Psoralea corylifolia seeds contain many bioactive compounds commonly used in traditional Chinese medicine. In this study, the antibacterial activity and possible mechanism of P. corylifolia seed ethanol extract (PCEE) against foodborne pathogens were investigated. Both methicillin-resistant Staphylococcus aureus (MRSA) and Listeria monocytogenes had similar minimum inhibitory concentrations and minimum bactericidal concentrations of PCEE at 50 and 100 μg/mL, respectively. Furthermore, elevated OD₂₆₀, protein concentration, and electric conductivity indicated irreversible damage to the cytoplasmic membranes of PCEE-treated cells. Indeed, the treated cells displayed disrupted membranes, incomplete and deformed shapes, and rupture as visualized by scanning electron microscopy. Multidrug-resistance efflux pump gene expression was also analyzed by quantitative reverse transcription PCR. Although the mdrL, mdrT, and lde genes of L. monocytogenes and the mepA gene of MRSA were upregulated, there was no significant difference that indicated an attempt by the efflux pumps to discharge PCEE. MRSA norA expression and abcA expression were significantly downregulated (p < 0.05). A possible mechanism for PCEE may be to cause an energy depletion, either by inhibiting adenosine triphosphate binding or by disturbing the proton gradient, resulting in membrane damage.

The effects of microbial transglutaminase (MTGase) cross-linking on the physicochemical characteristics of individual caseins were investigated. MTGase was used to modify three major individual caseins, namely, κ-casein (κ-CN), α_S-casein (α_S-CN) and β-casein (β-CN). The SDS-PAGE analysis revealed that MTGase-induced cross-linking occurred during the reaction and that some components with high molecular weights (>130 kDa) were formed from the individual proteins κ-CN, α_S-CN and β-CN. Scanning electron microscopy (SEM) and particle size analysis respectively demonstrated that the κ-CN, α_S-CN and β-CN particle diameters and protein microstructures were larger and polymerized after MTGase cross-linking. The polymerized κ-CN (~749.9 nm) was smaller than that of β-CN (~7909.3 nm) and α_S-CN (~7909.3 nm). The enzyme kinetics results showed K_M values of 3.04 × 10⁻⁶, 2.37 × 10⁻⁴ and 8.90 × 10⁻³ M for κ-CN, α_S-CN and β-CN, respectively, and, furthermore, k_cat values of 5.17 × 10⁻⁴, 1.92 × 10⁻³ and 4.76 × 10⁻² 1/s, for κ-CN, α_S-CN and β-CN, respectively. Our results revealed that the cross-linking of β-CN catalyzed by MTGase was faster than that of α_S-CN or κ-CN. Overall, the polymers that formed in the individual caseins in the presence of MTGase presented a higher molecular weight and larger particles.

Purpurogallin (PPG) is a phenolic compound known for its high antioxidant properties in plant-based food materials. However, there is no easy and reliable method for direct determination of PPG in brewed beverages owing to its hydrophobicity, which makes it hard to separate from the background hydrophobic components. Therefore, a method employing solid-phase extraction (SPE) and liquid chromatography-mass spectrometry (LC-MS) was developed for detection and quantification of PPG in brewed beverages, and PPG content was quantified in commercial coffee, cocoa, and tea samples. The limits of detection and quantification were 71.8 and 155.6 ng/g dry weight (dw), respectively. The recovery with SPE was 26.6%. When combined with acetonitrile extraction (ANE), the recovery was 6.8%, higher than 2.6% with water extraction (WTE). Test tube extractions were better than moka pot brewing (MPB) for PPG quantification. Total PPG content of ground coffees prepared by ANE, WTE, and MPB ranged between 635 and 770, 455 and 630, and 85 and 135 ng/g dw, respectively. PPG was detected in two English breakfast tea samples (335–360 ng/g dw) using WTE, but not in cocoa samples. ANE showed higher (p < 0.05) PPG levels, but WTE (r = 0.55, p < 0.01) correlated better with MPB than ANE (r = 0.43, p < 0.01). The result indicated that WTE is the best method to determine PPG in brewed beverages. This work demonstrated that PPG was significant in brewed coffee, and our pioneer study in developing the method for beverage sample preparation and LC-MS analysis has made possible industrial applications and provided new perspectives for future research.

The effects of ultrasonic treatment (UT)-assisted transglutaminase (TGase) catalysis on the physicochemical properties of individual α_s-casein (α_s-CN), β-casein (β-CN), and κ-casein (κ-CN) were investigated. After 60 min of incubation at 30 °C, α_s-CN, β-CN, and κ-CN were cross-linked with TGase (6.0 units/mL), and high molecular weight polymers (>200 kDa) were formed. The use of TGase in conjunction with UT (20 kHz, power of 400 W, and amplitude 20%) led to an increase in the rate of α_s-CN, β-CN, and κ-CN polymerization compared to the individual casein that contained TGase but did not undergo UT. SDS-PAGE scrutiny showed that the intensities of α_s-CN, β-CN, and κ-CN incubation with regard to TGase and UT at 30 °C for 60 min noticeably decreased to 5.66 ± 0.39, 3.97 ± 0.43, and 26.07 ± 1.18%, respectively (p < 0.05). Particle size analysis results indicated that the molecule size appropriation for the cross-linking of α_s-CN, β-CN, and κ-CN ranged from 6000 to 10,000 nm after 60 min incubation with TGase and UT. Transmission electron microscopy investigation showed network structures of cross-linking α_s-CN, β-CN, and κ-CN were formed from α_s-CN, β-CN, and κ-CN, respectively. As our results show, the comprehensive utilization of TGase and UT will be a superior method for the polymerization of α_s-CN, β-CN, and κ-CN.