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Physicochemical Analysis of Ice Cream using Zsinϕ and DTA Techniques
Summer is here and with UK temperatures currently soaring to >30°C, our Research and Development Department is hoping to answer the question on everyone’s mind – how should I store my ice cream to prevent large ice crystals forming but still allow for an easy scoop. In order to answer this, Biopharma utilises thermal analysis techniques to determine the best storage and scooping temperatures for two separate ice cream products.
Ice creams are complex, multi-phase emulsions typically forming a post-processing product containing four phases of air, ice, fat and serum . The serum component is a concentrated, unfrozen phase at the storage temperature of -20°C containing a mixture of milk solids, sweeteners, stabilisers, emulsifiers and other additives . The percentage volume of each of the phases varies widely across the industry as do the concentration and nature of components comprising the serum.
The unfrozen serum forms a matrix that contains ice crystals and an immiscible, partially coalesced fat network containing air bubbles . The serum phase forms the bulk of the ice cream by volume and includes a large volume of unfrozen water , . Degradation processes resulting in a loss of stability occur predominantly in the serum phase. Breakdown of the emulsion structure is particularly important as it is inherently unstable.
Soft-serve ice creams are used directly after the freezing stage, whereas hardened versions need to be stored at -30 to -40°C to allow more water to freeze . Using Zsinϕ (a derivative of electrical impedance) and differential thermal analysis (DTA) on samples of ice cream the physical reasoning can be illustrated.
Figure 1: Hard vs soft ice cream comparison
The Zsinϕ and DTA measurements were performed simultaneously using the Lyotherm3, and the samples used were Haagen Daazs Strawberry Cheesecake (HDSC) and Tesco’s Softscoop Raspberry Swirl (TSRS). Zsinϕ analysis provides a measure of the maximum molecular mobility pathway through the sample as this minimises the impedance to current flow. DTA provides a thermal measure of heat changes in the sample compared to a water reference. The samples were removed from storage at ‑20°C and quench cooled to -100°C at 10°C/min, before being heated at 2°C/min to room temperature, with a logging rate for all probes of every 3 seconds.
In Fig. 1 both hard and soft scoop ice cream varieties follow same pattern in both Zsinϕ and DTA curves which shows the consistency change between the two is purely physical and doesn’t affect the phase transitions present at different temperatures. Some ice creams such as HDSC are held at ‑30 to ‑40°C for the hardening process, when impedance is much higher (Z1), to allow more crystallization of water to strengthen the structure, and all ice cream is held at -20°C (Tstorage) in commercial freezers . The event D1 seen on both DTA curves is indicative of a crystallization exothermic event in the region of Tharden. There is some molecular mobility at this temperature which allows ice cream to be taken straight out of the freezer and served easily. In Figure 1 the melting point of the ice cream can also be seen at around -10°C (D2). Most water is liquid at this temperature due to the effects of other solutes causing freezing point depression .
Using these two techniques simultaneously allows for accurate physicochemical analysis of ice cream samples and the softening behaviour of the serum phase over the critical temperature range. This can provide manufacturers with information to analyse reformulation routes, quality and consistency testing, as well as the substantive effects of different ingredients on the thermal profile of the resulting formulation.
In conclusion, to prevent ice crystal growth within the ice cream the Lyotherm data shows that both ice creams should be stored below -38°C [D1]. Ice crystal growth is faster at higher temperatures so by maintaining the temperature below -38°C ice crystals remain small and contribute to a ‘creamier’ texture. The higher storage temperatures in domestic freezers (-18°C to -20°C) promotes ice crystal growth within the ice cream giving it a grainy texture if stored for longer periods of time.
Significant mobility changes within the frozen structure are seen at -32°C where the mobility of the molecules starts to increase [Z1] i.e. where the ice cream moves from a hard to a soft material. As the temperature of the ice cream continues to increase, the Zsinϕ anaylsis shows impedance approaching ‘0’ indicating maximum mobility within the frozen structure. This is the ideal ‘scooping’ temperature [Z2] (-9°C for TSRS and -8°C for HDSC) is below the melting point of the ice cream [D2] (-6°C) for both samples.
|||P. Udabage, M. Annaugustin, L. J. Cheng and R. P. Williams., “Physical behaviour of dairy ingredients during ice cream processing,” Le Lait, INRA Editions, pp. 383-394, 2005.|
|||D. J. McClements, Food Emulsions: Principles, Practices, and Techniques, Second Edition, CRC Press, 2004.|
|||A. Chiralt, Food Engineering Vol. II – Food Emulsions, 2005.|
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