Answer
Each gram of water that that freezes releases 80 calories of energy to the air of the cellar (page 330, Figure 17.18). This energy helps keep the temperature above $0^{\circ}C$.
The canned food contains sugary and salty water, which lowers the freezing point below $0^{\circ}C$ (page 327). The canned food won't freeze before all the water in the tub does.
As long as the farmer keeps replenishing the tub water before it completely freezes, the canned food won't ever freeze.
