Can a person make seltzer on shobbos or yom tov with something like sodastream?
According to the rulings of R' Shlomo Zalman Auerbach, yes.
From SodaStream's website:
Are your products kosher? SodaStream products are kosher (OU) certified (click to see OU certificate). SodaStream seltzer is kosher for passover (click to see OUP certificate). Also, please note that your home soda maker can be used on the Sabbath. For more information, please contact us send an email to firstname.lastname@example.org or call 1-800-763-2258.
I contacted the company, and they pointed me to Shulchan Shlomo O"C II 313 (page 137). Shulchan Shlomo is a work put together by a student of R' Shlomo Zalman Auerbach, based on his rulings. (I am assuming the scanned page they sent me is legitimate.)
- Adding flavorings -- anyone who uses tea essence on Shabbos or chooses to mix wine with water does this. Not a problem.
- Changing the gas cartridge -- when replacing the cartridge, you're not "fixing" a "broken" machine, as that's doing what it should. More akin to refilling a salt shaker with salt.
- Carbonating the water -- doesn't seem to fall under any prohibited act. You could try to argue "uvda d'chol" ("it's a mundane-day activity, not Shabbos-like"), but we don't just make those up for every new situation.
So it appears R' Shlomo Zalman Auerbach allowed it. I haven't heard any other opinions on the matter. Though if you wanted to be thorough, you could ask a British Jew, as according to wikipedia, "In the UK (where it was first sold) the SodaStream machine is strongly associated with 1970s/1980s childhood nostalgia."
The Beis Yitzchak (at the very end of Yoreh Deah part 2) writes that making soda water is forbidden on Shabbos due to molid. This view is cited in Minchas Shlomo
who notes (as above) that people tend to be lenient.
It is permissible to make seltzer on Shabbat, provided that no electricity is involved (and seltzer machines generally do not involve electricity), and that the seltzer is needed on Shabbat.
Rabbi Dovid Ribiat ("The 39 Melochos" mleches makeh bpatish p. תתלב fn. 104) cites differing opinions as to whether making seltzer qualifies as the forbidden melocho of makeh b'patish (according to the Yerushalmi that includes food items in the prohibition). He cites the Shemiras Shabbas K'hilchasa (11:35) who is lenient. He doesn't explicitly mention why molid (ala e.g. infusing clothing with perfume) isn't an issue. Also, at least in the case of the SodaStream sodamaker, the instructions make explicit use of the sound created by the device to determine when the seltzer is ready (3 loud presses) which would seem to raise an additional issue of the Rabbinic prohibition of hashmaas kol.
ילקוט יוסף קצוש"ע אורח חיים סימן שכב - דין נולד בשבת
ה. מותר לעשות מי - סודה בשבת על - ידי הכלי המיוחד לכך שנקרא "סיפולקוס" או "סודה סטרים", ואין בזה שום חשש לא של עובדין דחול, ולא משום נולד, ולא משום מכה בפטיש. ובלבד שיעשה זאת לצורך השבת. ואם הותיר לאחר השבת אין בכך כלום. [ילקוט יוסף שבת כרך ג' עמוד תי].
Chacham Ovadia allowed this as well.