Mass balance measurement infers food loss and waste levels by comparing inputs (e.g., products entering a grocery store) with outputs (e.g., products sold to customers) along with changes in standing stock levels. At its most basic, this method estimates FLW by subtracting the outputs from the inputs, with the difference being considered the amount of FLW.
A summary of the strengths and limitations of mass balance measurement is shown in Table A13.
Table A13. Factors to Consider when Using Mass Balance to Quantify FLW
|Strengths||Limitations / Points to Consider|
How to Use Mass Balance for FLW Quantification
Step 1: Define your inputs, outputs and stocks
Three key figures – the inputs, the outputs and the stocks – form the basis of the mass balance calculation.
In a manufacturing plant, the inputs would be the ingredients used, the outputs would be the products produced and the stocks would be whatever ingredients or products are held on site. At a state or country level, the inputs would be domestic food production and imports, and the outputs would be food consumption, exports and nonfood uses such as seed, feed, fuel and pet food.
Step 2: Identify data sources
After determining the inputs, outputs and stocks, find appropriate sources of data to estimate those numbers. Data can come from sources such as product inventories, shipping and storage records, invoices and other documentation. See the “Records” section above for more information on gathering records.
Once the data sources have been identified, make sure that all data is in the same units. If it is not, you will need to standardize the units.
Step 3: Account for any variations
If the weight of the inputs changes during processing or cooking, you will need to adjust for it in the mass balance equation. For example, in some cooking processes (e.g., preparing a sauce), significant amounts of water will evaporate, while in others (e.g., cooking pasta), water will be added. These weight changes must be identified so they do not skew the overall waste figure.
Step 4: Perform the mass balance analysis
Once the data have been collected and standardized, conduct the mass balance analysis. The calculation is based on the following equation (FLW Protocol 2016a):
FLW = Inputs – Outputs ± Changes in Stock ± Adjustments
The terms in this equation are defined as follows:
Inputs: the ingredients or food products that enter the facility or geographic region during the measurement timeframe.
Outputs: the ingredients or food products that leave the facility or geographic region during the measurement timeframe.
Changes in stock: any variation, positive or negative, in the amount of ingredients or food products held by the facility or geographic region during the measurement timeframe.
Adjustments: any change in weight, positive or negative, to the ingredients or food products, most commonly due to added or removed water.
The result of this equation is an estimate of the FLW level, since the unexplained variation between inputs and outputs can be inferred to be due to loss and waste.
Common Data Challenges when Using Mass Balance
Inaccuracies in data. If any of the four key variables in a mass balance equation are inaccurate, the final FLW number will also be inaccurate. Therefore, it is crucial to make sure these data are accurate and to note any points of uncertainty when reporting the final FLW figure.
Additional Resources on Using Mass Balance
FLW Protocol. 2016. Chapter 8. “Mass Balance.” In Guidance on FLW quantification methods. <http://flwprotocol.org/wp-content/uploads/2016/06/FLW_Guidance_Chapter8_Mass_Balance.pdf>.
TU Wein. n.d. Stan2Web. Vienna, Austria: Technische Universitat Wien. <http://www.stan2web.net>. (STAN [short for subSTance flow Analysis] is a free software for conducting a mass balance measurement.)