Bitterness is a widespread sensory defect during cheese ripening caused by the accumulation of low molecular weight peptides ($\lt 6000\text{ Da}$) rich in hydrophobic amino acid residues (such as leucine, proline, phenylalanine, and valine), primarily derived from the enzymatic cleavage of $\beta$-casein and $\alpha_{s1}$-casein by coagulant and starter proteases. If starter peptidases fail to degrade these hydrophobic peptides into free amino acids, bitterness develops.

Option analysis:

• Hydrophobic peptides: Correct. Bitterness is directly elicited by hydrophobic peptides stimulating bitter taste receptors on the tongue.
• Hydrophilic peptides: Incorrect. Do not elicit a bitter taste.
• Amino acids: Incorrect. Free amino acids contribute savoury/brothy and umami taste, not cheese bitterness.
• Sulphur compounds: Incorrect. Provide pungent, garlicky or cheddar-like sulfur notes.

During cheese ripening, the initial firm, rubbery curd matrix is transformed into a smooth, pliable, and meltable body primarily through Proteolysis. Residual coagulant (chymosin), native plasmin, and starter lactic acid bacteria enzymes cleave intact alpha-s1-casein and beta-casein into peptides. The progressive degradation of the paracaseinate protein scaffold weakens the curd network, binding free water and governing the evolution of cheese body and texture.

Option analysis:

• Proteolysis: Correct. The premier biochemical event dictating cheese body and texture changes during ripening.
• Lipolysis: Incorrect. Primarily contributes to pungent and piquant flavor development (free fatty acids), with secondary impact on texture.
• Lactose fermentation: Incorrect. Occurs rapidly in early cheese manufacture, establishing pH.
• Lipid oxidation: Incorrect. Spoilage defect leading to off-flavors.