Glp 21 ph meter

The pH levels of protein hydrolysates were read at room temperature using a GLP 21 pH meter (Crison Instruments S.A., Barcelona, España). Technical buffer solutions of pH 4.00, 7.00, and 10.00 were used to calibrate the equipment before the measurements.
The browning intensity of protein hydrolysates were determined following a method described by Laroque et al. [22 (link)] with some modifications. Briefly, protein hydrolysates were diluted 10 times using distilled water, and the absorbance was measured at 420 nm using a Jasco V-750 spectrophotometer (Madrid, España).

Fluorescence spectra were measured using a Shimadzu (Tokyo Japan) Model RF-5000 spectrofluorophotometer with a Model DR-15 controller unit and a 150 W Xenon lamp as a light source. The UV-visible absorption measurements of the samples were carried out on a Shimadzu UV/Vis-160 spectrophotometer. The pH was measured with a Crison (Barcelona, Spain) GLP 21 pHmeter. Transmission electron microscopy (TEM) was performed on a ZEISS EM-900 device (Oberkochen, Germany). X-ray powder diffraction (XRD) patterns were recorded on a Siemens (Berlin, Germany) D500 X-ray powder diffractometer equipped with graphite monochromatized high-intensity Cu-Kα radiation (λ = 154.050 pm). The refraction index of the samples was determined with an Abbé refractometer (refractometer.com/abbe-refractometer). The infrared spectra (IR) of the samples were measured using a FT-IR Perkin-Elmer Model 1700 spectrometer and the KBr pellets technique with 100 mg of KBr and approximately 0.5 mg of the substance that is being studied.

The pH of the samples was measured as a parameter of spoilage, since some hydroperoxide decomposition products are acidic. The pH value of six grams of beef patties was determined according Fan et al. [30 (link)] in triplicate using an electronic pH meter (Crison Instruments, GLP 21 pH METER, Barcelona, Spain). The experiment was performed on days 0, 6 and 13.

Before analysis, the samples were placed in a heated water bath to reach room temperature and, following that, they were thoroughly mixed by gentle inversion of the sample container multiple times without causing frothing. Milk pH was measured using a glass electrode with a built-in temperature sensor (5014T electrode, Crison, Barcelona, Spain) in a Crison GLP 21 pH-meter (Barcelona, Spain), which was calibrated with standard buffer solutions at pH 4.0 and 7.0 according to the manufacturer’s instructions. The electrical conductivity of the samples was measured by a GLP 31 conductometer (Crison Instruments, Barcelona, Spain) using a Sodium Ion-Selective Electrode 9650, calibrated with 147 μS/cm, 1413 μS/cm, and 12.88 mS/cm buffer solutions. Refractive index and Brix value were determined using a digital refractometer (DR6000-T, Krüss, Hamburg, Germany) set at 20 °C.

OMW was first centrifuged (Eppendorf 5804, Hamburg, Germany) for 30 min at 4000 rpm in order to separate the remaining solid particles, obtaining a liquid phase with a pH value of 4.85 (GLP-21 pH-meter, Crison, Barcelona, Spain). Afterwards, based on the method described by Bazzarelli et al. [41 (link)], acidification with HCl (37%) was performed by 0.003% (v/w) addition to reach pH = 1.8. After 24 h, the pretreated OMW was used in a two-stage membrane treatment process including ultrafiltration (UF) followed by nanofiltration (NF).
Both UF and NF were performed in batch concentration mode using a stainless steel HP4750 high-pressure stirred cell of 300 mL capacity supplied by Sterlitech Corporation (Kent, WA, USA). For this purpose, an UF flat sheet polysulfone membrane (US100, 100 kDa, Microdyn-Nadir, Wiesbaden, Germany) was used and the permeate obtained from UF was entered to the same stirred cell module equipped with a NF flat sheet polyamide-thin film composite membrane (NF90, 200 Da, Dow Filmtec, Minneapolis, MN, USA). Transmembrane pressures of 5 bar and 10 bar for UF and NF processes, respectively, were supplied by a nitrogen cylinder. Membrane surface area was 14.6 cm² and both UF and NF treatments were performed at room temperature. The retentate solution obtained by NF treatment was used as the inner aqueous phase in the optimal formulation of W/O/W nanoemulsions.

The pH of the supernatant phase of each PA14 and Newman monoculture and coculture biofilm was measured using a GLP 21 pH meter (Crison®, Hospitalet de Llobregat, Barcelona). For this experiment, biofilms were grown in a volume of 3 mL in 6-well polystyrene plates. pH measurements were taken in triplicate for each culture medium and time point, directly in the microplate well where the biofilm was growing.

Analyses of the chemical oxygen demand (COD), total suspended solids (TSS), electrical conductivity (EC), and pH were performed following standard methods [16 (link)]. The methods were triplicated and performed with analytical-grade reagents (99% minimum purity).
A Helios Gamma UV–visible spectrophotometer (Thermo Fisher Scientific, Granada, Spain) was used for the analyses of the COD. EC and pH were analysed with a Crison GLP31 conductivity meter and a Crison GLP21 pH meter. TSS were analysed following a standard filtration method [16 (link)].
Also, the effluent was characterised according to the saturation index (SI), determined following the ASTM International method [17 ].