My first crush

When life gives you wine grapes, make wine!

In case I haven’t mentioned it yet, my parents grow wine grapes. Their vineyard is small and new—a few acres of Malbec in Calaveras County (Sierra Foothills AVA), only a few years old. At the moment, they sell most of the grapes to local winemakers, but this year the complexities of the harvest schedule led to several rows not being included in the main harvest—several hundred pounds of grapes without a home.

So my dad, sis, and I gleaned ~425 lbs of grapes in about 2 hours, at the crack of dawn on Sunday morning (best to pick grapes while the day is still cool, to lower the amount of microbial activity that starts the moment they’re piled up in the bins).

Here’s an overview of how we got the fermentation started on Monday afternoon—not because I think that everyone should necessarily be interested in my family’s home fermentation activities, but because I figured that a blog post about the basics of winemaking would be a fitting start to a new year in the SWS wine science-and-other-stuff blog. With luck, the primary fermentation will be complete in time for our intro wine workshops, so if you attend workshop #2 you’ll get a chance to taste a Malbec in the making, including samples from different stages of production.

Importantly: this was our first time! Well, Dad and I have each had minor experiences (and varying success rates) in various fermentations before, but this is the first time we’ve ever done anything on this scale. If you’re a home fermenter and would like to provide input, it’s more than welcome! Most of what I write here is what I’ve learned in the last few days from reading stuff (most notably, the MoreWine! manual).

 

 

1) Crushing/destemming

First, we separated the grapes from (90% of) their stems, and got the skins open to allow microbes access to the juice inside. We rented a crusher-destemmer was rented from MoreWine! in Los Altos; whole bunches go in the top, stems are pushed out the side, and the must-- crushed grapes and their juice, including small stems, skins, seeds, etc-- comes down the chute at the bottom.

The process of actually crushing and destemming was shockingly quick-- the time it took to run all our grapes through the machine was on the order of 10 minutes. However, those 10 minutes were preceded by about a half-hour of prep and an hour of cleanup. Prep consisted of figuring out how we were going to fit all our must into the food-safe buckets we had, sanitizing them with something called Star-San (phosphoric acid + dodecylbenzenesulfonic acid = low pH + detergent), and dragging the quarter-ton of grapes into the backyard. The cleanup required an unfortunate amount of (used-as-efficiently-as-possible) water... and also figuring out where to put store the buckets of must as they fermented. For that, we settled on the guest bathroom.

We ended up with 5 buckets that held 5 gallons each, and one big Rubbermaid bin that we thought held 12.5 gallons. And sticky grape juice everywhere.

 

 

 

2) Chemical analysis

Before fermentation, we wanted to make sure that the characteristics of the grape must fell within a certain expected range. Of course, it’s all a matter of taste, but it’s also a matter of chemistry, and if some of these things are off, you can expect a finished product that most people don’t consider to be quality wine.

 

a. Sugar

For a red wine, the sugar content of the must should start out somewhere between 22º and 25º Brix. (What is a degree Brix? It’s 1g of sucrose in 100g of solution; read more about the complexity of this fascinating unit on Wikipedia.) As the initial sugar is what is eventually transformed into alcohol, so you can quite straightforwardly predict how alcoholic your wine will end up with if you know how much sugar you start with.

            C6H12O6 (glucose or fructose) --> 2C2H5OH (ethanol) + 2CO2 (carbon dioxide)

However, yeast can only tolerate a certain amount of ethanol before they, like everything else, die of it. If you start out with very high sugar levels, there’s a possibility that your yeast will produce enough alcohol to kill themselves before they get through all the sugar, leaving you with sweet wine.

Also, at really super-high sugar concentrations, yeast struggle because of the osmotic stress-- the sugar pulls water out of their cells, so they have to work hard to resist shriveling up—though I think this really become problematic at higher sugar levels than we had.

If the sugar level is too high, the easiest fix is to dilute with water before fermentation.

If the sugar content is too low, you’ll end up with low-alcohol wine, which I understand is not only a matter of taste and balance but also is less likely to keep or age well. The answer to that is usually to add sugar at the start (chaptalization).

We measured the sugar level in a blended sample of the must using a refractometer, which is a compact, brilliant tool that embodies Snell’s law: the concentration of sugar in a sample of grape must determines the refractive index of the must, and therefore the angle at which light bends when it travels through the must in the tool. A refractometer can also be used to check grape ripeness in the field, and to monitor the disappearance of sugar as fermentation proceeds-- just add a drop of juice to this device and hold it up to the light.

Our must was at 24.5º Brix, which is right within the recommended range.

