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Slightly bitter coffee with Oscar Nuova
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atytse
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Joined: 9 Oct 2013
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Posted Tue Dec 17, 2013, 6:26pm
Subject: Slightly bitter coffee with Oscar Nuova
 

Hi all, I purchase this beans that everyone else said was sweet with slightly acidic note but when I made mine it was slightly bitter.

I use a Vario for grinder and extracted for about 25 secs (the flow looked fine). Tamped normally.

I use an Oscar Nuova so I wonder what else could be causing the bitterness (the beans are fresh as well).

Have not measured (dont' know how) the pressure of Oscar, also can't really tell the temperature of the water.  What would be your advice?
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GVDub
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Posted Tue Dec 17, 2013, 8:55pm
Subject: Re: Slightly bitter coffee with Oscar Nuova
 

How long of a cooling flush are you doing before pulling the shot? The nature of any heat exchanger (or HX) machine, like the Oscar, is that the water that sits in the heat exchanger tube that passes through the boiler gets overheated when it's allowed to sit for longer than a couple of minutes and must be flushed out before pulling your first shot after the machine has been on but resting. Otherwise the shot is too hot, and too hot equals bitter.
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atytse
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Posted Wed Dec 18, 2013, 12:14am
Subject: Re: Slightly bitter coffee with Oscar Nuova
 

That's a great tip. I usually flush for at most only 3 seconds because I don't want to waste so much water...

How long should the flush be?
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SStones
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SStones
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Posted Wed Dec 18, 2013, 5:05am
Subject: Re: Slightly bitter coffee with Oscar Nuova
 

Watch the shower screen during the flush.  You'll see the temperature change from boiling (Water bubbling/boiling as it escapes) to just below boiling (Water running in a liquid stream, or almost no bubbling at all).  Run the flush long enough to get that below boiling range and try brewing one there.  That should be very close to the temp you want.  You can then experiment and see if it cools further with a longer flush, but once that boiling to non-boiling step is achieved, the difference made by a longer flush is not as obvious in your finished product.
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atytse
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Posted Wed Dec 18, 2013, 6:16am
Subject: Re: Slightly bitter coffee with Oscar Nuova
 

Thanks for your reply!

Also wanted to ask if it's normal that the water doesn't come out as a single stream, that it sometimes sprays across before converging to couple streams. Hopefully nothing wrong with my grouphead.
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GVDub
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Posted Wed Dec 18, 2013, 9:02am
Subject: Re: Slightly bitter coffee with Oscar Nuova
 

My flush on an Ellimatic (Simonelli's predecessor to the Oscar) is about 15-20 seconds when it is up to stable temperature and has been idle more than 10 minutes. If I pull a couple of shots within a short period of time, the second shot only requires a couple of seconds of flush, which you'd do anyway to clear old grinds off the dispersion screen.

Also, you're not wasting the flush water if you use it to preheat the cup you'll be pulling a shot into.

That "spray" you're seeing is likely the "water dance" where the overheated water is boiling once it hits atmospheric pressure.

Check this video from caliblacksmith (Wayne P) to see exactly what the cooling flush should look like. Pay attention to what the water flow looks like, as the time will be different on every machine.
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boar_d_laze
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Posted Wed Dec 18, 2013, 9:12am
Subject: Re: Slightly bitter coffee with Oscar Nuova
 

The Oscar is an HX machine.  HX is short for heat exchanger.  

A "heat exchanger" is a coil or straight tube which sits inside the steam boiler.  In most HX machines the HX is usually partially submerged in water and partially exposed to steam. When the pump is not pumping, the water inside the HX gradually equlibrates to the boiler's internal temperature -- which is well above boiling.  It is able to do that, because when not pumping, the HX is a closed, pressurized system.  

As the machine pumps water out of the HX in order to brew, cool water enters the HX from the reservoir or line at one end, mixing with the hot water still in the HX -- while the rest is pumped out through the group.  In other words, when the machine is pumping water through the group and out the hood, cool water is mixing with hot water inside the HX.  The barista (you) takes advantage of the mixing process in order to "temp" the machine and get the brew water to the proper temperature.  

