2 Water and Food Properties Free Online FlashCards

How are charges distributed in a water molecule?

Water is polar: its oxygen end carries a partial negative charge, while its hydrogen ends carry partial positive charges.

How do water’s hydrogen bonds support food ingredients?

Water forms hydrogen bonds with suitable groups in proteins and carbohydrates, helping dissolve salts and sugars and hydrate food macromolecules.

Why can freezing damage delicate food tissues?

Ice has an open hydrogen-bonded crystal structure. Its expansion during freezing can damage delicate food tissues.

Are closely associated and mobile water two completely separate categories?

Water associated closely with food components and more mobile water are points on a continuum, not two perfectly separate kinds.

How can moisture act as a plasticizer in dry foods?

Water acts as a plasticizer: added moisture can increase molecular mobility, making dry, glassy foods softer or stickier.

What does moisture content measure?

Moisture content measures the total amount of water in a food; it does not indicate by itself how available that water is.

What does water activity indicate?

Water activity, denoted $a_w$, describes how available water is for microbial growth and chemical reactions. Pure water has an activity of $1.0$.

How is water activity defined by vapor pressure?

Water activity is the ratio of the water-vapor pressure above a food to that above pure water at the same temperature.

How is equilibrium relative humidity related to $a_w$?

At equilibrium, $a_w$ equals the surrounding equilibrium relative humidity divided by $100$.

Why can jam and an unsweetened food with similar moisture have different $a_w$?

Foods with equal moisture content can have different $a_w$ because ingredients bind or dissolve water differently. Sugar in jam lowers water availability.

How can water activity be lowered, and what does this not guarantee?

Drying, adding salt or sugar, and freezing can lower water activity and restrict microbial growth, but lowering it does not necessarily destroy microorganisms.

How does dry-condition tolerance differ among microbes?

Some molds and yeasts tolerate drier conditions than many bacteria, so low water activity alone does not affect all microbes equally.