Can yeast affect OG readings?

I have been making one gallon batches of cider for a few years now, and love it. Usually have 4-6 going at any given time. I got a refractometer for xmas, so i have been testing everything (i have never really taken a OG on cider before)

Anyway, i was transfering a cider to secondary, and pitching a new one on the yeast cake. I took an OG reading of the juice, and it came in at 1050 on the dot. I then added the juice to the fermenter. I boiled up a bit of water, mabe 2 cups. Steeped some black tea in it, and added 1/2 cup brown sugar. I poured this in the fermenter and shook vigorously. Remember i pitched on the old yeast cake. I then took another OG reading to see how much the 1/2 cup of brown sugar would boost the OG. This reading came in at 1045.......

So my question is, can the yeast from the yeast cake alter the gravity readings in a refractometer? Because there is no way adding sugar to my cider made the gravity drop 5 points

 

No. The dilution with water would have an effect though. Still, that seems like a lot.

Edit: I've never used a refractometer, so if that works differently than a hydrometer I dunno.

 

i know the water can dilute it, but there is no way a cup of water with a half cup of sugar is gonna lower my 1050 OG. I just dont get it.....

 

Yuh, seems really odd. Can you double check with a hydrometer?
And how long after putting it on the yeast did you check it? Right away?

 

You said you shook your cider vigorously. That presumably stirred up the yeast cake, and your cider was cloudy. Never used a refractometer myself, but the handheld variety are based on transmission of light through your sample. If you've got a bunch of lees swirling around scattering and absorbing the light, it might affect the readings.

I searched for something like this and found this statement here (http://www.globalw.com/support/refractometer.html):

"Because handheld refractometers work with transmitted light, highly colored or opaque samples may not give very sharp borderlines. The position of the borderline then becomes a subjective judgment of the user, which reduces the reproducibility of the handheld refractometer. Automatic bench refractometers surmount this problem by measuring the reflected light from a sample surface, rather than transmitted light. Thus, for example in the tomato paste industry, automatic bench refractometers are considerably easier to use and much more reliable than handheld refractometers where borderlines are often not discernible."

Other things that can affect the reading: calibration and temperature (probably not an issue unless your tea/sugar heated the cider)

It's probably too late to get an accurate hydrometer/refractometer reading of OG, but you should be able to calculate the estimated OG based on the sugar added and the volume.