In aqueous solutions of concentrated lactic acid (>30 wt%), oligomers of lactic acid can produce via the intermolecular esterification of lactic acid, and thus the amount of titratable acidity is often less than that of total acidity. In this paper, the hydrolysis of oligomers of lactic acid was investigated at different temperatures in detail. The experimental results showed that when the content of total acidity is lower than 30 wt% in aqueous lactic acid solutions, the oligomers of lactic acid can been completely hydrolyzed to give monomeric lactic acid and the monomeric lactic acid is stable at temperatures of 8–33 °C, thus the total acidity is equivalent to the titratable acidity and can be determined quantitatively by a direct potentiometric titration. The results for determining total acidities were in good agreement with those obtained from a back titration by a two tailed t -test. Compared with the back titration, the novel method's procedure is simpler and reducing sugars do not interfere with measurements. The relative standard deviation ( RSD ) was less than 0.7% and 1.1% for the direct potentiometric and back titrations, respectively, which indicated that the direct potentiometric titration has a higher precision.
The production of 5-ethoxymethylfurfural (EMF) from cheap and abundant glucose feedstock has attracted a great deal of attention because of its potential applications. Herein, three porous tetra-component metal oxide solid acid catalysts (SO 4 2− /ZFSA-A, SO 4 2− /ZFSA-F-10, SO 4 2− /ZFSA-F-20) containing both Brønsted and Lewis acid sites have been successfully synthesized and can efficiently catalyze the synthesis of EMF from glucose. Particularly, the SO 4 2− /ZFSA-A, SO 4 2− /ZFSA-F-10, and SO 4 2− /ZFSA-F-20 catalysts were prepared under the template of sodium alginate, 10% F127, and 20% F127, respectively. The prepared catalysts were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), N 2 -adsorption–desorption isotherm, X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG), temperature-programmed desorption of ammonia (NH 3 -TPD), and pyridine infrared spectra (Py-FTIR). The characterization results indicated that all three catalysts have a porous structure. The pore distribution of the SO 4 2− /ZFSA-A catalyst is mainly macroporous and mesoporous, while the SO 4 2− /ZFSA-F-10 and SO 4 2− /ZFSA-F-20 catalysts are mainly mesoporous. The SO 4 2− /ZFSA-F-20 catalyst exhibited the highest catalytic performance due to its large specific surface area, desirable mesoporous structure, high total acidity (especially strong and super strong acidity), and appropriate Lewis/Brønsted acid ratio. By applying SO 4 2− /ZFSA-F-20, the formation of EMF from glucose was successfully catalyzed and EMF was obtained at a yield of 34.56% after 12 h at 160°C. This yield is much higher compared with most other results achieved by solid acids catalysis. In addition, the reaction parameters such as reaction temperature, reaction time, glucose amount, and catalyst dosage were shown to have a significant effect on the catalytic performance. Notably, the SO 4 2− /ZFSA-F-20 catalyst exhibited good reusability, as its activity did not significantly diminish after four rounds of repeated experiments and was successfully reactivated after calcination.
Jie Xiong,Ya-ying Wang,Muhammad Yousaf,Dong-mei Liu
Summary The fermented vegetable is a long-history food, and the addition of probiotics in recent years has pushed it in a novel direction. This study explored the impact of a Direct-vat set starter (DVS), comprising Lactiplantibacillus plantarum MC14 and Leuconostoc mesenteroides , on pickled radish (PR). The addition of DVS triggered a rapid reduction in pH and titratable acidity, leading to lowered nitrite levels and increased viable count. Textural analysis showed that DVS increased the springiness and chewiness of PR by about 55.07% and 25.73%, respectively. Organic acid analysis unveiled eight compounds. The addition of probiotics did not significantly affect the antioxidant properties but was able to significantly enhance the XO inhibitory activity of PR. DVS decreased undesirable flavor compounds such as isothiocyanates and sulfides, while elevating esters, aldehydes, and acids. Our results suggested that DVS is a promising starter culture that can enhance the texture and flavor of fermented vegetables.
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