Human milk spoils due to bacterial, yeast, or mold contamination. Current domestic methods of assessing milk spoilage are subjective or rely on time and temperature‐based guidelines. A key unmet food safety need remains the objective assessment of human milk spoilage. Experiments were conducted using a simplified human milk spoilage model based on goat’s milk as a human milk surrogate, spiked with a single bacterial strain (Staphylococcus epidermidis ), in which pH and carbon dioxide (CO2) concentration were measured along with bacteria count over 160 hr. Bacteria count correlated highly with CO2 but not with pH. A 0.21% CO2 concentration threshold could be defined for milk spoilage (correlating to a bacteria count threshold of 105 CFU/ml), with sensitivity and specificity above 84%. These findings suggest that CO2 measurement is a promising method to detect S. epidermidis growth in milk which merits further investigation for the objective and quantitative assessment of milk spoilage.