From my experience, the important aspects to consider are to take the matter seriously, and deal with it both promptly and professionaly, but do not to crucify yourself by putting yourself in a position that makes you a target of litigation.
Simply express regret at the situation, advise that the sample wiil be / is being analized, and that you are always concerned about the satisfaction of your clients, and offer to replace the product or provide a refund to them.
As a chef, many years ago i had a case of a customer complaint that a hair was found in their salad.
the facts: i shave my head, and personally washed all the lettuce myself, and the server had long blond hair.
the offending hair was long and black, the same as the customers, who had a big beehive hairdo.
I offered to replace the meal and to take the hair for analysis.
the customer was immediately shocked at my suggestion of DNA analysis, and refused to co-operate, and demanded free lunch for her group of diners.
I stated that without analysis that it could not be proven that the hair is any ones in particular, and that without such proof that i was unable to accept the free lunch for her entire group as suitable resolution.
She then became agressive and said she will refuse to pay, and i advised that this would be a case of theft and the police would be involved as she is not legally entitled to refuse payment without genuine basis.
the end result was she paid, and her friends apologised stating they believed the hair was hers and had fallen into her own meal.
very sensitive area, so must be very delicately and professionally handled.....
in a case where i had inferior product, yoghurt, that was almost all mushed strawberries, and not much yoghurt, i called the company just to advise they may need to check their production line, and they were kind enough to offer me a $10 refund direct to my bank account.. without me asking for it.
as can be seen, how a complaint is deleivered, and how it is received can result in very different outcomes..