Of note—this number was quite different than the ºBrix we measured in individual berries, and also in different regions of the vineyard. Grapes are diverse biological products.

 

b. Acid

As far as I understand, the role of acid is slightly less vital to the success of the fermentation process, although the low pH of grape juice is important to preventing the growth of some spoilage organisms, creating an environment in which yeast can thrive (one reason wine yeast are so great—they’re remarkably tolerant of acidity). Importantly, the total acid level is important to the final balance of the wine, and tartaric acid specifically (which constitutes most of the acid in grape must) also contributes to color stability. For a full run-down on wine acids, see my previous blog post on the topic.

In wine, we care both about titratable acidity and pH.

Titratable acidity (TA) is a measure of how much acid is actually in the wine—that is, how many functional groups there are attached to the tartaric acid molecules (and also the malic acid and citric acid molecules) that are likely to donate H+ ions into solution.

However, each of those functional groups has a different willingness to donate H+; also, there are other buffering compounds in grape must that mop up excess H+. So pH is a measure of how much H+ is actually floating around in solution. Typical pH of grape must is 3.5-3.8.

Titratable acidity is the thing that we measured, with an aim to change it if necessary, and hoped pH would follow along. This is the recommendation of the home winemaking manual we read, and also a result of the fact that we had an inexpensive kit that made it easy to measure titratable acidity, but we had no good way of measuring pH. (pH paper is not very precise and digital meters are expensive. This was admittedly a backward experience for me, as I work in a lab where I think of pH measurement as second nature, but titration as relatively painful.)

The kit we used for TA measurement essentially worked as follows:

1) measure out 5 mL of homogenized grape must, using the included syringe (precision is important here)

2) dilute with ample distilled water (precision is not important here, since there’s no acid in the water to titrate, but diluting the must makes color changes easier to see)

3) add some phenolphthalein solution, an indicator that is clear below pH 8.2 but pink above (the deprotonated form absorbs at 553 nm).

4) titrate by gradually adding 1 molar sodium hydroxide (NaOH, a strong base) until the mixture just begins to turn pink; record precisely how much NaOH it took to make the color change.

The point of titration is to figure out how much acid is in solution by how figuring out how much NaOH is needed to neutralize it. The color change in phenolphthalein occurs at a pH conveniently on the way between acid (grape juice by itself) and base (when we’ve added enough NaOH to match the acid and then some). So we measure how much NaOH is necessary to incur a color change, and from that we can calculate the original concentration of acid... if we take some constants and unit conversions into account. Conveniently, the titration kit has done that for us, and provides a handy conversion factor: for every 1 mL of NaOH added, multiply by 1.5 to come out with the grams/L of titratable acid in the solution.

Yes, it is a little difficult to look for a pink color change in a mixture containing red grape must! But it’s not as hard as one might think.

 

The winemaking handbooks recommend starting with a TA of 6.0-8.0 g/L (or 0.60-0.80%). Our titratable acidity was a shockingly low 4.2 g/L. The handbook we used also provided the rule of thumb that adding 3.8 g of tartaric acid will raise TA by ~1 g/L in 1 gallon of must... with the caveat that everything’s a little unpredictable, so you should start by adding less acid than you think you need, then measure, taste, and figure out whether you still need to add more.

For starters, we added ~60% of what we thought we should add, and that ended up being enough. I though the flavor difference in the must before and after acid addition was remarkable; it went from being a very sweet and flavorful juice to a very sweet and also refreshing flavorful juice.

 

 

3) Adding stuff

After figuring out what we had, we then had to figure out what to add. Stuff that one typically adds to wine when beginning fermentation includes:

 

a. Sugar

As I explained above, sugar levels in the must can be adjusted either by adding sucrose or by diluting with water. Happily, we didn’t need to do either!

 

b. Acid

As described above, titration + some trial and error helped us figure out how much tartaric acid to add.

 

c. SO2

Sulfur dioxide (SO2) is a potent tool that both prevents the growth of spoilage organisms and acts as an antioxidant to protect wine flavors. It’s a very common additive at multiple stages of winemaking; if you want to know all about it, I recommend this thorough article by Practical Winery & Vineyard. Controversies about allergies and additives aside, I find SO2 use fascinating from an evolutionary and ecological point of view. Wine yeast is a very special microorganism in that it’s resistant to sulfite levels that kill many other fungi and bacteria—and the yeast we use today probably acquired that resistance after many generations of selection in the high-sulfite environment of winemaking (Pérez-Ortı́n 2002).

We added SO2 in the form of potassium metabisulfite (K2S2O5). We dissolved a measured amount of the powder in water first... and they’re right; the smell is pungent—you’d recognize it as something you’ve probably inhaled from certain wines, or from sulfured dried apricots. Apparently there’s a good deal of chemistry that can go into calculating your SO2 needs if you really want to get it right, but we ballparked it based on the crudest of guidelines.

