Ph meter

Soil pH, soil water content (SWC), total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), total potassium (TK), nitrate nitrogen (NO₃⁻-N), ammonium nitrogen (NH₄⁺-N), available phosphorus (AP), and available potassium (AK) according to routine methods (Bao, 2000 ). Soil pH was determined with a fresh soil to water ratio of 1:5 using a pH meter (Thermo Fisher Scientific, MA, United States). SWC was measured by the oven-drying method. TOC was determined by applying a traditional dichromate oxidation titration. TN was quantified following the Kjeldahl method. The NH₄⁺-N and NO₃⁻-N were determined by extraction with 1M KCl, steam distillation, and titration following the alkaline diffusion method. TP was extracted using 1 M HCl after ignition, and AP were extracted using 0.5 M NaHCO₃, and then, they were measured following the Mo-Sb colorimetric method. AK extracted by 1 M CH₃COONH₄ and TK were measured using the flame photometry method.

Four hundred uniformly sized Nile tilapia; Oreochromis niloticus (O. niloticus) weighing 23.87 ± 0.5 g were procured from El-Abassa Fish Hatchery, Sharkia, Egypt. They were then transported to the Fish Research Unit at Faculty of Veterinary Medicine, Zagazig University, Egypt. Prior to the beginning of the experiment, fish were acclimated to the laboratory rearing conditions for two weeks and received the control diet twice daily (Table 1). After that, the experimental fish were allocated in 20 glass aquaria; 20 fish per aquarium and each aquarium was supplemented with dechlorinated tap water. Along the acclimation and experiment periods, all aquaria were kept in constant rearing conditions involving dissolved oxygen (6.7 ± 0.5 mg/L), which was adjusted via an oxygen meter (YSI Company model 56, Yellow Springs, OH, USA), pH (7.2 ± 0.1), which was measured by pH meter (Orion, Thermo Fisher, San Francisco, CA, USA), temperature (24 ± 2 °C), nitrate (5.4 mg/L), nitrite (0.034 mg/L), ammonium (0.23 mg/L), and photoperiod (12 h light: 12 h darkness). Moreover, the water quality parameters recommended by the American Public Health Association were considered.

The pH meter (Thermo Fisher Scientific, Waltham, MA, USA) was calibrated using buffered standards of pH 4, 7, and 10. Approximately 10 g of each formulation was taken in a suitable container and tapped to remove entrapped air. The pH for all the formulations was recorded, and the probe was washed with the DI water and 70% v/v ethanol after each measurement.

As major factors affecting the growth and development of E. coli, pH and water activity were measured prior to the development of the predictive growth model. pH was measured using a pH meter (Thermo Fisher Scientific, Waltham, Massachusetts, USA) after homogenization by mixing 90 ml of deionized distilled water with 10 g of sample. Water activity was measured using a water activity meter (Novasina, Lachen, Schwyz, Switzerland) after filling the sample cup (40 mm diameter × 12 mm deep) to approximately two‐thirds with the sample chopped into small pieces.