Acid Stable Yeast Cell-Associated Tannase with High Capability in Gallated Catechin Biotransformation

Previously, nine tannin-tolerant and tannase-producing yeasts were isolated from Miang; all produced cell-associated tannase (CAT) during growth in tannin substrate. Among which, only CAT from Sporidiobolus ruineniae showed better stability than its purified form. Yet, it is of particular interest to directly characterize CATs from the latter yeasts. In this study, four CATs from yeasts, namely Cyberlindnera rhodanensis A22.3, Candida sp. A39.3, Debaryomyces hansenii A45.1, and Cy. rhodanensis A45.3 were characterized. The results indicate that all CATs were produced within the same production yield (11 mU/mL). Most CATs exhibited similar pH and temperature optima and stabilities, except for CAT from Cy. rhodanensis A22.3. This CAT was assigned as acid-stable tannase due to its unusual optimum pH of 2.0 with pH stability and half-life thermostability in the range of pH 2.0–4.0, and 70 °C, respectively. All CATs demonstrated high substrate specificity toward epigallocatechin gallate and epicatechin gallate, thus forming epigallocatechin and epicatechin, respectively. Moreover, they showed operational stability to repeated use for up to five cycles without loss of the initial activity. Therefore, CATs from these yeasts could be useful for the extraction and biotransformation of tea catechins and related applications.

(E)-1-(3,4-Dimethoxyphenyl)-2-methyl-3-phenylprop-2-en-1-one: A P-Type Acid-Stable Photochromic α-Methylchalcone

The α-methylated chalcone 3 with an electron-donor substituted A-aryl ring and an unsubstituted B-phenyl ring was synthesized by base-catalyzed aldehyde/acetophenone condensation. Compound 3 can be photo-switched from E→Z by irradiation with long-wavelength light λ > 350 nm, whereas irradiation with shorter wavelengths leads to photo-stationary states (PSS) with lower amounts of the Z-isomer. The limiting wavelength for fully equilibrated E⮀Z (PSS = 1) can be achieved around 240 nm. The stability of both E- and Z-isomers at the wavelength-dependent PSS under UV-irradiation between 250 and 350 nm is remarkably high as observed from UV and NMR spectroscopy. Compound 3 is fatigue resistant even after more than 10 days continuous irradiation and is also oxygenation-stable under singlet oxygen sensitization conditions. In remarkable contrast to many other α-methylated chalcones, no change in the E/Z-ratio was detected when PSS samples were treated with Broensted acids. The negative photochromic E→Z switch of 3 is accompanied by a conformational switch from the E-form in its preferred s-trans conformation to the Z-form in a distorted s-cis conformation (Es-c→Zs-t).

Acid Stable α-Amylase Supplementation in Sourdough Enhanced Lactic Acid Bacterial Performance and the Quality of Bread

The effects of an acid-stable α-amylase AmyE from Aspergillus niger on the growth and fermentative ability of sourdough-fermenting Lactobacillus plantarum (LP) were studied and the accompanying changes in the dough rheological properties, specific volume of bread, crumb texture, and the microstructure of dough and bread were analyzed. Addition of acid-stable α-amylase AmyE in the sourdough at a concentration of 15 U/g significantly increased population of LP to 11.61 log CFU/g and accelerated acidification process compared to the commercial α-amylase supplemented sourdough and the control (non-amylase treated) during 24 h fermentation. Tensile test showed that adding AmyE induced an increase in maximal dough resistance to stretching and a decrease in extended distance. From scanning electron microscopy (SEM) images, the microstructure of dough displayed an advanced stage of granules swelling and much more deformation with increasing levels of α-amylase. The microstructure of breads prepared with AmyE was more homogeneous with increasing number of gas cells and resulted in improvement of specific volume and softness of bread crumbs. Given the aforementioned benefits of AmyE for lactic acid bacterial performance and bread quality, this acid-stable α-amylase AmyE has the potential as an improved enzyme additive for sourdough bread production.