Measurement of Water Activity

Summary

This video delves into the concept of water activity (aw), which is pivotal for assessing food safety and shelf life by gauging the availability of free water for microbial growth. It introduces four primary methods for measuring water activity: the desiccator technique using sulfuric acid or salt solutions to maintain low humidity, the Winkler method for dissolved oxygen measurement in freshwater systems, the dynamic vapor sorption (DVS) which directly measures vapor pressure for fast and reliable results, and the spring sorption balance method that tracks moisture content changes in a controlled environment. The video underscores the significance of water activity in predicting food spoilage and ensuring product longevity.

Takeaways

• 💧 Water Activity (AW) is a measure of the free water available for microbial growth in a system.
• 📊 AW is defined as the ratio of the vapor pressure of water in a material to that of pure water at the same temperature.
• 🍽️ High AW indicates more water available for bacteria, which can lead to faster spoilage of food.
• 🌡️ AW is crucial for understanding food safety, shelf life, and the longevity of food products.
• 🔬 The desiccator method uses sulfuric acid or salt solutions to maintain a low humidity environment for moisture-sensitive items.
• 🧪 The Winkler method measures dissolved oxygen in freshwater systems, indicating the health and productivity of water bodies.
• 📉 The dynamic vapor sorption (DVS) method directly measures the vapor pressure of a sample to calculate water activity, overcoming traditional method drawbacks.
• 🧬 DVS is useful in various industries including pharmaceuticals, foods, feeds, fertilizers, agrochemicals, fine chemicals, detergents, cosmetics, and electronics.
• 🌱 The spring sorption balance method involves observing the extension or contraction of a calibrated coil spring due to moisture gain or loss in a constant relative humidity atmosphere.
• 📈 By plotting the sorption isotherm with samples of various moisture contents, one can determine the food's water activity.
• 🛡️ Controlling water activity helps predict and prevent food spoilage, ensuring the safety and quality of food products.

Q & A

What is water activity and why is it important?

Water activity, denoted by 'aw', is a measure of the free water in a system available for microbial growth. It's important because it directly impacts food safety and quality, influences microbial growth, shelf life, and chemical reactions within food.

How is water activity defined in the context of food safety?

Water activity is defined as the ratio of the vapor pressure of the water in a material to the vapor pressure of pure water at the same temperature. It's like a water score for food, indicating how much water is available for bacteria to use, which can affect how quickly the food spoils.

What is the purpose of a desiccator in relation to water activity?

A desiccator maintains a low humidity environment for moisture-sensitive items such as fruits and vegetables. It contains substances like sulfuric acid or salt solutions to absorb moisture and preserve the integrity of chemicals, electronics, or hygroscopic materials.

How does the Winkler method measure dissolved oxygen?

The Winkler method involves suspending a sample in a vessel with a saturated salt solution and continuously weighing it. Oxygen reacts with manganese hydroxide to form higher hydroxides, which release iodine upon acidification. This iodine is then titrated by a standard sodium thiosulfate solution using starch as an indicator.

What is the formula used to calculate dissolved oxygen using the Winkler method?

The formula to calculate dissolved oxygen using the Winkler method is: Dissolved Oxygen (mg/L) = (Volume of titrant * Normality of titrant * 8.8). The 8.8 is a conversion factor that helps convert the volume and normality of the thiosulfate solution into the amount of oxygen in milligrams per liter.

What is the dynamic vapor sorption (DVS) method and how does it measure water activity?

Dynamic vapor sorption (DVS) is a method for measuring water activity by directly measuring the vapor pressure of a sample. Water activity is calculated from the ratio of the sample's vapor pressure to that of pure water. DVS is fast, reliable, and overcomes drawbacks of traditional methods.

How does the spring sorption balance method measure water activity?

The spring sorption balance method involves suspending a food sample from a calibrated coil spring. The extension or contraction of the spring, which is caused by moisture gain or loss in a constant relative humidity atmosphere, is followed by a cathetometer or recorded electronically.

What are some applications of the dynamic vapor sorption (DVS) method?

The DVS method is used in various industries such as pharmaceuticals, foods and feeds, fertilizers and agrochemicals, fine chemicals, detergents and cosmetics, and building materials and electronics to measure water activity in materials.

How can water activity measurements be used to predict and prevent food spoilage?

By controlling and measuring water activity, we can predict the conditions under which food is most likely to spoil. This helps in ensuring the safety and longevity of food products by preventing microbial growth and spoilage.

What are some sources that provide more detailed information on water activity measurements?

For more detailed information on water activity measurements, one can refer to reviews by Gal (1967) and To (1973). Additionally, methods of partial pressure measurement using simple manometric techniques are given by Taylor (1961) and Carol (1955).