Qualitative evaluation of cold brewing factors

Coffee preparation techniques besides espresso like pourover.
wearashirt
Posts: 228
Joined: 9 years ago

#1: Post by wearashirt »

Hi all! I would just like to share my qualitative findings with my little cold brew experiment.

Introduction:
I wanted to finally settle, once and for all, the effect of grind size and cold brew time for a specific coffee. Particularly, to seek an optimum flavor as an effect of fine vs coarse grind size, and 10 h vs 20 h brew time.

Furthermore, in my experience in my kiosk, the cold brew would get sour or undesirable on the 36th hour onwards of contact time. Thus, it's become an SOP to throw away cold brew that wasn't consumed the following day after it was set to brew. So this experiment also aimed to check if separating the brew from the grinds after brew-time would lengthen its drinkable period.

I also happen to vacuum-seal my cold brew jars, so I wanted to check if vacuuming really had an effect on flavor and its longevity in the brew.

Materials and methods:

Coffee: from the Mt Apo region of Southern Philippines; tastes of tropical fruit and gin-alcohol (the alcohol part beats me...but it's great as a cold brew). If you smell a palmful of the green beans (which appear yellow), it would smell light, fruity. Picking is not perfect as there are some defects. So I guess it remains to be seen whether the alcoholic part would remain if picking was improved at the farm level.

The experimental set up consisted of four 25-g grinds of coffee (see Fig 1) subject to the following brew conditions:

(1) 10h, coarse
(2) 10h, fine
(3) 20h, coarse
(4) 20h fine

A
B
Fig 1 A-B. Fine vs coarse grinding via the conical grinder of my Breville 870XL

Each brew condition was set in a medium size mason jar. 350 mL of cold water was used to cold brew each coffee (1:14 ratio). The slurries were stirred to degas them sufficiently. Each mason jar was then vacuumed with a Foodsaver device, then set in the fridge overnight.


Fig 2. Size of mason jars and appearance of slurries

The following day (2nd day), the cold brewed results were filtered through V60 filters at the designated stopping of the brew time (10h and 20h) and tasting samples were obtained (point A). The cold brewed coffees were placed in new mason jars, vacuumed again with a Foodsaver device, and replaced in the refrigerator.

The following day (3rd day), tasting samples were obtained again for evaluation (point B). This time, however, 2 of the 4 jars were left unvacuumed and replaced in the refrigerator. The other two jars were vacuumed again.

The samples were tasted in its entirety on the 5th day for a final taste evaluation (point C).

RESULTS and DISCUSSION

Instead of overly detailing my tasting evaluations A, B, and C....I'll just give you the totality of my thoughts. So, to summarize:

1. Effect of grind - coarse grinds gave way to better acidities and aromas but just a tad less body than the fine grinds
2. Effect of brew time - minimal; I theorize that coffee is dissolvable hot or cold -- the brew temperatures facilitate diff chemical species. However, I believe overextraction is still a problem to be dealt with, so it's important to separate the grounds and brew product by 24 hours contact time, to prevent off-tastes and souring. This arises largely from my own experience in the shop, since I did not previously separate the cold brew and the grinds in the jar. This also comes in contrast with the comment of an HB'er somewhere here, "it's very hard to overextract cold brew coffee" -- I now disagree.
3. Effect of vacuum - slightly better preservation of aromas; worthwhile to keep doing as part of the recipe.
4. Effect of storage - The cold brew was most delicious at tasting points A and B, across the 4 jars. However, I noticed slightly lessened aromas and "vitality" on tasting point C, so I wouldn't recommend selling-off 4- or 5-day old cold brew.

With regards to the brew sets-up, if only to choose one of the four, I might prefer set-up C (20h, coarse). I think just a tad more of the aromas came through. The coarsely ground cold brew also had less of a palpable heaviness, which was good for the acidities and aromas since it's a cold drink.


Fig 3. Difference in translucence at tasting point A. I know it's qualitative, and my pic is bad, but I gave it a good look against the sunlight and they looked the same. Below in the recommendations, I indicate that a refractometer is needed for comparison. Taste wise, as I would conclude, there's not a lot of difference!

CONCLUSION

I simply would like to think that coffee is dissolvable hot or cold, and it's a matter of finding the optimum contact time between water (any temp) and coffee. In addition, cold water is just as prone to overextracting coffee, simply because of its contact time with the coffee grinds.

Depending on the roast level, different grind sizes will favor different styles of end product-goals. For light roasted coffees with citrusy/tropical fruit flavors, coarse grinding will very slightly favor aromas and acidities. However like I said, when I was tasting across the 4 jars at tasting points A & B, the differences didn't seem to matter much. I suppose cold brew is forgiving this way!

I know that most cold brew products in cafes don't use the vacuum, but I would recommend that if you can, maybe you can go ahead anyway, in the spirit of avoiding oxidation. The Foodsaver method remains to be disputed with a replicated qualitative study -- like, does the vacuuming negatively affect the longevity of aromas? As of this little experiment, I think vacuuming positively affects cold brew results.

RECOMMENDATIONS
The use of refractometers to test TDS across brew-times; granule size and contact time for different coffees (malic coffees, nutty coffees); using lukewarm water to brew pourover coffees (hey it's an idea...)

Thanks guys! That's all. Chime in your comments.

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Unrooted
Posts: 279
Joined: 8 years ago

#2: Post by Unrooted »

That's some cool data!

I just cold Brewer some coffee after my mom raved about her boyfriend's new toddy Brewer, and the lack of bitterness. I was curious and I had a bag of over-roasted beans that I would never use otherwise.

I just put the grinds and water in a large Rubbermaid container and let it sit overnight. I slowly poured the water through my chemex and was amazed at how much coffee "sludge" I had, about 75% of the chemex filter from 10oz of course ground coffee, i guess my grinder isn't doing a great job...

Amazingly the coffee is much more drinkable like this, very little bitterness and sweeter than I would have imagined, fun experiment that I will likely replicate with my home roasts that I let go too far!

I would be interested in knowing how much difference vacuum sealing during the brew phase makes.

wearashirt (original poster)
Posts: 228
Joined: 9 years ago

#3: Post by wearashirt (original poster) »

Unrooted wrote:home roasts that I let go too far!

I would be interested in knowing how much difference vacuum sealing during the brew phase makes.
Thanks mate!

The weird thing with cold brew coffee being "less bitter" is a good analogy I read in a discussion about iced espresso. Coldness actually lessens the sweetness (whether percieved or chemically), which accounts for the unpleasing taste of iced espresso (described as metallic, blech). The analogy is ice cream -- when solid, it's good. When melted, it's too sweet (and I actually like it melted...)

And I agree with this analogy...hot coffee has more acidities, with whatever roast. For cold brewing, I'm still experimenting with chunkier roasts, but for now most of my customers like the light roasted cold brews. :)

Yeah, go for a Foodsaver machine! Otherwise there are those cool jars with a button lid that comes with a manual siphon vacuum. :)

weavdaddy
Posts: 38
Joined: 9 years ago

#4: Post by weavdaddy »

I wish I could do this kind of comparison. I'm brewing in a CoffeeSock at half capacity - 2.5 pounds of coffee to 2.5 gallons of water. I'm using the largest grind size possible and steeping for 24-27 hours. I remove the grounds from contact at that time. The remaining liqueur has a supposed shelf life of 3 weeks but the longest it has lasted is 12 days. Consistent taste throughout that period of time.