The residue is naturally occurring yeasts. Grapes have it, Plums have heavy layers. You can make natural wine by just crushing the fruit, no yeast added. But you do run the chance of also getting an acetobactor, which will turn the alcohol you want into vinegar. Since I have no need for a 10 gallon batch of plum vinegar, I add potassium metabisulphite to kill off all of the natural stuff, and replace with champagne yeast. She's a-bubbling still, so it is working.
Yes many fruits including grapes, apples plums support a wide range of wild fungi including wild yeasts, but you are confusing the broad category of yeasts with the rather narrow group of fungi than can be used for alcoholic fermentation (Saccharomyces cerevisiae, S. bayanus, B. dekkera, and a few rare others). Yes there are a few lucky places, parts of Rhone for example, where natural fermentation can produce a palatable product, but it's not common. Wild yeasts may not produce ethanol as their primary end-product, and many can introduce obnoxious flavors and aromas. For a lot of reasons I won't go into here, wild members of common wine/ale/bread yeasts, S.cerevisiae, tend to dominate (they are potent competitors for simple sugar solutions), but wild yeasts can themselves produce obnoxious off-flavors. There aren't many organisms that can tolerate the low pH, and oxygen-free environment that S.cerevisiae produce. A few surface fungi (where O2 is available) can survive, some Brettanomyces and then we have to switch from eukaryote fungi to prokaryote bacteria.
Some bacteria can live at the low pH and metabolize some of he simple organic acids from fruit. Famously Leuconostoc oenos converts malic acid to lactic acid in malo-lactic fermentation commonly used in red wines. Lactobaccili and lactococcus may survive. Acetic acid bacteria, mentioned, are probably the wort case scenario for a wine maker. That can tolerate the low pH and metabolize ethanol into CO2 and acetic acid. They require free oxygen to metabolize EtOH so they can only proceed where some oxygen is introduced.
Metabisulfite salts release free SO2- ions into the beverage, while commercial winemakers may add SO2 from tanks. Common wine yeasts also can release small amounts of SO2- ions. The SO2- ions are a strong anti-oxidant and also, at concentration, an antiseptic
So what's an effective, practical way to lose the harmful microbes? Under the faucet cold rinse? Hot? With soap?
When processing apples for cider it is recommended that the wash water must be warmer than the fruit to prevent the gas in the fruit from contracting and sucking bacteria into the inaccessible interior. That's probably a good idea generally.
The biggest culprit on fresh foods is perhaps E.coli O157:H7. Escherichia coli is a common mammalian gut bacteria, with a few dangerous and potentially lethal serotypes. Vectors might include poor meat processing methods, cattleyard run-off and the deer and the antelope (or even goats, rabbits, horses) playing in the garden or orchard.
So pasteurizing milk, cooking ground meat appropriately, using proper kitchen sanitation are the basics. Yes washing fresh produce.
The "Fit Produce wash " product mentioned on this forum has an MSDS sheet here:
http://www.tryfit.com/downloads/MSDS Liquid Fit Institutional.pdf showing the product contains
WATER 7732-18-5
CITRIC ACID 77-92-9
ETHANOL 64-17-5
GRAPEFRUIT OIL TERPENES 68917-32-8
SODIUM LAURYL SULFATE 151-21-3
The red-flare items are grapefruit terpenes and sodium lauryl sulfate.
Grapefruit oil terpenes are used as an aroma agent, isn't water soluble (but is soluble in ethanol and oils). There is evidence this can be a skin irritant perhaps a stomach irritant. At the levels present it's undoubtedly a "don't care" for all but one in a million. You probably get as much handling grapefruit.
Sodium lauryl sulfate (aka sodium dodecyl sulfate) is a anionic detergent created from esterified products of lauric acid - a mid-chain length saturated fatty acid.
http://www.nicnas.gov.au/publicatio..._chemical_information_sheets/ecis_sls_pdf.pdf
Short and mid-chain fatty acids aren't any more innocuous than drinking kerosene - they act as detergents capable of rupturing the lipid bi-layer of cells - general cell killers. OTOH this property is greatly reduced as the Carbon chain length increases and after esterification.
Despite this the sodium lauryl sulfate has anti-microbial properties and possibly anti-viral properties. It's used in toothpastes, antiseptic handwash, laxative enemas. It has an LD-50 about 1000mg/kg for some rodents, but appears innocuous and non-carcinogenic at 100mg/kg. It may cause oral ulcers.
In all - and at the concentrations present - there isn't very more troubling that brushing your teeth or handling a gratefruit; which isn't to say "zero risk".
I have serious doubts that an agent like "Fit" can be effective at killing microbes on produce given the concentrations - but I'd be willing to read any studies. Still a mild antimicrobal detergent to wash off debris is a good idea. At any concentration that might be effective as a microbicide, I'd want to make sure I rinsed the Fit product off produce very well.
