Two separate problems arise, and they need separating because one is a quality issue
and the other is not.
The quality problem is physical. Thawing releases cell fluid; refreezing forms new and
larger ice crystals, which damage more cell structure. The result is greater drip loss,
softer texture, colour deterioration and, in the pack, clumping and visible ice. Each
cycle makes it worse and none of it is recoverable.
The safety problem is microbiological. Freezing does not kill most micro-organisms, it
suspends them. During a thaw, whatever was present resumes growing at a rate set by the
temperature reached and the time spent there. Refreezing suspends them again but does
not undo the growth or remove any toxins produced. So the state of a refrozen product
depends on a thaw history that nobody can reconstruct after the fact.
That is why the operational rule at goods-in is not to judge the product but to judge
the record. Clumping, ice crystals inside the bag and block formation in an IQF product
are indicators of temperature abuse, and the temperature trace is the evidence. Reject
against the record, not against a subjective assessment of the pack.
Consumer-facing frozen packs normally carry a do-not-refreeze instruction for the same
reason. In a commercial chain the equivalent is a written acceptance criterion and a
recorder somebody actually downloads.