Agropur cooperative

Epigallocatechin-3-gallate (EGCG), an antioxidant present in green tea, could play an important role in the prevention of cancer. However, its bioavailability is low due to its instability in gastrointestinal environment. β-Lactoglobulin (β-Lg), the major protein in whey, is known for its capacity to bind bioactive molecules and could protect them from oxidation. Interaction between β-Lg and EGCG was investigated at pH 7.0 and 4.0 using fluorescence and Fourier transform infrared spectroscopy, and its impact on EGCG antioxidant activity was determined using the ferric-reducing antioxidant power method. EGCG bound to native or heat-denatured β-Lg by hydrogen bonding and possibly hydrophobic interaction at pH 7.0 and 4.0. The affinity of EGCG for heat-denatured β-Lg at pH 7.0 was greater than for native β-Lg and greater than its affinity for the protein in either state at pH 4.0. Complexing with β-Lg decreased the antioxidant activity of EGCG under all conditions investigated. However, the protein provided limited protection of EGCG against degradation over time, the heat-denatured form being slightly more effective at pH 7.0. This study provides insight into the characteristics of β-Lg binding with a flavonoid and its impact on the antioxidant activity of flavonoids in food systems.

The lactic acid bacterium Streptococcus thermophilus is widely used by the dairy industry for its ability to transform lactose, the primary sugar found in milk, into lactic acid. Unlike the phylogenetically related species Streptococcus salivarius, S. thermophilus is unable to metabolize and grow on galactose and thus releases substantial amounts of this hexose into the external medium during growth on lactose. This metabolic property may result from the inability of S. thermophilus to synthesize galactokinase, an enzyme of the Leloir pathway that phosphorylates intracellular galactose to generate galactose-1-phosphate. In this work, we report the complementation of Gal(-) strain S. thermophilus SMQ-301 with S. salivarius galK, the gene that codes for galactokinase, and the characterization of recombinant strain SMQ-301K01. The recombinant strain, which was obtained by transformation of strain SMQ-301 with pTRKL2TK, a plasmid bearing S. salivarius galK, grew on galactose with a generation time of 55 min, which was almost double the generation time on lactose. Data confirmed that (i) the ability of SMQ-301K01 to grow on galactose resulted from the expression of S. salivarius galK and (ii) transcription of the plasmid-borne galK gene did not require GalR, a transcriptional regulator of the gal and lac operons, and did not interfere with the transcription of these operons. Unexpectedly, recombinant strain SMQ-301K01 still expelled galactose during growth on lactose, but only when the amount of the disaccharide in the medium exceeded 0.05%. Thus, unlike S. salivarius, the ability to metabolize galactose was not sufficient for S. thermophilus to simultaneously metabolize the glucose and galactose moieties of lactose. Nevertheless, during growth in milk and under time-temperature conditions that simulated those used to produce mozzarella cheese, the recombinant Gal(+) strain grew and produced acid more rapidly than the Gal(-) wild-type strain.