Acid, Alkaline or Neutral
The acidity of any substance is measured according to the pH scale. The abbreviation "pH" comes from "power of hydrogen," because acidity is directly related to a material's concentration of hydrogen ions. The pH scale ranges from zero to 14. The middle of this range, pH 7.0, corresponds to a substance that is neutral, meaning neither acidic nor alkaline. Values above 7.0 apply to alkaline substances, also known as bases, and values below 7.0 apply to acidic substances. The acidity or alkalinity of a substance becomes stronger as pH values deviate further from 7.0. Examples of common acids are honey (pH 3.9) and vinegar (pH 2.7); examples of common bases are bleach (pH 9) and household ammonia (pH 12).
The typical pH of raspberries is far below the neutral point of 7.0, but this would not be sufficient reason to classify raspberries as an acidic food, because almost all foods are somewhat acidic. Meat, dairy products and vegetables tend to be slightly acidic, garden fruits and some tropical fruits are moderately acidic and most temperate-region fruits and berries are highly acidic. The pH of raspberries varies from 3.2 to 4.0, meaning that raspberries can certainly be identified as an acidic food because they are significantly more acidic than most other foods.
Acid Fruit, Not Acid Soil
The acidity of a food crop is not directly related to the optimal pH of the soil in which that crop is grown. Though many fruits and vegetables have pH values below 5.0, few of these crops tolerate soil pH below 5.0. Furthermore, some crops with highly acidic produce -- including raspberries -- prefer soil that is slightly acidic or neutral. One exception to this trend is blueberries (Vaccinium spp., adapted to USDA zones 3 to 10), which are both highly acidic (pH 3.2) and adapted to soils with pH as low as 4.5.
Death From a Jar
The primary relevance of food acidity for the home gardener is the relationship between the pH of produce and the appropriate way to preserve the harvest. Home canning with the water bath method brings the internal contents of the canning jar to the boiling point of water, which is 212 degrees Fahrenheit at sea level. This kills almost all microorganisms, but unfortunately, one of the most virulent microbes in the world -- Clostridium botulinum -- can survive these temperatures. This bacterium, which thrives in the oxygen-deficient environment of a sealed canning jar, produces a toxin that causes the deadly illness known as botulism. Clostridium botulinum cannot grow in food with pH of 4.6 or below. With pH values ranging from 3.2 to 4.0, raspberries are considered safe for water bath canning.