Microorganism Growth: pH Factors

Factors Affecting Microorganism Growth

• Microorganism growth and survival are affected by pH and water activity.

General Information About pH

• The pH value measures the acidity of a food on a log scale.
• Each unit on the pH scale represents a 10-fold difference in acidity.
• A food with a pH of 6.0 is 10 times more acidic than a food with a pH of 7.0.

pH and Microorganism Growth

• Every microorganism has an optimal pH and a pH range for growth.
• Ionic strength of media impacts both the external and internal environment of cells.
• Cells are impermeable to ions, whether acidic or basic.
• The optimal growth range for a microorganism is between 2 to 3 pH units.
• Exposure to a pH above or below the optimal pH adversely affects growth and survival.
• Yeasts and molds have a greater tolerance of pH extremes compared to other microorganisms.
• Most pathogens cannot grow at a pH less than 4.0.
• Internal pH is near neutral pH values.

Biological Effects of Low pH on Cells

• Low pH causes the hydrolysis of nucleic acids like DNA, RNA, and ATP.
• It also causes structural damage to the membrane and proteins of cells.
• Disruption of ion concentration gradients across the cell membrane occurs
• Undissociated organic acids pass freely into the cell
• Once inside the cell, they may dissociate
• Organic acids become more ionized as pH increases. RCOOH < pKa < RCOO- + H+

Biological Protection Against Low pH

• Cells maintain an intracellular pH above some critical pH.
• Homeostatic response: increasing the activity of proton pumps when the external pH is greater than 6.0.
• Acid tolerance response (ATR): induces more protein involving ATPase, pumping protons when the external pH is between 5.5 and 6.
• Acid shock proteins: induce special proteins supporting cell survival when the external pH is between 3 and 5.
• Stationary phase cells are more tolerant to low pH.
• Cells employ proteases and chaperones to control protein damage and perform specialized enzymatic reactions.

pH of Foods and Control of Bacteria

• Low-acid foods have a pH above 5.2
• Medium-acid foods have a pH between 5.2 and 4.6
• Acid foods have a pH between 4.6 and 4.0
• High-acid foods have a pH below 4.0.
• Food with a pH of less than 4.5 poses no public health risk.
• This pH is too low for the growth of C. botulinum and almost all other pathogens.
• Any acid can be used to achieve this pH level, so long as it is approved and within legal limits.

Control of Yeasts and Molds

• A change in pH alone has little effect on yeasts and molds.
• Yeasts and mold growth is inhibited by weak organic acids adjusted to a low pH.

Food Preservation with Acids

• HA ⇌ H+ + A-
• Ka=[H+][A-]/[HA]
• 1/[H+]=1/Ka[A-]/[HA]
• pH = pKa + log [A-]/[HA]
• When pH = pKa, 50% of acid is in HA (undissociated).
• When pH > pKa, the proportion of HA decreases and dissociation increases.
• When pH < pKa, the proportion of HA increases and dissociation increases.

Information on Water Activity

• All organisms need H₂O to survive.
• Too much or too little H₂O is detrimental to them.
• Water activity is important for predicting microbial growth, while survival is less affected.
• Water activity (aW) refers to the amount of H₂O available to an organism (aw = Pwater above product/Ppure water).
• Water content is the total water in the food.
• The relationship between water content and water activity depends on the composition and relative quantities of all H₂O binding substances present.

Calculating Water Activity

• Water activity = Molarity of water/(Molarity of water+Molarity of solute)
• Molarity of water is 55.56 M.
• Water activity is based on solute concentration
• Foods are a mixture of many solutes.
• It is almost impossible to quantify all the solutes in a given food.
• Some solutes bind H2O more effectively than others.
• The water activity of 1 M NaCl is ~0.97
• The water activity of 1 M sucrose is ~0.98

Effect of Water Activity on Survival

• aW protection from lethal treatment depends on species/strain differences, the physiological state of the cell, and the composition of the food-matrix and other environmental parameters.
• Limiting the growth with low water activity does not mean lowering the survival rate.
• A high water activity might reduce survival (encourage growth).

Food Preservation

• This is often done through drying and freeze-drying ( aW < 0.6).
• It can be achieved with intermediate moisture foods ( 0.6-0.85).
• This can be done using salt or sugar
• Blending low aW ingredients (honey or peanut butter)
• Addition of humectants

Humectants

• These are compounds that bind H2O very efficiently for low aw without creating the sensation of dryness.