pH and temperature on the growth and acidification characteristics of Streptococcus thermophilus 404 and Lactobacillus bulgaricus 398 were studied in order to optimize starter production. A quadratic two-variable model for each of these parameters is proposed. Optimal growth conditions were found at pH 6.5 and 40° C for S. thermophilus and pH 5.8 and 44° C for L. bulgaricus. Maximum acidification was obtained at pH values and temperatures higher than the optimal growth conditions. In addition, the two strains were generally more sensitive to pH effect.