Objective

To determine the refractive index and pH of beverages      

Background

A thorough understanding of food and their properties are required in order to make the right decisions regarding how they should be processed and handled. For example, a fruit that has a soft texture (a physical property) will require more careful transport in order to prevent mechanical injury. A food with a high protein and moisture content (chemical properties) may spoil more quickly and therefore likely to require refrigeration or freezing, for example, meat and milk.

Properties of food can be divided into physical, chemical, microbial, sensory and functional properties. These are defined below.

Physical property: Properties that do not change the chemical nature and makeup of the food e.g. its weight, size and shape

Chemical property: Properties that change the chemical nature and makeup of the food e.g. pH, acidity and nutritional content 

Microbial property: Microbiome in or on the food that could cause spoilage e.g. bacteria, viruses, yeast, molds and parasites

Sensory property: The experience you get from eating food e.g. texture, flavor, taste, smell etc.

Functional property: How foods behave during their processing and its effect on the finished product. The functional property of a food will be dependent on its chemical and physical properties. Examples of functional properties are shown in figure 2 below.

In this lab you will be measuring physical and chemical properties of food, namely refractive index (physical property) and pH (chemical property). Refractive index is the ability of a substance to bend light. Have you ever noticed that when you put a pencil in a jar of water it appears to break? This is because the water causes a bend in the light as it passes from air to the water. If you dissolve sugar in the water, the break will become even more pronounced due to greater light bending. This principle is used to determine how much dissolved sugars is present in a liquid. A refractometer is used to measure the light refraction. The instrument converts this value to percentage sugar or brix.

Light Refraction

The next value you will measure is pH. This value ranges from 1 to 14 and tells you how acidic or alkaline a substance is. A pH of 7 is neutral. Neutral is the pH of water, which is neither acidic nor alkaline. Below 7 is acidic and above 7 is alkaline. Some foods such as mayonnaise, ketchups and other sauces must be prepared at a pH of 3.4 or less in order to prevent the growth of dangerous bacteria that will make you sick. Hence, managing pH is critical to food safety. In addition, controlling pH can also adjust the taste of the food. That is because certain foods taste better when they are acidified. In the case of jam and jelly production, an acidic pH is essential in creating its gel-like structure, another function of pH control.

Materials and Equipment

  1. Refractometer
  2. Digital pH meter
  3. pH paper
  4. 250 ml beakers, cups or jars
  5. 1ml pipette 
  6. Orange beverage samples
  7. Freshly squeezed orange juice
  8. Orange juice from concentrate
  9. Orange drink
  10. Carbonated orange soda

Procedure

pH Reading

  1. Transfer 150 ml of beverage samples, into four 250 ml beakers. Note: You could also use cups or glass jars
  2. Remove a strip of pH paper and dip into each sample to determine the pH. Use the color chart to help you determine what the pH is, based on the color change
  3. Insert the probe of the digital pH meter into each sample and read the pH

Refractometer Reading 

  1. Using a 1 ml pipette, transfer 1-2 drops of beverage sample onto the refractometer well
  2. Close, press read and record the percentage sugar (brix). Be sure to clean the instrument with a Kimwipes or other non-scratch paper between each measurement.
Refractometer Reading

Lab Questions  

  1. Sample readings (11 points)
    • Freshly squeezed orange juice: pH (strip)____________, pH (meter) __________, Brix _____________
    • Orange juice from concentrate: pH (strip)___________, pH (meter) __________, Brix _____________
    • Orange drink: pH (strip)____________, pH (meter) __________, Brix _____________
    • Carbonated orange soda: pH (strip)____________, pH (meter) __________, Brix _____________
  2. Which product had the most acid? _____________________ (1 point)
  3. Which product had the most sugar: ____________________ (1 point)
  4. What type of food acid is found in oranges? __________________________ (1 point)
  5. Describe two functions of this acid in orange juice? (2 point)
  6. Give an example of when it would be best to use a pH meter instead of pH strips in food production (3 points)

Courtney Simons
Administrator
Courtney Simons is a food science writer. He holds a BS degree in food science and a PhD in cereal science from North Dakota State University.
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