A cautionary tale of mice, microbes, and mechanical harvesting

Short: A recent medical report indicates that fresh juice from mechanically harvested grapes can (on very rare occasions) carry infectious diseases from animals picked up and crushed by harvesting machinery.

Long: Winery work is dangerous. Especially (but not only) during harvest, wineries are production facilities with heavy objects, slippery floors, potentially hazardous chemicals, machinery with moving parts and, often, forklifts. The positive side of that situation is that winemaking professionals know that winemaking — like any similar form of factory or food production work — is dangerous, and they take steps to mitigate risks. An enormous part of mitigating risks is knowing what those risks are, which is why a recent letter to the editor about mechanical harvesting in the New England Journal of Medicine is worth knowing about.

Potential dangers of mechanical harvesting are usually discussed in terms of dangers to the grape vines (of being beaten up by the machines), to the wine (though mechanical harvesting is far from being always a bad thing for quality) or, maybe, to vineyard soil compressed by heavy machinery or animals caught by harvesting equipment. Endangering animals, however, might on rare occasion also endanger humans — and not just if a harvester has an inadvertent run-in with a neighbor’s cat. Some critters commonly found around vines can carry diseases that affect humans.

When six of 29 harvest workers employed at a vineyard in Rhineland-Palatinate, Germany, came down with the same symptoms — trouble swallowing, swollen lymph nodes in the neck, diarrhea, general aches and chills — regional specialists worked them up for infectious disease and came down with a diagnosis of tularemia. Tularemia is what happens to humans when infected by a bacteria, Francisella tularensis, carried by some rabbits and rodents. Most cases occur in hunters. Symptoms vary with how the bacteria entered the body, but the bacteria are never transmitted from human to human, only from animal to human. Or, in this case from animal to grape juice to human. All six workers who became sick remembered drinking fresh juice from the same load of mechanically harvested grapes before becoming sick. Epidemiologists found DNA from F. tularensis and from common field mice in the freshly fermented wine made from that batch of grapes.* Their concluding hypothesis was that an infected mouse or two had been picked up by the harvester and crushed along with the fruit, contaminating the fresh juice.

The wine was confiscated and prohibited from sale, needless to say, though probably without real cause. Infectious disease transmission via wine is unheardof, thanks to the combination of high alcohol and low (acidic) pH that makes wine an inhospitable environment for could-be pathogens. Though I don’t know (and I’m not sure anyone knows) about F. tularensis‘s ability to survive in wine, it’s important to note that the epidemiologists found F. tularensis DNA in the wine, not intact infectious bacteria, and that workers suffered from drinking grape juice, not wine.

The authors of the NEJM letter — most of them German public health officials — concluded that “raw food stuff should be treated before consumption.” If that means “we recommend that you don’t drink fresh juice from mechanically harvested grapes,” that sounds pretty reasonable. Like recommendations about not eating raw cookie dough, many will choose to accept the risk; a lot of us will accept the very small chance of getting ill from bacteria in raw eggs or raw flour for the sake of the certain pleasure of enjoying delicious cookie dough. But, importantly, we know that the risk exists, and if we become sick we might even think to tell our physician about what we ate, which could speed up receiving appropriate treatment.

Tularemia is endemic but rare across the United States and Europe (it’s mostly a Northern Hemisphere disease), with only a few hundred cases per year across the United States. The lesson here would NOT seem to be “don’t drink fresh mechanically harvested grape juice or you might get tularemia” but, rather, “know enough about your environment to have a clue about what might have happened on the very, very rare occasions that something goes wrong.” Also, watch out for mice.


*Did any of the harvest workers remember seeing field mice in the vinyard, and did anyone in the cellar see a dead mouse or two pour into a fermenting vat? Unfortunately, the public health officials who wrote the report didn’t say.

New, better pictures of what bacteria are doing during fermentation

Short: Microbiologists, using new techniques, are finding that actively fermenting grape musts contain a much wider variety of bacteria than we’ve previously recognized.

Longer: Yeast, being the main actors in alcoholic fermentation, are going to get most of the attention during it. But that’s not the only reason why we don’t know a whole lot about bacteria during fermentation. They’re hard to grow. Doing microbiology the classic way means collecting samples and growing bacteria from them in petri dishes, then identifying whatever grows in the dish. I used to spend measurable fractions of my life “making plates” by mixing Jell-o for bacteria and pouring it into hundreds of little plastic dishes. A main activity of microbiology is waiting for stuff to grow on your plates. The logic here is simple, understandable, and incredibly silly. Bacteria are unbelievably diverse and incredibly specific to their environments. It’s balderdash to think that all of them are going to grow happily, on command, in a dish, in a week or two (if you don’t just throw Monday’s plates out on Friday, which sometimes happens).

The petri dish method is just fine* for working with well-identified and properly house-trained bacteria. It’s pretty horrible for trying to identify all of the unknown bacteria growing in some mystery environment. Only when alternatives became available did microbiology really start coming to terms with the magnitude of what it had been missing. Today, looking for bacterial “unknowns” means identifying bacterial DNA, which is more direct and gives you a better chance of picking up punk microbes that aren’t willing to grow nicely in captivity. Search for bacterial DNA in a vat of actively fermenting grape and you’ll find evidence of a lot more bacteria than the conventional mechanism ever had us thinking about.

Using these techniques, Bokulich, Mills, & co. at UC Davis have been mapping bacterial communities in wineries around the calendar year, wineries across California, and wines with more and less SO2. New research (open-access article), from a (mostly) Washington state-based group, has pointed out something simpler and yet very worth knowing: fermenting wine contains scads more bacteria then we’d ever thought about before. They used “next generation sequencing”** techniques to take snapshots of bacterial communities five times through two weeks of fermentation.

The authors make some questionable comparisons of patterns of bacterial growth between their two study conditions – all the grapes involved were organically grown Riesling, but half were fermented “organically” without added SO2 and the other “conventionally” with SO2. But the experiment involved only a single comparison: two vats, same batch of grapes. Limited replications is no doubt a trade-off with fermenting in realistic 15,000 gallon volumes instead of the completely unrealistic five-gallon carboys too common in much wine research. Regardless, it’s going to take many more comparisons before it’s possible to talk meaningfully about differences in bacterial abundance with and without added SO2.

Here’s why this research is still important. Right now, wine bacteriology is mostly two things: malolactic fermentation, where bacteria are the good guys (unless you’re trying to prevent MLF and it’s happening anyway), and spoilage by a pretty well-known set of culprits, especially acetic acid bacteria. That’s a bit like saying that all Americans are either New York City firefighters or drug dealers. There is a whole lot more going on in both cases. And some of our persistent wine mysteries – why some fermentations stick, some go faster or slower, some produce one aroma and others another – may owe something to that unseen majority. If microbiologists start seeing them, maybe we’ll find out what.

*It’s also time-consuming, labor-intensive, and incredibly wasteful in terms of the masses of plastic that get thrown away. Sometimes you want to do an experiment and can’t because you don’t have enough plates. Or the results you get at 6:00 pm suggest an experiment you should do tomorrow for which you’ll need more plates and you stay until 9:00 pm going through the several-hour process of making more, or someone else uses your plates without telling you, or your plates get contaminated with mold and you have to throw a big batch out.

**As opposed to “deep space nine” sequencing techniques, which are expected to come out next season, will take longer and be more sophisticated, but will never be quite as cool as its predecessor because Patrick Stewart isn’t involved.