Dairy ingredients in ice-cream

Abstract

The effects of the use of whey solids in place of SMP in ice-cream formulations have been reported. The limitations for incorporation of high levels of whey powders in ice-cream stem from their higher level of lactose compared to skim milk powder and the flavour of whey. Huse et al. (1984) demonstrated that acceptable ice-creams could be made with 50% substitution of skim milk by whey solids and that hydrolysis of the lactose in whey solids improved the texture of the ice-cream.

Headnote

Abstract

The source, type and amount of each ingredient in an ice-cream formulation and the manufacturing process affect the structure, stability and sensory perception of ice-cream. A range of new dairy ingredients is being used as alternatives to the traditional milk ingredients in the manufacture of ice-cream. The role of ingredients commonly used in ice-cream formulations and the effects of the manufacturing process on ice-cream properties are reviewed.

Dairy ingredients in ice-cream

Introduction

Ice-cream contains 35-42% total solids (TS), which comprises 10-16% milkfat, 9-12% milk solids non-fat (MSNF), 14-20% sugars, 0-0.4% stabilisers and 0-0.25% emulsifiers. The final ice-cream is a complex structured mixture comprising ~25% ice, ~50% air, 5% fat and ~20%) unfrozen serum. The source, type and the amount of each ingredient, as well as the processing conditions, affect the ice-cream's properties (Keeney 1974; Berger 1990; Goff 1997a; Marshall and Arbuckle 1996; Campbell and Pelan 1997). This paper reviews the effects of ingredient formulation and processing on ice-cream properties. Special attention is given to the role of dairy ingredients in ice-cream.

Manufacture of ice-cream

The processing of ice-cream involves the mixing of ingredients, homogenisation of the mixture, pasteurisation and ageing at 4[degrees]C before freezing in a scraped surface heat exchanger and hardening (Berger 1990). All processes involved contribute to the transformation of the ingredients in the mix into the final structured ice-cream product. The structural elements of ice-cream are ice crystals, air cells and fat aggregates, and they have a major influence on the sensory and the textural properties of ice-cream (Gelin et al. 1996; Goff 1997b). The proteins, through their influence on the stabilisation of the structural elements of ice-cream, also affect the ice-cream texture.

Homogenisation

Homogenisation of the ice-cream mix ruptures the original fat globules in milk and results in the formation of smaller globules with new exposed surfaces; these are stabilised by the proteins and the low-molecular-weight emulsifiers. The fundamentals of homogenisation and its applicability in ice-cream have been previously discussed (White 1981).

Pasteurisation

Various heat treatments (e.g. 71[degrees]C for 30 min, 82[degrees]C for 35 s or UHT) may be used to destroy undesirable micro-organisms. These heat treatments cause the destruction of pathogens in the ice-cream mix, improve the solubility of ingredients and melt the fat. The application of different heat treatments can affect ice-cream properties. Pasteurisation at lower temperatures for longer times generally results in the improved solubility of stabilisers and gives ice-creams better melt resistance (Loewenstein and Haddad 1972a, 1972b).

Cooling and ageing

Cooling and ageing of the ice-cream mix cause crystallisation of the fat. Cooling and ageing also promote the displacement of the proteins, which are adsorbed onto the fat globules during homogenisation, by low-molecular-weight surfactants in the ice-cream mix. It has been shown that decreasing the temperature of an emulsion increases the displacement of proteins from an oil-water interface by low-molecular-weight surfactants and further, that the temperature-induced changes in interface composition are dependent on the type of emulsifier used (Krog 1991).

Freezing with agitation

On freezing with agitation and aeration, the serum phase is concentrated, ice crystals are formed and the partial coalescence of fat globules is promoted. The partially coalesced fat, together with the serum phase protein, intact fat globules and emulsifier, results in the formation of a composite layer consisting of fat, protein and emulsifier which stabilises the air cells and gives the ice-cream a stable structure. The amount of air incorporated, the time the ice-cream mix spends in the freezing barrel, the temperature of the freezer outlet and the dasher speed all affect the final structure development of an ice-cream during the dynamic freezing process. The size of the ice crystals and their growth are generally dependent on the freezer outlet temperature and dasher speed. Ice crystal size decreases with increase in total solids (Donhowe et al. 1991; Flores and Goff 1999a). A low drawing temperature at freezing and aeration, an increased dasher capacity of freezer and dasher speed, and a higher degree of aeration all contribute to increased fat destabilisation, which results in good melt resistance and greater shape retention in the ice-cream (Sakurai et al. 1996; Bolliger et al. 2000) and a drier ice-cream (Zama 1994).

Role of ingredients in ice-cream

The properties and stability of ice-cream are influenced by the characteristics of the individual ingredients and their interactions with other components in the mix. Some of the common dairy ingredients traditionally used in ice-creams include whole, concentrated or evaporated milk, cream, anhydrous milkfat, butter, sweetened condensed milk, skim milk powder (SMP) and whey protein (WP). Many newer specialised dairy ingredients for ice-cream, such as skim milk powder alternatives and cream powders, are now being used. A range of sugars and sweeteners (sucrose, corn syrup, honey), stabilisers (gelatin, guar gum, locust bean gum, carrageenan and carboxymethyl cellulose), emulsifiers (lecithin, mono/diglycerides and polysorbates) and mineral salts (sodium salts of citric acid and phosphate) are also used in ice-cream formulations.

