Syrup and botulism

[Rangdale]

We got 10 gallons of organic blueberries so I decided to try my hand at blueberry syrup which started me worrying about botulism if I didn't do it right. After some research it appears blueberries are too acidic for the botulism spores to grow which got me thinking about maple syrup. I don't believe maple syrup is all that acidic so why isn't there a botulism risk? or is there? I got curious but couldn't really find an answer. Does anyone know?

[GeneralStark]

High concentrations of sugar (and salt) inhibit microbial growth. There are several reasons for this but the primary mechanism is related to osmosis. If a bacterium were to find itself in a highly concentrated sugar solution (maple syrup), the water in its cell would flow through its cell membrane to the higher sugar concentration on the other side, thereby causing dehydration of the cell and death.

One other factor is available water, and this is another reason (in addition to sugar concentration) why it is important to get your syrup to the proper density for storage.

[DrTimPerkins]

High concentrations of sugar (and salt) inhibit microbial growth. ...

It all comes down to the water activity (and pH in some cases) of the food item. In the case of maple syrup, it is water activity that is important (and pH is typically not except in the case of extreme fermentation). Water activity (aw) is basically the vapor pressure (like humidity, or moisture content) in a food item. Pure water has an aw of 1. As food items gets drier, water becomes less available (or more tightly held to the food material), then aw gets lower. Different microbes require different amounts of water to live. Some need it really moist....others can deal with it being drier. Bacteria typically need a fairly high aw, but some fungi and molds can survive lower aw levels, although they might not grow very fast if aw is low.

Botulism, which is caused by Clostridium botulinum bacteria (a real nasty little beastie), typically don't do very well below about aw 0.91. While it may persist below that, it doesn't reproduce well or produce the botulism toxin. Maple syrup has an aw of 0.87-0.88 (Perkins and van den Berg, 2009. Maple Syrup—Production, Composition, Chemistry, and Sensory Characteristics. p. 101-143. Advances in Food Nutrition, Elsevier Publishing), so there isn't much cause for concern.

This is why both proper packing density and proper packing temperatures are critical in preventing and retarding spoilage of maple syrup. Heating to 185 deg F kills or inactivates most of the microbes, and proper packing density (which reduces water activity) keeps things from growing (or growing rapidly).

U. Maine has been (Kathy Hopkins and her colleagues), and is continuing to do some interesting research (funded by NAMSC Research Grants -- the penny per container producers can voluntarily contribute) on identifying what microbes can live in maple syrup, and whether any of them have the potential to produce mycotoxins. Really neat stuff.

More info at http://extension.psu.edu/food/preservation/issues/water-activity-of-foods/water-activity-of-foods-table

[GeneralStark]

Very interesting! At about what density would maple syrup potentially become problematic? I assume the .87-.88 aw value is using VT's standard?