 

d. Yeast

The most important part! Well, there are definitely wines that are made without the addition of commercial yeast—many great winemakers depend only on the native yeasts that inhabit the grapes and the winery, and colonize the grape must on their own. I love that idea. However, many studies seem to indicate that grapes don’t come with many useful yeast on their own, and rather the winery is the most important contributor to that wild yeast colonization, having had resident populations established for years (Ciani et al. 2004). Given that our fermentation is occurring in a bathroom rather than a winery, we figured we’d probably better make our own contribution to the yeast population.

Choosing a wine yeast strain can be a fun process (browse the Lallemand website for starters), but we arrived at MoreWine! late enough in the game that were few suitable wine yeast left in stock to choose from. So we settled on Lalvin BM45, an Italian yeast isolated in 1990; its description notes that it should contribute good mouthfeel to the wine, and has high nutrient requirements. I have a feeling that it’ll take a significant amount of experience in getting the other parts of winemaking right, before we know enough to know what we want out of the yeast strain we choose.

Our yeast came in dried form, and had to be woken up before being added to the must. We did this by first mixing it with water and a yeast nutrient medium called Go-Ferm (smelled a bit like malt extract and a bit like Vegemite; perhaps it contained both), until it started to bubble (exactly as you would with your active dry yeast before breadmaking). Then we added some grape must and let the yeast get used to that and foam again for 20 minutes, before dividing it among the buckets.

 

e. Yeast nutrients

I mentioned that BM45 has high nutrient requirements. Yeast need a balanced diet just like every other eukaryote; as a medium for growh, grape must can veer toward the skittles-and-starburst end of the nutrition spectrum, and nitrogen shortage is a common problem. When this happens, yeast get stressed, and they may 1) produce unpleasant compounds such as hydrogen sulfide; or 2) quit fermenting altogether (this is called “stuck fermentation”), leaving you with a low-alcohol sweet mixture that screams for other fungi and bacteria to come take up residence.

Accordingly, there are yeast nutrient mixtures on the market to prevent these problems. Fermaid-K is such a mixture; it provides a lot of nitrogen in the form of diammonium phosphate, as well as building blocks for proteins (amino acids) and healthy cell membranes (fatty acids and sterols), and several B-vitamins. And overall nutrition in the form of “inactivated yeast”—apparently live yeast are perfectly happy to be nourished with dead yeast.

Addition of nutrients like Fermaid-K is technically optional if you have really great grapes and you know what you’re doing, but it was sold to us as an easy insurance policy, which we bought.

 

f. Other stuff

There are other products that one can add at the start of fermentation, to do things such as aid in grape skin breakdown, boost red fruit character, or contribute to tannins. Whether to add these depends partly on your philosophy and partly on whether you even know enough about winemaking to know you want them, and we definitely didn’t have the second part going for us so we opted out. But you can find out more at the MoreWine! website.

 

4) Waiting

We finished setting up the fermentation on Monday night, and from now on we just wait and watch. We’re keeping things lightly covered with bucket lids or saran wrap, punching down the “cap” (skins and stems that float to the top) on a regular basis to prevent colonization by aerobic microbes, monitoring the sugar level regularly to keep track of how fast it’s getting turned into alcohol, and crossing our fingers that the buckets don’t overflow once the gas production really gets going. The primary fermentation should be complete—that is, the sugar should all have been converted to CO2 and ethanol—within 1-2 weeks. Then comes the next stage—pressing. Stay tuned for updates.

And come to our intro wine workshop #2, where we can talk more about acid, sugar, and alcohol!

 

 

Some references:

Ciani, Maurizio, Ilaria Mannazzu, Paola Marinangeli, Francesca Clementi, and Alessandro Martini. 2004. “Contribution of Winery-Resident Saccharomyces Cerevisiae Strains to Spontaneous Grape Must Fermentation.” Antonie van Leeuwenhoek 85 (2): 159–64.

Henderson Pat. 2009. "Sulfur Dioxide: Science behind this anti-microbial anti-oxidant, wine additive". Practical Winery & Vineyard Journal, Jan/Feb 2009: 1-6.

Pérez-Ortı́n, José E., Amparo Querol, Sergi Puig, and Eladio Barrio. 2002. “Molecular Characterization of a Chromosomal Rearrangement Involved in the Adaptive Evolution of Yeast Strains.” Genome Research 12 (10): 1533–39.

 

And thanks to my dad, K. Mark Lee, for most of the grape photos accompanying this post.