If you have ever opened a can of coffee, a bag of potato chips, box of cereal, etc. and been dismayed by the amount of empty space at the top you have experienced what is generally known as "slack fill".
The FDA defines it like this:
A container that does not allow the consumer to fully view its contents shall be considered to be filled as to be misleading if it contains nonfunctional slack-fill. Slack-fill is the difference between the actual capacity of a container and the volume of product contained therein.
The problems start when...
So – if you can see the empty space before opening the package or the empty space is functional everything is OK. The first part makes sense – if a clear glass jar lets you see how much empty space you are buying it's not really misleading. But when could empty space ever really be considered functional?
The FDA offers six acceptable reasons...
(1) Protection of the contents of the package;
An example would be all the extra space in bags of potato chips that appear to be almost inflated; that extra air acts as a cushion to prevent the product from being broken during transport.
(2) The requirements of the machines used for enclosing the contents in such package;
The mechanics of the equipment involved in packaging might not allow for the containers to be filled to the very top
(3) Unavoidable product settling during shipping and handling;
Think brown sugar, which could be shipped loose but become packed during shipping, or cereal which can settle during transit. Another example is bagged goods (think dog food) that are shipped packed very flat, and then appear much less full as they are isolated and allowed to assume a more efficient (in terms of volume to surface area) rounded shape.
(4) The need for the package to perform a specific function (e.g., where packaging plays a role in the preparation or consumption of a food), where such function is inherent to the nature of the food and is clearly communicated to consumers;
An example would be instant noodles, where the packaging (largely empty) is intended to be filled with hot water and used as a serving vessel.
(5) The fact that the product consists of a food packaged in a reusable container where the container is part of the presentation of the food and has value which is both significant in proportion to the value of the product and independent of its function to hold the food, e.g., a gift product consisting of a food or foods combined with a container that is intended for further use after the food is consumed; or durable commemorative or promotional packages;
Souvenir tin/box, etc., the key idea being that the container itself has significant value.
(6) Inability to increase level of fill or to further reduce the size of the package (e.g., where some minimum package size is necessary to accommodate required food labeling (excluding any vignettes or other nonmandatory designs or label information), discourage pilfering, facilitate handling, or accommodate tamper-resistant devices).
Think about the small foil packages that contain just one or two painkiller tablets, though far bigger than the product they need to be able to display product information, bar code, etc. Larger packages can also be used to discourage shoplifting.
In those situations slack fill is considered functional (and permissible), in most others it is assumed to be misleading.
For more information visit the relevant section of the FDA website.