Stale bread is confusing

Quick Overview

Bread goes stale because starch molecules retrogradate, reforming crystalline structures and squeezing out water, making it hard, while Oreo cookies remain soft because their low water activity (aw=0.15) prevents starch retrogradation, and reheating stale bread with moisture reverses the process, unlike cookies which only get softer when left out.

Key Points: Bread stales because starch molecules retrogradate, reforming crystalline structures and forcing out water, causing the bread to become hard, as demonstrated by a high water activity (aw=0.95) bread losing moisture to the air (aw=0.50). Reheating stale bread in foil with a sprinkle of water reverses retrogradation by reintroducing water to the starch crystals, temporarily restoring freshness. Oreo cookies, with a low water activity (aw=0.30), do not typically stale by retrogradation; instead, they soften by absorbing moisture from the air (aw=0.50). The UK tax authority controversially classified Jaffa Cakes as cakes (0% tax) rather than biscuits (20% tax) because cakes must go stale (harden) over time, which Jaffa Cakes do not do, whereas biscuits (like Oreos) harden when left out. Jean Baptiste Boussingault proved in 1852 that bread sealed hermetically still hardens over time, indicating retrogradation is an inherent process, not just moisture loss to the air. Commercial bread recipes often include ingredients like enzymes and emulsifiers that interfere with starch retrogradation, keeping soft bread fresh longer than artisan loaves. Freezing bread stops retrogradation entirely, as the water is locked in place, but refrigerating it accelerates the process, making bread stale faster.

Context: The video addresses a viewer's question about why stale bread becomes hard while stale crackers (like Oreos) become soft, exploring the scientific concepts of starch retrogradation and water activity (aw) that govern the texture changes in baked goods over time. The video uses animated stick figures and real-life food examples, including bread, Jaffa Cakes, and Oreos, to illustrate these physical chemistry principles.