Michel Britten

Microentrapment of probiotic bacteria in a Ca2 +-induced whey protein gel and effects on their viability in a dynamic gastro-intestinal model

Entrapping probiotic bacteria in gels with ionic cross-linking is typically achieved with polysaccharides (alginate, pectin, carraghenan). In this study, whey proteins were used for this purpose by carrying out the Ca2+-induced gelation of pre-heated whey protein isolate (WPI). A Lactobacillus rhamnosus cell suspension was added in a denatured WPI solution in a 30 : 70 volume ratio. Gelation was carried out by extrusion of the cell suspension in a CaCl2 solution. Beads of ∼3 mm diameter were formed. The population in the beads was 8.0 × 108 cells g−1. Entrapment efficiency in gel beads was 96%, with a survival level of 23%. Scanning electron microscopy of beads before freeze-drying showed a tight protein network containing encapsulated Lb. rhamnosus cells homogeneously distributed throughout the matrix. The survival to freeze-drying of the bead-entrapped cells was 41%. Viability of microentrapped cells in a dynamic gastro-intestinal (GI) model was studied and the results were compared to free cells freeze-dried in a milk-based cryoprotective solution, as well as in a pre-denatured WPI solution. Results showed that protein gelation provided protection against acidic conditions in the stomach after 90 min, as well as against bile after 30, 60 and 90 min in the duodenum. Moreover, the milk-based cryoprotective solution was equally effective after 90 min in the duodenum. It is concluded that the gelation of whey proteins induced by Ca2+ ions can protect the cells against adverse conditions of the GI system. However, certain stages in the entrapment process, particularly extrusion in the solution of CaCl2, still need to be optimized in order to reduce the mortality of the cells during gelation.

Deposit formation on heated surfaces: effect of interface energetics

Deposit formation was measured in a model laboratory plant in which whole milk was in contact with a heated surface at 100 °C for 1 h. The effect of the interfacial properties of various poly mer-coated surfaces on the amount and the adhesion strength of deposit was determined. The nature of the surface influenced the formation of deposit only slightly, but had a large effect on its adhesion strength. From correlation analysis, the polar contribution to surface energy was identified as the main factor influencing the deposit adhesion strength. These results suggest that the type of interactions at the surface govern the ease of removal of deposit.

Fat-free yogurt made using a galactose-positive exopolysaccharide-producing recombinant strain of Streptococcus thermophilus

To prevent textural defects in low-fat and fat-free yogurts, fat substitutes are routinely added to milk. In situ production of exopolysaccharides (EPS) by starter cultures is an acknowledged alternative to the addition of biothickeners. With the aim of increasing in situ EPS production, a recombinant galactose-positive EPS(+) Streptococcus thermophilus strain, RD-534-S1, was generated and compared with the parent galactose-negative EPS(+) strain RD-534. The RD-534-S1 strain produced up to 84 mg/L of EPS during a single-strain milk fermentation process, which represented 1.3 times more than the EPS produced by strain RD-534. Under conditions that mimic industrial yogurt production, the starter culture consisting of RD-534-S1 and (EPS(-)) Lactobacillus bulgaricus L210R strain (RD-534-S1/L210R) led to an EPS production increase of 1.65-fold as compared with RD-534-S1 alone. However, the amount of EPS produced did not differ from that found in yogurts produced using an isogenic starter culture that included the parent S. thermophilus strain RD-534 and Lb. bulgaricus L210R (RD-534/L210R). Moreover, the gel characteristics of set-style yogurt and the rheological properties of stirred-style yogurt produced using RD-534-S1/L210R were similar to the values obtained for yogurts made with RD-534/L210R. In conclusion, it is possible to increase the production of EPS by ropy S. thermophilus strains through genetic engineering of galactose metabolism. However, when used in combination with Lb. bulgaricus for yogurt manufacture, the EPS overproduction of recombinant strain is not significant.

