This study investigated the effects of brine solution pre-treatment and addition of calamansi juice extract on oyster mushroom kropek sensory and physicochemical (pH, color, moisture, water activity) properties. Results revealed, that the pre-treatment and addition of calamansi juice extract significantly affected (p<0.05) the appearance (3.44 ±1.43 - 5.21 ±1.71), salty taste (3.35 ±2.29 - 4.92 ±2.17), sour taste (2.51 ±1.98 - 5.42 ±2.51), and spicy taste (2.98 ±2.35 - 4.56 ±2.60) of the kropek. The acceptability test also revealed a significant difference (p<0.05) on product appearance (7.09 ±1.22 - 7.47 ±1.46) and overall acceptability (6.40 ±1.18 - 7.18 ±1.07). Although, in terms of consumer preference, no differences were noted among sample ranks (p>0.05), indicating equal preferences. However, since treatment 1 (5% brine & 10ml calamansi juice extract) had the lowest rank among samples, it was chosen as the most preferred and was subjected to laboratory tests along with the control. Physicochemical tests revealed a significant decrease (p<0.05) in the pH values of both raw (6.17 ±0.02 - 4.17 ±.01) and cooked (7.01 ±0.02 - 5.44 ±0.02) kropek with the increased amount of calamansi juice extract. Similarly, color parameters such as L* (67.03 ±0.84 - 71.60 ±1.04), a* (5.63 ±0.55 - 9.30 ±0.36), b* (21.90 ±0.62 - 24.07 ± 0.55) and Hue° (1.20 ±0.01 - 1.32 ±0.02) values were significantly affected by pretreatment and addition of calamansi juice extract. Likewise, in general, the increase in brine solution and calamansi juice extract resulted in a lighter color (redyellow), as displayed by treatment 4 (15% brine and 20ml calamansi juice extract), although treatment 3 (15% brine and 20ml calamansi juice extract) exhibited darker color. Nevertheless, the moisture content (1.08 ±0.07 - 1.13 ±0.35) and water activity (0.53 ±0.09 - 0.54 ±0.01) of the most preferred sample and the control were considered low and not significantly different (p>0.05), indicating longer shelf stability. Therefore, the results of this study are important as they give baseline information about the effect of pre-treatment and addition of calamansi juice extract on product key parameters that would help solve problems during its processing, preservation, storage, distribution, and even food consumption. However, it is recommended that further studies may be conducted, particularly in terms of the correlation of some physicochemical properties with the results of sensory tests

Keywords : Oyster Mushroom, Calamansi, Kropek, PreTreatment, Physicochemical Properties, Sensory Properties.