Grapes aren’t the only things that end up in your picking bin, and not all the grapes that do are the kind you want entering your fermentation. Editing which grapes make it to the fermenter is, in my opinion, not an option, even when working with the cleanest of pickers. Most importantly, this is a way to make sure as few grapes as possible harboring quality-damaging microbiota enter your fermentation cycle, and secondarily, as a place you can easily increase the quality of your wine. From bugs to leaves to damaged to underripes to overripes, let’s dive in and see what you do and don’t want bubbling its way through your fermentation.
As a home winemaker you will likely be sorting whole clusters, which, I think without a large team of people to sort through individual berries (or a cost-prohibitive color sorter), is the way to go for commercial producers as well. Destemming and sorting individual berries, to complete in any realistic period of time, requires lots of hands; a quick moving conveyer or vibrating table with too few hands means poorly sorted grapes, and a slow moving one with any amount of hands means a thick mass of layered berries you can’t efficiently sort.
To start, let’s go over the most crucial part of sorting: Getting out the bad stuff. These are the things that can be detrimental to your wine’s quality — either directly or indirectly via the flora they harbor. Here we’re talking about damaged or diseased grapes. These should all end up in the compost bin, so let’s go over their variety, causes, and what they can do to a wine if included in fermentation.
Afterward, we’ll talk about underripe and overripe grapes, as well as material other than grapes (MOG), the effect they can have, and then figure out how important their removal is to your wine. Depending on the amount of grapes you are working with, it may be impossible (within a realistic timeline, as well as to maintain sanity) to get everything out, and in those cases I think it is important to have in mind a hierarchy of necessity for removal. The first priority being a focus on what absolutely needs to be removed (damaged or diseased fruit!). The second priority lies in things that pose less threat but may (albeit on a micro scale) work against your style goals. The third priority should consist of those factors that pose a threat to your wine but may work (again, on a micro scale) with your style goals.
The level of importance you distribute to these three categories should be adjusted for the amount of material in each category you have in your grapes. For example, if you have a ton of Botrytis, this should be your first priority to remove, and you may want (or need) to let more of the second and third priority material pass into the fermenter. Conversely, if you have little or no diseased fruit, you have much more time and energy to take out less threatening material from category two or three. Let’s begin!
It’s rude not to finish your meal
Damaged grapes are the things nightmares are made of. As soon as your grapes begin veraison they become a target for birds, bees, wasps, deer, boars, and, if you happen to own a world-famous vineyard, grape thieves. Damage to grapes, particularly perforated or half eaten berries, exposes the nutrient-rich inside of the grape to microbiota, which inoculate the berry, and, by the time they reach your fermenter, can harbor significant populations of quality-damaging flora — either by bringing unwanted bacteria (particularly acetic acid bacteria) into your fermentation, or the microbiota’s by-products, which can be problematic in larger quantities.
While some animals may do a better job of eating entire grapes than others, the birds and the bees poke holes and, true to their fame, lead to the potential for new life! This new life, however, is not the kind we want inside our grapes. Microbiota is everywhere — everywhere! — and can come from anywhere. It’s on the outside of the grape, the leaves, the bark of the vine, the soil, blowing in on winds from adjacent areas, in our picking bins, every winery surface, on our hands and under our nails, and so on, and so on. Recent UC-Davis research has shown that the surfaces of different grape varieties are home to differing microbiota, which is consistent (by variety) across various regions! Bees and wasps actually carry yeasts and bacteria in their gut, derived from their parents and from past meals, which are then passed on to their young via regurgitation feeding and are continued in the following vintage.
Damaged grapes will often stink of volatile acidity by the time harvest comes around. Some berries will have been hollowed out, only the skin remaining, the pulp having been eaten by fauna or flora, juice long since having been fermented to alcohol and then vinegar. This is not the kind of grape you want to make wine from.
In addition to the variety of bacteria and yeast strains that can be found in damaged grapes, you can also find a variety of fungi: Penicillium, Aspergillus, Trichothecium, and more, which contribute off-flavors and mycotoxins, some of which are cytotoxicins and carcinogens. We’ll discuss their effects in the disease section.
Diseased grapes can cause just as much trouble in your fermentation, and as little as 1% of your crop being affected can lead to negative effects! There are a myriad of bad grape-affecting diseases, so I will only discuss the two most common and widespread: Botrytis and powdery mildew.
Botrytis cinerea (and a few very similar fungi), also known as bunch rot or noble rot, is a nasty disease with far-reaching effects on grapes (some massive, others subtle). It is a complex topic that I’m sure fills many university dissertation shelves. It is a common fungus on plants and can overwinter and reside in the vineyard itself, or blow in from surrounding plants, forests, etc. While under certain circumstances, in certain climates, in certain hands, amazing wines can be crafted from Botrytis-infected grapes (Sauternes and Barsac in France, Beerenauslese and Trokenbeerenauslese in Germany, and many more beyond), but for the vast majority of grapes and winemakers, it is a big problem.
