MICROBIOLOGY Study Guide - 2012

CHAPTER 15. FOOD MICROBIOLOGY

15.5. MICROBIOLOGY OF GRAIN AND FLOUR

15.5.2. Microflora of flour

Microorganisms residing on grain enter flour during the milling process. In freshly milled flour, Bacteria of the species Erwinia herbicola predominate, accounting for up to 90%. Small numbers of bacilli (B. subtilis, В. mycoides, В. megatherium, В. cereus), micrococci, lactic acid and acetic acid bacteria, Yeasts, and Molds (spores of filamentous Fungi) are also detected. The microbial load in flour is generally lower than in grain, as a portion of the microorganisms is removed during grain cleaning, along with the seed coats and mechanical impurities.

In flour with a moisture content of 15% or less, stored under conditions of low relative humidity and constant Temperature, microorganisms do not multiply and gradually die off. Prolonged storage leads to a decline in the population of Erwinia herbicola. However, a slight increase in flour moisture (by 1–2%) triggers a rapid proliferation of bacteria and fungi capable of causing various types of spoilage, such as molding, spontaneous heating, souring, and rancidity.

Molding is the most common form of flour spoilage during storage. Fungi of the genera Aspergillus, Penicillium, and others can develop in flour at lower moisture levels than bacteria (at a_w = 0.7). Flour begins to mold when the relative humidity of the air exceeds 79%. Moldy flour becomes spoiled, darkens, and acquires an unpleasant musty, moldy odor. Molding reduces the Nutritional Value of the flour, increases titratable acidity, and impairs gluten quality; the gluten darkens, becomes brittle, loses its elasticity, and washes out poorly. The spread of mold into the inner layers of the product can lead to spontaneous heating, causing the flour to lose its flowability and cake into lumps. Furthermore, moldy flour may accumulate mycotoxins.

Souring typically develops deep within the bulk of the product. This type of spoilage is caused by lactic acid bacteria, and less frequently by Butyric acid and other bacteria that ferment sugars with The production of acids. Sugars in flour are produced from starch through the action of flour Enzymes (amylases) and bacterial enzymes. As a result, flour acquires a characteristic sour odor and taste, its titratable acidity increases sharply, and its baking qualities deteriorate.

Rancidity may result from The oxidation of flour Lipids by atmospheric oxygen, facilitated by the flour enzyme lipoxygenase. It is also associated with the Enzymatic Hydrolysis of lipids mediated by microorganisms possessing active lipases, primarily molds. Fat hydrolysis leads to the accumulation of free Fatty acids in the flour, imparting an off-flavor and off-odor, altering the color of the flour, and destroying its characteristic freshness.

Because flour is highly hygroscopic, it should be stored at a relative humidity not exceeding 65% and a constant temperature of 15–20 °C. The moisture content of the flour must not exceed 14–15%.