High temperatures bring out coffee's bitter flavors.  Low temperatures, bring out the sours.  The "proper temperature" represents the barisata's idea of the best balance.  While that can vary from barista to barista most of us are in close agreement that we brew for maximum sweetness -- which is where the bitters and sours more or less cancel.  As a measurable temperature, it's usually pretty close to 198F, but different coffees have different balance points, with a temp range of (about) 195F to (about) 206F (as measured on a Scace).

After the machine has been fully warmed up, and has idled for more than a few minutes, the temperature of the water inside the HX -- as already noted -- is already well above boiling.  When it's pumped out of the HX into the surrounding atmosphere, the pressure drops, and some of the superheated water turns to steam.  The steaming, bubbling, water comes out of the head as a spritz which spreads across the entire screen, making a hissing sound as it does so.  

This boiling resulting from superheated water leaving a pressurized environment is called "flash boiling."

As it cools, the water coalesces into a stream, and the hissing stops.  The change of the appearance and sound gives rise to the expression "water dance" as a description for temping an HX.    

The barista uses the end of flash boiling as information to help temp the machine to the appropriate brew temperature; but hitting that usually requires running a bit more water.  The activity is called "the cooling flush."  You absolutely, positively must perform a cooling flush of some length anytime the water in the HX is too hot in order to brew espresso which is not horribly bitter.

Many people (me among them) find that sound is a more sensitive and accurate indicator of the end of flash boiling.  

Before temping any HX, (especially any HX with "thermo-siphonic" stabilization like an Oscar) draw a long flush -- at least two double shots' worth -- and allow the machine a few minutes to recover.  This will get the various parts of the group and the HX singing in the same key.  

In order to temp your machine after idle, or whenver "dialing in" a new machine (you may want to print these instructions out and leave them by your machine until you get the hang of temping:
  1. Dose and prep the basket.  Set it aside;
  2. Flush until flash boiling stops, then stop the machine instantly;
  3. Flush again for a brief, timed interval.  My machine hits the sweet spot for most coffees after around six seconds.  Yours may be quicker or slower.  It won't take you long before you learn its idiosyncrasies.  You don't have to use a stop watch.  Counting in your head, "one cappuccino, two cappuccino..." and so on is accurate enough.
  4. Lock the pf into the group head and pull a shot;
  5. Taste, sink (throw out) the shot; and
  6. Allow the machine a minute or so to recover*, then repeat until you get the temp right; using the knowledge that Bitter = Too Hot, and Sour = Too Cold as the guide to whether you should temp hotter or cooler.  Unless I'm very close when I start out, I find it easier to use big jumps to make sure that I can taste the difference between too hot and too cold, then zero in with successively smaller jumps.

After you've done this a few times you'll be able to find the generic "good" temp, and the right temp for a coffee you've dialed in without much thought and with great precision.  But temping is something which must be done EVERY TIME with an HX.  

*Note 1:
Sometimes, if you've been banging out shot after shot, or if you've gone too far with the cooling flush, the water in the HX will fall below the appropriate temperature, and the HX must be given time to recover.  Unsurprisingly this is referred to as "recovery time."  

Until you've developed a real feel for your machine, it's a good idea to let it recover all the way back to flash boiling so that you can perform the final part of the cooling flush from the known reference point of the instant that flash boiling ends.

Some machines are crankier than others in this respect; and an unmodified Oscar is one of the more demanding.

Note 2:
People new to the game but still know everything, frequently call this process "temperature surfing," but it is not. Temperature surfing is an SBDU specific term and activity.  

Note 3
Most of the methods noobs use to measure water temp (thermometer in a cup, e.g.) are more misleading than helpful.  If you really want an accurate water temperature reading, you'll have to buy, rent or build a Scace.  However, taste is the most reliable indicator of how well you're temping and is more than good enough for anyone who doesn't make a fetish out of measurements.  

VERY IMPORTANT Note 4:
If you're not "wasting" a LOT of water when you're using an HX, you're doing it wrong. Nature of the beast.

Hope this helps,
BDL
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atytse
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Joined: 9 Oct 2013
Posts: 26
Location: Beijing
Expertise: Just starting

Posted Wed Dec 18, 2013, 4:26pm
Subject: Re: Slightly bitter coffee with Oscar Nuova
 

This is such an awesome answer. Thanks a lot!
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