Effect of Buttermilk Made from Creams with Different Heat Treatment Histories on Properties of Rennet Gels and Model Cheeses

Although many studies have reported negative effects on cheese properties resulting from the use of buttermilk in cheese milk, the cause of these effects has not been determined. In this study, buttermilk was manufactured from raw cream and pasteurized cream, as well as from a cream derived from pasteurized whole milk. Skim milks with the same heat treatments were also manufactured to be used as controls. Compositional analysis of the buttermilks revealed a pH 4.6-insoluble protein content approximately 10% lower than that of the skim milk counterparts. Milk fat globule membrane (MFGM) proteins remained soluble at pH 4.6 in raw cream buttermilk; however, when heat was applied to cream or whole milk before butter making, MFGM proteins precipitated with the caseins. Rennet gel characterization showed that MFGM material in the buttermilks decreased the firmness and increased the set-to-cut time of rennet gels, but this effect was amplified when pasteurized cream buttermilk was added to cheese milk. The microstructure of gels was studied, and it was observed that gel appearance was very different when pasteurized cream buttermilk was used, as opposed to raw cream buttermilk. Model cheeses manufactured with buttermilks tended to have a higher moisture content than cheeses made with skim milks, explaining the higher yields obtained with buttermilk. Superior retention of MFGM particles was observed in model cheeses made from pasteurized cream buttermilk compared with raw cream buttermilk. The results from this study show that pasteurization of cream and of whole milk modifies the surface of MFGM particles, and this may explain why buttermilk has poor coagulation properties and therefore yields rennet gels with texture defects.

Characterization of adsorptive fouling on ultrafiltration membranes by peptides mixtures using contact angle measurements

Abstract Severe protein fouling is usually encountered during the ultrafiltration of enzymatic hydrolysates from casein on polysulfone membranes. Previous work on the effect of physico-chemical parameters on the flux decline during ultrafiltration suggested that the adsorption of peptides onto the membrane material is responsible for the early stages of fouling and also strongly affect the subsequent evolution of the flux. In the present work, surface energy parameters of polysulfone (PS) and polyethersulfone (PES) membranes were determined from advancing contact angle measurements with three different solvents on hydrated membranes. Changes in membrane surface energetics upon adsorption of components from casein hydrolysates were measured. Adsorption experiments were performed at three pH levels (6,0, 8,0, and 10,0), with and without EDTA. The non-polar contribution to the surface energy was slightly lower for PES than PS. Both materials were highly monopolar, exhibiting a strong electron donor component. The interfacial pressure associated with peptide adsorption was higher for PS than PES, suggesting a higher tendency to peptide adsorption on PS. Interfacial pressure decreased with increasing pH during adsorption on PS, while the reverse trend was observed for PES. The addition of EDTA had very slight effects on the interfacial pressure values of PES, but it reduced that of PS at pH lower than 10,0. Water molecules in the hydration layer showed a high degree of orientation (>85%) for both membranes, suggesting a strong entropic contribution to the adsorption of peptides on PS and PES.

Effect of pepsin-treated bovine and goat caseinomacropeptide on Escherichia coli and Lactobacillus rhamnosus in acidic conditions.

Caseinomacropeptide (CMP) is a 7-kDa phosphoglycopolypeptide released from κ-casein during milk digestion and in the cheesemaking process. The objective of the study was to analyze the effect of pepsin-treated CMP from cow and goat milk on the resistance of Escherichia coli and Lactobacillus rhamnosus during acid stress. Bacterial cells in the exponential growth phase were suspended in acidified phosphate buffered saline with or without pepsin-treated CMP. Viability was determined during a 90-min incubation period. Pepsin-treated CMP exhibited bactericidal activity at pH 3.5 when added in a dose-dependent manner to E. coli, decreasing survival by more than 90% within 15 min at 0.25 mg/mL. At pH >4.5, the bactericidal activity disappeared, indicating that pepsin-treated CMP was efficient at low pH only. The effectiveness of pepsin-treated CMP at pH 3.5 was not affected by the presence of glycoconjugates linked to CMP or by the bovine or caprine origin of milk. In contrast, L. rhamnosus, a probiotic, was more resistant to acid stress when pepsin-treated bovine or caprine CMP was added to the media. Viability reached 50% after 60 min of incubation at pH 3 compared with 5% survival in the media without added pepsin-treated CMP. Neither glycosylation extent nor sequence variations between CMP from bovine milk and caprine milk affected the protective activity of hydrolyzed CMP toward L. rhamnosus. This suggests that encrypted bioactive peptides released by the pepsin treatment of CMP had an antibacterial effect on E. coli in acidic media, but improved the resistance of L. rhamnosus to acid stress. The peptide fragment accountable for bactericidal activity is the N-terminal region κ-casein f(106-124).

