Simplicity is the key. The four larger storage drawers are designed to accept our insulated Coolbox inserts that with the addition of multiple ‘ice packs’ — of the kind used by campers — inserted at intervals between the milk containers milk is kept cool. In testing so far we have found this to be very affective throughout a long day. The ice packs are then simply put into a domestic home freezer overnight to be “recharged” (frozen) and ready for use again the next day.
The regulations about keeping dairy products cool vary between UK, Australia, USA etc., but are based on the same principles and very similar temperature/time factor limits. We have simply made the simplest, robust and best performing thermal system we can within the space restrictions of the Velopresso, and it generally works incredibly well, particularly if you ensure your milk is very cold (ideally 5 degrees C) to begin with. Here follows a detailed description of the system and some anecdotal test results for guidance:
Milk is kept cool in the four large aluminium drawers – as described in our machine Specification pdf. The coolbox inserts are highly insulated flat pack fabric assemblies that are like camping coolbags, and they are designed to fold from flat into a rectangular box that fits exactly into the any of the 4 large drawers. Two of the freezer blocks fit into net sleeves on the under side of the insulated lid of the coolbox insert, two more go at the bottom, and then you add at least one more between the milk containers, so cooling is above, below and between the milk containers, minimising heat loss.
There are four large drawers on the Velopresso (and two small ones above for tools, beans, cups etc.). Three x 2litre (4pint) milk containers can fit in each drawer with the coolbox inserts fitted, so if you use all four large drawers for milk you can have 24 litres (48 pints) of milk on board at one time. We use a minimum of 5 ice packs per insulated coolbox drawer as standard, and add more on hot days.
The milk should be used in sequence, so you start by using the milk in the ‘active drawer’ (usually middle right), transferring fresh milk from the other drawers to your ‘active’ drawer only when required to minimise the number of times the majority of the drawers are opened and therefore minimising heat gain.
Using this method in our own tests milk has been maintained at a temperature of between 5 and 8 degrees celcius (C) for 6–10 hours in temperatures ranging from 23 to 28* degrees C. For a particular example: at a London trade show (inside a building) the ambient temperature was 23 degrees C. The milk in our ‘non-active’ drawers remained at 5.1 to 5.9 degrees C for over 10 hours with just 5 of our freezer blocks in each drawer. The legal requirement in the UK is to not exceed 8 degrees for more than 4hours, so we are well within that performance range.
*We have not tested above this temperature but Velopresso owners around the world will have operated above this temperature and may have more feedback to offer. Hotter countries do pose a greater challenge, but generally speaking less hot milk-based drinks are served in hot weather, and you can always improve performance by adding more ( 1 or 2) freezer blocks and possibly reducing the amount of milk in order to make room – as needed – for them.