17 Questions
What is the relationship between water activity (aw) and the shelf-life of food?
Water activity is a parameter closely linked to the shelf-life of food and is more accurate than total water content.
Which of the following foods has the highest water activity (aw) based on the provided information?
Meat/fish
What is the typical water content (% MC) for honey based on the given information?
18%
Which bacteria's growth is favored by higher water activity levels?
Pseudomonas fluorescens
How does water activity (aw) affect food deterioration rate according to the text?
Higher water activity leads to faster food deterioration.
Which form of water in food is linked to the food components and is not available for the development of microorganisms or chemical reactions?
Bound water
What is the relationship between water activity (aw) and the growth of bacteria, yeasts, and molds in food?
Higher water activity encourages growth
How is the measurement of water content in food typically expressed?
Percentage
What does a low water content in food indicate in terms of perishability?
High perishability
What does the water activity (aw) measure in food?
Available water for growth
In the context of food, what term is used to describe the water that is not easily removed or frozen due to its linkage with food components?
Non-freezable water
What is the role of water in microbial growth in foods?
Water is essential for microbial growth as microorganisms need it in an available form
Which type of food has high moisture content and needs to be stored at refrigerator or freezer temperature?
Perishable food
What is the classification of food based on moisture content that includes flour, grains, dried fruits, and dry mixes?
Semi-perishable food
How long can semi-perishable foods remain unspoiled if properly stored and handled?
About one year
Which type of food has very low moisture content and includes sugar, dried beans, and spices?
Non-perishable food
What happens to non-perishable foods if stored over a long time in ideal conditions?
They lose sensory quality
Study Notes
Intrinsic Factors and Moisture Content
- Moisture content in food is typically measured by heating a sample under controlled conditions (usually 105°C overnight) and weighing it before and after drying, expressed as a percentage.
- Water content can be classified into three categories: free water, bound water, and non-freezable water.
Water Activity (aw)
- Water activity is a measure of the amount of water not tightly bound to the food matrix, available for microbial growth, chemical reactions, and phase transitions.
- It is defined as the ratio of the water vapor pressure of the food substrate (P) to the vapor pressure of pure water (P0) at the same temperature.
- Water activity is a key factor in determining the shelf-life of food and is a more accurate indicator of stability than total water content.
Classification of Food Based on Moisture Content
- Perishable foods: high moisture content (e.g., meat, poultry, fish, milk, eggs, raw fruits and vegetables), require refrigeration or freezing to prevent spoilage.
- Semi-perishable foods: intermediate moisture content (e.g., flour, grains, dried fruits, and dry mixes), can remain unspoiled for 6 months to 1 year if stored properly.
- Non-perishable foods: very low moisture content (e.g., sugar, dried beans, spices), do not spoil unless handled carelessly.
Microbial Growth and Intrinsic Factors
- Microorganisms need water in an available form to grow in foods.
- Intrinsic factors affecting microbial growth include: moisture content or water activity, acidity and pH, redox potential, presence of nutrients, biological structures of food, and antimicrobials (natural and added).
Typical aw Values for Different Products
- Meat/fish: 0.99 (60% water)
- Soft cheese: 0.95 (37% water)
- Hard cheese: 0.85
- Sausages: 0.83 (30% water)
- Jam: 0.80-0.88 (30% water)
- Pudding: 0.8
- Dry fruit: 0.6-0.76
- Honey: 0.75 (18% water)
- Biscuits: 0.3
- Instant coffee: 0.2
Relationship Between Water Activity and Microbial Growth
- aw values of 0.97-0.90 allow for growth of bacteria, including Clostridium botulinum, E. coli, and Salmonella spp.
Test your knowledge on measuring moisture content in food. Learn about the intrinsic factors affecting the moisture content and why wetter foods are more perishable. Understand the procedure of measuring water content in food samples.
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