Sunday 31 October 2010

Study vs life, and the difficulty of answering questions.

Life's been winning lately. In some ways, this is a good thing; I've been, in general, pretty happy and upbeat of late. I've been concentrating on me-stuff - getting to know my job and my workmates, exercising, playing games, and generally not taking on any stress.

However, the bill for that is just about to come due. I've got an exam on Tuesday, and since I effectively haven't (and haven't effectively) studied since early August, I'm not rating my chances of doing well on this exam particularly highly. Wine is a complex subject, and I don't have quite enough chemical background to really easily understand the wine chemistry I'm doing at the moment. If I had a week, and no other distractions, and my focus was working, I'd be confident. As it is, I have about 48 hours, a focus shot to hell, distractions left, right and centre, and a sprinkling of other things to do.

It doesn't help that what I'm trying to study and understand is so dry, and the resources given to me to help me understand are, in my opinion, quite insufficient. For instance, a question that I'm currently trying to answer: Hydrometry cannot provide an accurate measure of the sugar content of a fermenting wine. Why?

I think the answer lies somewhere in the following chunk of text:

The absolute density of any substance, expressed in units of grams per cubic centimetre, or grams per milliliter, is defined as: Density of substance = (Weight of substance / Volume of substance). Direct measurement of volumes may present a problem, especially where gases are concerned. As a result, it becomes convenient to use the ratio of the density of a substance to that of a recognised reference such as water. This relationship, known as specific gravity, is expressed as: Specific gravity = (weight of * mL of substance) / (weight of * mL of water) .
The density of water at 4degC is, for all practical purposes, 1g/cm^3. Because the weight of any substance will change as a function of temperature, any complete definition of specific gravity must include the temperature at which the determination was made, as well as the reference temperature for water. The temperature of the measured sample is noted above that of the reference. For example, the notation 15deg/4degC indicates that the specific gravity of the solution in question was made at 15degC relative to water at 4degC.
The concentration of dissolved substances in solution is related to the specific gravity but one should know, however, assume a simple and direct correlation between observed specific gravity and concentration in all cases because molal volumes of substances in solution may vary in a complex and unpredictable manner. Tables are available that relate concentration of dissolved substances to apparent specific gravity; those most commonly encountered in analysis of wine are for Brix, Baume, degrees Oechsle, and alcohol.
Tables usually reference only one or two standard temperatures; one must either measure the specific gravity at the defined temperature or, alternatively, employ a temperature correction factor. For the most accurate work, it is recommended that the solution be brought to defined temperature prior to measurement.
Hydrometric determinations are based on the principle that an object will displace an equivalent weight in any liquid in which it is placed. The volume displaced by an object is inversly proportional to its density. Hence a solution of high density will show less displacement than on of lower density. This relationship defines the basic principle of hydrometry.
A hydrometer consists of a calibrated scale within a glass tube that is usually constructed with a mercury or shot-filled terminal bulb to maintain it in an upright position. Hydrometers are available to read either specific gravity or the concentration of some component in solution. Examples of the latter include the familiar saccharometer and the salinometer.

Excerpted from Zoecklein, B. W., Fugelsang, K. C., Gump, B. H. & Nury, F. S. (1995) Wine analysis and production, Aspen Publishing, Gaithersburg.

The answers I'm coming up with here are musings along the lines of the way temperature tends to vary in a ferment, the fact that ferments are complex beasties, with lots of different things doing their own dance, and that gas (specifically, carbon dioxide) is given off as part of the fermentation process.

But I'm not sure that's correct, or complete. And I don't have anything that says 'this is the answer we are looking for'. And, due to the joys of distance ed and a really small degree program, I don't have any classmates to ask. And I don't want to be wrong. I guess I'll just skip that question - like I have about half the questions - and move on, hopefully to something I can be confident that I can understand. I may be overthinking my answers to these questions, but that's the thing: I don't know, and I have no way of finding out. Well, except by having a question on it in the exam, and getting it right or wrong - if I get the marked exam back. Which, incidentally, I didn't last semester, so I still don't know where I gained or lost marks there.

Basically what this adds up to is I'm stressed, anxious, worried - although not yet depressed. If there's anyone out there that feels like answering this and other questions for me, or if you can point me at resources which I can read and hopefully understand the processes going on, that'd be great. If not - well, wish me luck. I feel like I'm going to need it.


  1. Hope you get past the stress...

    I'm no chemist, but my thought on reading the text + the question again was (although simplistic so probably wrong):
    hydrometry reports density, the sugar content of fermenting wine cannot be told by density alone because it will depend on exactly how much CO2 has escaped out of solution?


  2. That's one of the conclusions I've come to, yes. Thanks :)

  3. I guess the simplest answer is that hydrometry only works when you have a single substance dissolved in the water. So if you have a sugar solution, you can measure the density and look it up in a Brix table; or if you have an alcohol solution, you can measure the density and look it up on a table relating density to alcohol content. But a fermenting wine contains significant quantities of both alcohol and sugar, so you can't use either of these tables.

    There's CO2 in the solution, too, but the max solubility of CO2 in water is something like 0.1% w/v, whereas the concentrations of sugar and alcohol can both be several %. So when you're measuring the sugar content, the alcohol will cause more inaccuracy in your results than the CO2 will.

    P.S. Disclaimer: I am not a chemist. The above is mostly based on my half-forgotten memories of high-school chemistry.