I do a lot of R&I for Uel Zing Cold Brew and from my experience, cold brew has a lot of misconceptions surrounding roast, grind, time, temp… Cold brew does have its limits and though depending on method (drip vs immersion) and water temp.
Cold brew is just another method of brewing and like espresso it can accept every roast level that suits your preferences. The main factor is not roast level but roast development. As long as the beans are properly developed you should be fine brewing up any bean cold. For us we use all light roasted beans and have even cold brewed coffees from Tim Windelboe and Jacu with wonderful results - Super floral delicate notes from the Ethiopian, and tart juice goodness from a Kenyan.
Our ratios are 12.5:1 for Ready-T-Drink coffee and 6.25:1 for concentrate that is twice as strong as the RTD all using an immersion cloth filter method. This brew ratio is determined and linked directly to extraction. This is the first limit of cold immersion. It seems cold brew when done as an immersion hits its limit at around 18.5% extraction. So with this in mind we have settled on these ratios to deliver a 1.5%TDS RTD and 3.0%TDS concentrate. Cold drip does have it’s advantage when it comes to efficiency by using less coffee, and also be able to create even stronger concentrates. But it does have the added effort of dialing in grind.
Grind for immersion should be super coarse. Especially when done on bigger scales. The coarse grind will act as a more pours structure and allow the water to penetrate the grounds throughout the steeping process. A finer grind will yield you a steeper extraction curve for the first hour but after that it will level off more quickly and you run the risk of allowing the fines coffee to clump together and preventing the free flow of water.
Time- We do all cold brew at fridge temperatures. This requires a 24 hr steep vs 12 hrs but the result is better shelf life and a slower extraction that plays well with the inherently slow brewing process of cold brew.
Temp- The one thing I have found that I dislike about cold brew is when brewing very acidic coffees that have a dry or tart taste naturally i.e. Kenya and Malawi, the cold water tends to accent the dry, tea, tart flavors. In this case I find a hot bloom does help to develop the balancing sweetness of the coffee. For that we use 195 degree water and use twice the weight of the grounds, give it 5 minutes then too it off with cold water and place in the fridge. This will require a drastically less extraction period. More like 6-8 hrs.
I hope this is helpful information and feel free to hit me up on any follow ups.