Here is something I wrote on the topic a while back. (Hebrew) Footnotes posted in the next three post:
Introduction
Coffee on Shabbat
For the purposes of this discussion we will focus solely on the act of brewing ground coffee on Shabbat, without touching the matter of instant coffee which carries its own set of Halachic questions.
Seemingly simple, the act of brewing Coffee on Shabbat includes a plethora of Halachic dilemmas, as it touches upon some of the most complicated laws of Shabbat, namely cooking (Bishul) and sifting (Borer). When discussing the Halachic status of a certain action, it is important to understand the various aspects of the process, for all will be applicable to the Halachic decision.
The process of brewing coffee constitutes of ground coffee (from roasted coffee beans) brewed in hot water and separated from the liquid before drinking.
Part 1
Halachic Concepts
A. The prohibition of “cooking” on Shabbat is very broad and also includes any form of converting any uncooked food into a cooked food[1], even if the food was edible before being cooked[2]. Same applies to perfecting a cooked food. Therefore, boiling cold water on Shabbos is prohibited[3], as it constitutes “cooking” (Mevashel), and any hot water used for coffee should be heated before Shabbat, and kept warm in a permissible manner[4].
B. The prohibition of cooking doesn’t apply in when the product is completely dry and was previously cooked[5], unless it will partially dissolve when it’s heated (in which case it is advisable to be stringent[6]), this is because you cannot cook a cooked product (אין בישול אחר בישול). However, if the product was previously baked or roasted rather than cooked – there is a debate among the Halachic authorities as to whether there is cooking after baking or not (יש בישול אחר אפי'), with the Ashkenazi poskim prohibiting “cooking” any baked product on Shabbos, especially if the baked product will become soft during “cooking”[7], and the Sephardic ones permitting[8]. Based on the fact that coffee is fully roasted and edible Rabbi Ovadia Yosef rules that it is permitted to “cook” coffee on Shabbos, by pouring hot water directly unto the coffee grinds, though he recommends to use water from a second vessel (Kli Sheni)[9].
C. The temperature needed for “cooking” is that the heat should be enough to bring up the temperature to “Yad Soledes Bo” (a degree of heat from which the hand recoils). There is a dispute among Rabbinic authorities as to what the actual degree of heat is, ranging from 43 celsius (110 f) to 71 celsius (160 f)[10].
D. However, if the liquid was transferred from its original cooking utensil (Kli Rishon) to a second vessel (Kli Sheni), the water no longer has the power to “cook” regular foods that were not cooked yet. Nevertheless, the sages prohibited placing uncooked dry foods into a second vessel because it seems like the person is cooking[11]. However, spices and other condiments (except for salt, sugar and other dissolvable spices) can be placed into a second vessel, because it doesn’t look like he intends on cooking these spices[12]. Some are lenient and permit placing any uncooked dry foods into a second vessel[13]. Based on this logic, if Coffee grinds were indeed to be considered a spice or a condiment – it would be permissible to place it into hot water which was put in a second vessel, and that was the opinion of R’ Moshe Feinstein, though he advised to be stringent and only place it into a third vessel (Kli Shlishi)[14].
E. Even according to the stringent opinion in the previous paragraph, it is still permissible to pour (עירוי) hot water from a second vessel (even if its temperature is above Yad Soledes Bo) unto any uncooked dry foods, unless they are specific foods which are easily cooked (“Kalei Habishul”) even in a lower degree of heat, such as certain kinds of fish, which their cooking is done (or completed) easily[15]. Based on this logic, if coffee grinds are fully cooked and not easily cooked again (but they become soft when placed in hot water) – it would be no problem to pour hot water unto them from a second vessel[16]. However if they were indeed to be considered as not being fully cooked – then it would be prohibited to pour hot water from a second vessel unto coffee (either because they are roasted and therefore can be cooked again; or because the roasting is not complete and it’s not edible until it’s mixed with hot water[17]), and the only solution would be to pour the hot water into an empty third vessel (Kli Shlishi) and then pour the coffee into it. Though, some disagree and prohibits even using water from a third vessel for coffee because we see it gets cooked in the third vessel too[18].
F. An additional problem with mixing the coffee with the water is that the water is colored from the coffee, and coloring on Shabbos is biblically prohibited. Although the Halacha is that there is no coloring with regards to food[19] and most say the same applies to drink[20], some opinions hold that there is a prohibition to color drinks, and therefore would prohibit pouring coffee into hot water, rather they would require to pour the water unto the coffee, thus the person is not coloring the water[21].
G. An additional problem with mixing the coffee with the water is that some consider this action as Makeh BePatish (striking the final blow to complete an action), which is biblically prohibited[22]. In this instance, since the coffee is not edible before it’s mixed with water – it is prohibited[23]. But, most Halachic authorities are of the opinion that this prohibition doesn’t apply to food[24], or that this prohibition only applies when an action is done to change the food, not when two foodstuffs are mixed[25]. But, today it is a known fact that roasted coffee is indeed edible as a bean, even without being mixed with water[26].
After concluding discussing the mixing of coffee grinds with hot water, the next issue to tackle is separating the coffee grinds from the coffee liquid after brewing, and the status of a coffee filter.
H. One of the forbidden categories of work on Shabbat is Borer, which is the act of separating food from refuse. With regards to coffee, once the coffee is brewed the person needs to separate the coffee grinds from the liquid prior to drinking it, this can be done in a variety of ways, some which may be considered Halachically prohibited.
