Here, students determine the total phenolic content in beers using the Folin–Ciocalteu assay. The Folin–Ciocalteu reagent is a yellow complex, in an alkaline medium; it reacts with phenols and non-phenolic reducing substances to form a blue complex. Quantitative analysis was carried out using absorbance measured at 765 nm (standard method) and digital images (proposed method). In the proposed method, samples were placed in a 96 microwell plate; then, a plate image was obtained with a flatbed scanner. The ImageJ plugin ReadPlate extracts red (R)-values from all wells of the scanned image at the same time; then, R-values were exported to a spreadsheet that converted these values to absorbances. Using absorbances obtained with R-values, the spreadsheet also provides standard calibration plots, limits of detection (LODs), limits of quantification (LOQs), and beer sample total phenolic contents. The proposed method and the standard method were compared using two-tailed significance tests; students observed that both methods provide equivalent results, that both methods were linear in a 2–10 mg L–1 gallic acid concentration range, and that both methods had close LODs and LOQs, 0.55 and 1.70 gallic acid mg L–1, respectively. Beer sample total phenolic contents were compared using one-tailed significance tests. This laboratory experiment provides an effective instrumental alternative to spectrophotometric methods, which can be especially beneficial, where purchasing and maintaining a spectrophotometer is a challenge, and it also presents to the students one-tailed and two-tailed significant tests.
Few studies were performed on the volatile components’ properties of propolis from native Brazilian stingless bees. This work sought to extract, chemically characterize, and evaluate of the antimicrobial properties of volatile oils from propolis of the stingless bees Melipona quadrifasciata quadrifasciata (MQ) and Tetragonisca angustula (TA). The volatile oils from the pulverized propolis samples were obtained in a Clevenger apparatus and characterized by gas chromatography coupled to mass spectrometry (GC/MS) and to flame ionization detector (GC‐FID). All samples showed promising activity against Mycoplasma pneumoniae, with MICs from 103 to 224 μg mL−1, nonetheless, they were less active against cell walled bacteria. Activity against Candida species was moderate, but one MQ sample showed a MIC value of 103 μg mL−1 against C. tropicallis. Oils’ fractionation showed no improvement in antibacterial activity. The promising antibacterial effect against this microorganism is likely related to the synergism between the components of the volatile oils.
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