Step 1: Understanding the Concept:
Fruit beverages and concentrates are regulated by statutory composition and quality specifications under FSSAI and FPO standards.
Step 2: Detailed Explanation:
According to regulatory standards for fruit squash, the beverage concentrate must contain a minimum of $25\%$ fruit juice/pulp, minimum total soluble solids of $40\text{--}45^\circ\text{Brix}$, and an acidity range of $1.0\text{--}3.5\%$ (expressed as anhydrous citric acid).
The maximum permissible acid limit is $3.5\%$.
Step 3: Final Answer:
Hence, the maximum permissible acid content in squash is 3.5%.