The most reliable, time-tested traditional method to keep meat patties tender and juicy is using a classic panade. A panade is simply a mixture of starch (such as torn stale bread, fresh breadcrumbs, or rolled oats) soaked in a liquid (usually whole milk, heavy cream, or broth) until it forms a soft paste.
When ground meat cooks, its protein fibers naturally contract and squeeze out moisture, which is why meal-prepped patties often turn dense and dry upon reheating. The starches in a panade absorb the released juices and form a barrier between the protein strands, preventing them from binding too tightly together. This preserves a soft, succulent texture without changing the meat's natural flavor or requiring modern additives.
For roughly one pound of ground meat, a standard ratio is one slice of crustless white bread (or one-third cup of breadcrumbs) mixed with two to three tablespoons of milk. Allow the bread to soak for about five minutes, mash it into a uniform paste with a fork, and then gently incorporate it into your ground meat and seasonings.
Another wonderful traditional option is adding grated raw onion along with its released juices. The moisture from the onion hydrates the meat from the inside out while contributing subtle, savory flavor. Just remember to mix your meat gently and avoid compacting the patties too firmly when shaping them.