Botrytis is a grey mold that damages and dehydrates grapes, concentrating sugars and some acids. It creates certain phytotoxins that are thought to hinder yeast growth. It finds a home on damaged parts of a grape where it eventually creates holes in the fruit, and, similar to animal damage, other flora can find a home in the grapes, leading to a variety of undesirable sensory and health related outcomes. Volatile acid-producing bacteria aside, non-Botrytis fungi are happy to make a home within the damaged grape, and are thought to create the moldy aromas often associated with Botrytis-infected grapes. To name a few: Aspergillus and Penicillium may lead to moldy odors and Trichothecium can result in bitterness. Some of these fungi generate mycotoxins, some of which are known allergens and carcinogens.
Uncinula necator, also known as powdery mildew or Oidium, is just as bad, and can cause huge crop losses, though perhaps from a winemaker’s standpoint, has a narrower spectrum of potential dangers than Botrytis. It can destroy both green vine material (leaves, shoots, shoot tips) and grapes alike. It spreads across grape and green surfaces, shooting protrusions into them, which suck out nutrients. The areas of the plant they cover will eventually die. Leaf loss early in the season from powdery mildew (or anything else, for that matter), can lead to lower crop loads for the following year. If used for wine production, it can create a mushroom-y aroma, bitterness, a higher pH, and lower anthocyanin levels. Despite the fact that powdery mildew is often white, during the harvest period, the fungus may have a red or black color.
Underripes, overripes, & MOG
Individual grapes on a cluster, individual clusters within a canopy, and even different sides of a cluster, do not ripen evenly. Most instances of this are due to environmental factors, for example sun exposure (more exposure/development to the outside facing portion of the cluster, less on the interior side) and flowering or fruit set issues (for example “hens and chicks” — small and large berries within the same cluster). Thankfully, these differences are not normally so significant as to warrant removal. Uneven ripening can also be typical of a grape variety: Zinfandel is notorious for frequently having everything from green to overripe, wrinkling grapes on the same cluster. In every batch of grapes, you will likely find some underdeveloped clusters with tiny, acidic berries, and some overripe, dehydrated/raisining clusters, as well as clusters containing some of one, the other, or both. I will discuss these more in-depth below.
MOG (material other than grapes) is just that: Anything that winds up in your picking bin that is not a cluster of grapes (i.e. grape and grape-stems). This can be grape leaves, bugs (I usually find lots of earwigs and ants), dirt, agricultural tape/tools/etc.
Microbiota is everywhere, including on the surface of MOG; but luckily, the types of flora that thrive on the surface of leaves and in dirt are usually not the type to cause problems in a fermentation — however, the cleaner the better, of course. I will discuss bugs below, but any MOG that did not come from the grapevine (plastic, gloves, and so on), should absolutely be discarded.
Bugs, at least in small quantities, are not ideal, but not likely detrimental — as mentioned above they harbor microbiota, but it is their inoculating damaged berries with this microbiota, which leads to much larger populations in berries than the bugs can themselves harbor, that is the greatest concern. They are too diverse, and in differing geographic areas to practically discuss here. I encourage you to study any pests that are common in your area, and in what amounts they become a problem for quality. For example, mealy bugs in small quantities are not a huge concern, however in larger quantities — where removal is impossible as they can live out of sight, deep within clusters — can cause a moldy, mushroomy aroma in the finished wine. With earwigs a tasting during one study found that it was at 10 or more earwigs per kilogram of grapes that began to lower wine quality, or even just 0.6 g/kg of earwig feces! I wonder who drew the short straw at that tasting . . .
Harvest days are among the longest of the year, and energy becomes a scarce resource. That being the case, it never hurts to find harmless places to save time and energy. If your goal is volume, or a tasty, simple table wine, by all means save your effort and include all that is not damaged or diseased. Unless there are significant quantities of green or dehydrating grapes, it is unlikely to have notable affect on your wine. Underripe berries will (at least in theory, as opposed to a perceptible or measurable way) add a bit of acid (along with some bitterness) and overripe berries will add a bit of extra sugar (by volume) and dried fruit aromatics. In both of these cases, if one of these types of grapes would work against your wine style, I would prioritize their removal over the type that works with your style.
If the highest quality attainable is your goal, and the fruit you have is capable of great complexity, I am of the mind that the cleaner your fruit, the better your wine will be. One can argue here that with grapes that are only capable of producing a simple, pleasant wine, that some variation in composition of the wine (be it by differing fruit composition, spontaneous fermentation, blending, oak regimen, and so on) will add complexity — I can’t disagree.
Sorting grapes is a crucial step in the winemaking process that should not be skipped. At the very least, it is one of the more stationary and easier points in the process to minimize physical exertion, and perhaps put into practice the age-old winemaking adage “it takes a lot of beer to make wine”.