How does an osmometer make ice cream better?

Ice cream mixtures can be quickly monitored with an osmometer as part of a quality control program. Sometimes, the important ingredient is only 0.5% of a commercial ice cream mix. An osmometer can be used alone or in combination with other instruments like a cryoscope to ensure the proper consistency of the finished ice cream. Osmometers are cool!

Ice cream consumers generally don’t like large water ice crystals in ice cream. Using an osmometer can prevent that from happening. The key is measuring milli osmoles (mOsm). Osmolality measures the osmotic concentration in a solution, defined as the total number of solute particles within the liquid. A measurement of one osmole (Osm) means one mole of a fully dissolved substance (like glucose or sodium chloride) in water.

Osmolality measures the concentration of Osm. It’s typically measured in terms of weight and quantified as mOsm/kg. Osmolality is not related to the size, molecular weight, or other properties of the dissolved particles; it’s only a function of the number of particles.

It’s important for ice cream because it’s related to the mixture’s freezing point. In the case of water, a solution with an osmolality of 1,000 mOsm/kg has a freezing point 1.86° C lower than pure water.

An osmometer can measure osmotic concentrations in the mOsm/kg range (Figure 1). That’s a sufficient level of resolution for ice cream mix quality control purposes.

Figure 1. Osmometers like this can measure osmotic concentrations to a few mOsm/kg. (Image: *Gonotec/Wolf Labs*)

Ice cream quality control

An ice cream factory uses a mix that includes all the ingredients in the proper proportions as the quality control benchmark. Next, the osmolality of the control mix is measured and converted to a freezing point value when appropriate to serve as the base value.

During production, the mixture’s samples can be measured for osmolality. A variation of over ±45 mOs/kg of water or a freezing point variation over ±0.08 °C indicates that the mix is incorrect and improper quantities of soluble ingredients have been added.

Cryoscopy

Cryoscopy is the measurement of freezing points and is related to osmometry. Cryoscopy can be used to determine molecular weights by measuring the reduction in the freezing point of a solvent when a known quantity of solute is added.

Water is the main component in an ice cream mix and makes up most of the cream or milk used to make ice cream. Sugar is the primary ingredient used to lower the freezing point. The dissolved sugar interferes with the formation of ice crystals.

What are the types of osmometers?

The most common types of osmometers are freezing-point depression, vapor pressure, and membrane osmometers. While standalone tabletop osmometers are the most common, computerized, automatic, and handheld osmometer designs are also available.

A freezing-point depression osmometer consists of a temperature-controlled bath that can support sub-freezing temperatures, a thermistor probe, a Wheatstone bridge to measure the temperature of the sample and a thermistor readout circuit that combines a galvanometer and a potentiometer (Figure 2). Some freezing-point depression osmometers include a Peltier cooler and microprocessor control to improve measurement precision.

Figure 2. Sketch of the essential components in a freezing point depression osmometer. (Image: *National Center for Biotechnical Information*)

Vapor pressure osmometers measure osmolality by measuring the voltage difference between two thermistors. One thermistor is in the sample solution to be tested, and the second is a sample of the base solvent used to make the sample. The Osm measurement is based on the correlation between the solute concentration and the voltage.

Membrane osmometers measure the flow of the solvent, usually water, from a container of pure solvent across a semipermeable membrane into a solute solution with the same solvent. The semipermeable membrane only allows the flow of the solvent and blocks the flow of dissolved particles. The flow across the membrane is measured as the osmotic pressure of the sample, which is correlated to the concentration of solute in the sample.

A limited number of materials are suitable for use as the semipermeable membrane, and the selection of the material determines the osmometer’s measurement range and accuracy.

Summary

Osmolality measures the concentration of dissolved particles in Osm and is a function of the number of particles, not their physical, chemical, or electrical properties. Osmometers are used in ice cream making to compare the mix currently being used to a known-good mix for quality control. Several techniques for measuring osmolality exist, including freezing point depression, vapor pressure, and membrane designs.