Hi everyone.
We just got audited for FSSC and we were given a non-conformance on Food Fraud Mitigation. Mass Balance testing was declared as a control in the assessment of likelihood of detection. But we were not able to provide evidence of its implementation. The person who created our vulnerability assessment left 
Can someone please help me? How do I conduct Mass balance?
Thanks.
Mass balance is about making sure that the quantity coming out is equal to the quantity coming in, less any expected/explainable/understood losses due to e.g. production yields, wastage etc., over a specified period - could be a batch, a day, a period of weeks/months.
For example, if I start with zero balance of organic 100% beef burgers on January 1st, and in the period January 1st - March 31st I buy 300kg of organic beef burgers, but find that I have sold 350kg of organic beef burgers, one would potentially infer that I've fraudulently (or accidentally - but that isn't great either...) sold 50kg of organic beef burgers that are either not organic, or not beef, or possibly neither organic nor beef.
It's a fairly standard traceability approach used by e.g. some of the organic certification bodies in the UK.
It might be the case that your products/raw materials/supply chain are such that this type of verification activity has some specific benefit, but equally it might be worth reviewing the guidance mentioned by Charles and deciding whether there is a different approach that you prefer and that would work as well as, or better than, the mass balance approach if it's one with which you're not particularly comfortable.
@Charles - I've seen this used quite widely and had to do many such exercises in audits, with no direct/tangible food safety implications as such. It's always been entirely about demonstrating control of organic status, or similar "identity" claims, i.e. "fraud" in the basic misrepresentation sense of things.