A model heat-exchange apparatus for the investigation of fouling of stainless steel surfaces by milk I. Deposit formation at 100 °C

A model heat-exchange apparatus was used to investigate the factors affecting deposit formation from milk on a stainless steel surface at 100 °C. The structure and composition of the deposits were determined by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and chemical analysis after solution in alkali. The effects of changing the pH, preheating and skimming of the milk were similar to those observed in a small-scale continuous ultra high temperature plant. The time course of deposit formation showed that a lag phase did not occur, but the deposit which formed after more than 45 min was more porous than that formed after shorter times. Most (50–90%) of the fresh deposit was readily removed by sonication, leaving a sublayer richer in minerais than the original. The results provide evidence for the two-layer model for deposit formation proposed by Tissier & Lalande (1986).

Encapsulation of hydrophobic aroma in whey protein nanoparticles

Aroma-loaded nanoparticles (d < 300 nm) were prepared by cross-linking denatured whey protein through pH-cycling. The effect of nanoparticulation conditions and aroma concentration on the physicochemical characteristics of nanoparticles and aroma release profile was studied. Better retention of aroma was observed when ethyl hexanoate was added before nanoparticle formation. The highest aroma retention was obtained for nanoparticles produced at pH 5.0 and 5.5 without calcium addition. These nanoparticles are characterized by a less compact and more porous internal structure allowing a higher loading of aroma. Increasing aroma concentration increased the diameter and the voluminosity of the aroma-loaded nanoparticles. The percentage of aroma retention showed an increase from 7% to 24% over the tested concentration range while the value averaged 2% for native or denatured whey protein. Encapsulation of ethyl hexanoate in whey protein nanoparticles reduced the mass transfer of aroma at the surface of the matrix and improved its retention.

On the conventional cross-flow microfiltration of skim milk for the production of native phosphocaseinate

Abstract The use of conventional microfiltration for the production of native phosphocaseinate was studied at 50 °C using a microporous ceramic membrane of 0.22 μm pore diameter at a flow velocity of 6.9 m s−1 and a transmembrane pressure of 190 kPa. Flux declined by approximately 50% from an initial value of 180 L h−1 m−2 over the course of 3× concentration. Batch diafiltrations after dilution of the retentate with distilled water or 0.08 M Nad at pH 7.0 raised the flux for both media to the initial value or higher, followed by a similar pattern of flux decline during the concentration step. The procedure allowed the preparation of native phosphocaseinate containing 79% protein with a casein to total protein nitrogen ratio over 0.91.

Influence of dissolved gases and heat treatments on the oxidative degradation of polyunsaturated fatty acids enriched dairy beverage.

The combined effect of dissolved gas composition and heat treatment on the oxidative degradation of a dairy beverage enriched with 2% linseed oil was studied. The dairy beverage was saturated with air, nitrogen, or a nitrogen/hydrogen mixture (4% hydrogen) before pasteurization or sterilization. Saturation with either nitrogen or a nitrogen/hydrogen mixture decreased the dissolved oxygen concentration in dairy beverages (Delta = 7.7 ppm), and the presence of hydrogen significantly reduced the redox potential (Delta = 287 mV). Heat treatments also reduced the oxygen content and redox potential, sterilization being more effective than pasteurization. Both pasteurization and sterilization induced the oxidative degradation of the beverages. On average, the propanal concentration increased by a factor of 2.3 after pasteurization and by a factor of 6.2 after sterilization. However, during storage, sterilized beverages resisted light-induced oxidation better than unheated or pasteurized beverages. Furthermore, saturation with nitrogen or a nitrogen/hydrogen mixture significantly reduced oxidative degradation and provided some protection against color changes during storage.