I. The general rules of Borer are: (1) Removing the bad from the good is prohibited; (2) Using a utensil created specifically for the purpose of separation is prohibited; (3) Separating when there is no immediate need is prohibited. The permitted forms of Borer are when all the following conditions have been met: (1) The separation must be done by hand and not with any separation tool; (2) Removing only what is needed for immediate consumption; (3) Removing the good from the bad[27]. Additionally, according to some opinions it is permitted to remove refuse together with little food (for example: removing a fly from the soup with a spoon filled with soup) because it’s not the way it’s done usually[28]; but according to others – it is prohibited because it is clearly evident that the intent is to separate the bad from the good, and the only solution is to spill out the good until the bad comes out with it[29].
J. When discussing the filtering of liquids, there are a few important laws: (1) A drinkable liquid which has some sediment but it’s drinkable without filtering (to most people[30]) – may be filtered through a filter to clarify the liquid[31] (but according to the Rambam, a filter may not be used even in this instance[32]); (2) But, if there is lots of sediment and it is not drinkable without separation, it is prohibited to separate the liquid with a strainer (filter)[33]. If the purpose is to filter it for later use, then the solution is to pour the liquid slowly into another vessel so the sediment stays on bottom, and to stop when it slows down, leaving some liquid in the bottom of the original vessel with the sediment[34]. Alternatively, if no filter is used and it is filtered for immediate consumption – it is permitted to continue the pouring until all liquid (food) is separated from the sediment (refuse), and it is not prohibited despite the use of a vessel, because the vessel is not specifically made for the purpose of filtering[35] (though the Ben Ish Chai doesn’t have this distinction, and requires always to leave some liquid behind with the refuse[36]). Based on this, the Aruch HaShulchan states, that after brewing coffee a person may separate the grinds by pouring slowly the liquid into another cup while leaving some liquid behind, but if he wants to drink it immediately – he need not leave liquid behind[37]. The same logic would apply to removing the coffee from the cup in which the grinds are in a filter (according to the opinions below).
K. If someone wants to give a taste of wine to water, he may place wine sediment in a filter and pour water on it, causing the sediment to liquefy and mix with the water[38], giving it wine taste, and the reason is because the water enters the filter without any particles and exits the filter without any particles, thus there is no separation involved[39]. The placing of the sediment in the filter can only be done before Shabbos, because the purpose of placing the sediment is for filtering wine (so the sediment absorbs more particles from the wine) and that is prohibited on Shabbos, therefore this preparatory action cannot be done on Shabbos.
L. Based on the above, it would make sense to say that using a filter to brew coffee on Shabbos (with hot water according to the various opinions above) is permitted, for it can be compared either to a clear liquid which doesn’t need filtering (the water doesn’t need the filtering), or to the filtering for the purpose of obtaining taste. Some rule that it is permitted only if the coffee grinds were placed in the filter before Shabbos[40] (comparing it to the wine sediment[41]); some rule that it is permitted in any form, because there is no intention to filter anything[42]; some say that it’s permitted only if the hot water sits in the filter and doesn’t go to the drinking cup immediately (for then it doesn’t look like the purpose is the filtering)[43]; some prohibit it completely[44].
M. The next Halachic dilemma is removing the filter from the liquid (if the filter is still touching the liquid according to the stringent opinions above). According to the abovementioned opinions regarding removing the liquid and leaving the refuse behind – it would make sense to pour out the coffee and leave the filter in the original vessel (just as is done with tea bags[45]); many are of the opinion that it does not constitute a problem at all to remove the filter from the cup, as long as it’s not held over the cup to let it drip the coffee remaining in the filter[46], though it’s recommended to use a spoon to remove it with some liquid (as it’s permitted to remove a tea bag from a cup[47]); some recommend to hold the filter (or tea bag) with one hand, and the cup in the other hand, and pull the cup from under the filter, thus you are: (1) removing the food from the waste, (2) with your hand, (3) right before eating.
Part 2
Immersion Dripper and Coffee Plunger
A question was asked: what’s the Halachic view on the following two products; whether it is permitted to use them on Shabbos to make coffee.
1) Bonavita Porcelain Immersion Coffee Dripper, which is a cone in which you steep coffee grounds for a few minutes in a filter, then the liquid is released into a mug with a open/close valve on the bottom.
Halacha: According to the opinions that ground coffee can be mixed with hot water even in a filter, this seems to be the best option, as it has the filter sitting in the liquid; the liquid can be released without touching the filter; the vessel is not made for the purpose of Borer, rather for the purpose of brewing coffee; and additionally – it is in a closed container so there isn’t the problem of it “looking like Borer.”
2) French Press Coffee Maker, which is a glass vessel in which hot water is placed together with coffee grinds for a few minutes, followed by pressing a plunger which pushes the coffee grinds to the bottom, leaving the coffee on top.
Halacha: According to the opinions that ground coffee can be mixed with hot water, this seems to be permitted; although the plunger separates the coffee from the water, the good (liquid) is being pushed to the top i.e. separated from the bad (coffee), and therefore permitted (especially if some liquid is left on the bottom with the coffee grinds). The only problem is that the vessel might seem as a vessel made for separation, but it can be said that the separation is being done by the pressure of the hand pushing down, and therefore it is considered as done by hand.