Protein-based dairy ingredients

There is a growing interest in the replacement of traditional dairy ingredients, such as fresh skim milk or whole milk, with milk powders and whey-based ingredients with varying degrees of lactose hydrolysis in ice-cream formulations. These ingredient substitutions arc aimed at reducing the cost of production and increasing the ease of handling and storage of raw materials, while maintaining or improving the characteristics of the ice-cream that is produced (Westerbeek 1996).

The inherent and process-induced physico-chemical properties of the milk proteins affect the properties of the ice-cream mix and the textural and functional properties of the final product. The milk proteins have inherent water-holding capabilities and together with the stabilisers, enhance the viscosity of the base and reduce the iciness in the finished product. The proteins contribute to the viscosity of the unfrozen serum phase and alter the distribution of water and solutes within the ice-cream matrix. They also affect the formation of crystals (both ice and lactose) and air cell growth in storage.

Proteins have the ability to act at interfaces and the inherent interfacial properties of the various milk proteins are different. Hence, the use of different types and levels of milk proteins in ice-cream formulations has the potential to affect the emulsion stability of an ice-cream pre-mix and the properties of the final ice-cream.

The effects of the use of whey solids in place of SMP in ice-cream formulations have been reported. The limitations for incorporation of high levels of whey powders in ice-cream stem from their higher level of lactose compared to skim milk powder and the flavour of whey. Huse et al. (1984) demonstrated that acceptable ice-creams could be made with 50% substitution of skim milk by whey solids and that hydrolysis of the lactose in whey solids improved the texture of the ice-cream. With the addition of whey solids there is an increase in the content of lactose resulting in a depression of freezing point of the ice-cream mix and an increased tendency for lactose crystallisation, with consequent effects on the structure. Vulink (1995) examined the effects of substitution of SMP with partially demineralised whey powder and the use of microfine lactose in ice-cream formulations. Generally, the problem of lactose crystallisation arises when 15-25% of SMP is substituted by whey powder or when demineralised whey powder is used to substitute 30-40% SMP. The use of higher levels of whey-based powders results in a 'whey flavour'. Lactose crystallisation in the final ice-cream can be controlled by seeding the ice-cream mix with micronised lactose before freezing (Vulink 1995).

Various sources of milk proteins, including caseinates, ultrafiltration (UF) retentates and whey protein concentrates (WPC), have been examined for making emulsions and ice-cream mixes (Parsons et al 1985). Lee and White (1991) used UF milk retentate (~8% protein and 4% lactose) and WPC (~36% protein and 53% lactose) to replace different levels of skim milk solids in vanilla ice-cream formulations. They found that ice-creams made containing UF retentate had higher flavour, body and texture scores than samples containing WPC, but were similar to those containing only skim milk solids in these attributes. Replacement of skim milk solids with UF retentate resulted in ice-creams with improved heat shock stability. However, there were no significant differences in meltdown characteristics of ice-creams made with skim milk solids or those containing various levels of UF milk retentate or WPC.

The state of the milk protein also affects the properties of the ice-cream mix. This is exemplified in the work on ice-cream made with heat-denatured particulated whey proteins (Koxholt et al. 1999). While substitution of part of the skim milk solids with ultrafiltered whey powder (whey protein concentrate) increases the rate of meltdown of ice-cream, the use of particulated whey protein concentrate reduces the meltability of ice-creams (Koxholt et al. 1999). The increased resistance to meltdown was linked to both the increased water-binding properties of the whey proteins upon denaturation and the ability of the particulated protein to hinder drainage by being integrated with the foam lamellar layers in the ice-cream. It has been suggested that the use of these whey protein aggregates has the potential to reduce the amount of stabiliser required (Koxholt et al. 1999).

Further evidence of the effects of the state of the proteins on ice-cream properties was obtained in a study that examined the properties of ice-creams made from partially hydrolysed mixes (Chang etal. 1995). Partial hydrolysis of casein micelles in low-fat ice-cream mixes using chymosin resulted in an increase in mix viscosity. Ice-cream made from chymosin-treated mixes was found to have increased hardness, smoothness, cohesiveness and mouth-coating properties, but a decreased rate of melting, adhesiveness and graininess compared to untreated mixes.

A fundamental study of the effects of milk proteins in aerated milk protein emulsions highlighted the importance of considering not only the inherent interfacial properties of different milk proteins, but also the effects of other low-molecular-weight surfactants for an improved understanding of how protein ingredients affect ice-cream properties (Pelan et al. 1997). Model emulsions (20% w/w palm kernel oil) that contained SMP as the source of milk solids were found to be more stable to shear than emulsions made with sodium caseinate or whey proteins (Lactalbumin 70). The increased stability of emulsions with SMP was related to the higher level of proteins adsorbed at the oil droplet surface.

Milkfat

Milkfat influences the textural and flavour properties of ice-cream. The content of the fat, its solid fat content, the fat globule size and the properties of its surface all affect the properties of ice-cream. A network of fat is required for the formation of a structured ice-cream and is crucial for the resistance of the ice-cream to heat shock and melting. This network is formed by partially coalesced fat which results from the destabilisation of fat globules. The fat network is generally considered to be essential for maintaining the structure of ice-cream as it stabilises air bubbles and the foam structure of the ice-cream.

The melt resistance of an ice-cream increases with increasing levels of fat in the formulation (Campbell and Pelan 1997). In addition, an increase in the fat content improved the resistance of the ice-cream to heat shock, an effect that might have been expected considering the contribution of fat in the development of the structure of ice-cream (Prindiville et al. 1999)

The extent of coalescence of the fat is dependent on the solid fat content of the fat. Hartel et al. (1998) found an inverse correlation between the extent of fat destabilisation and the melting point of the milkfat component. Longer times were needed to develop a stable overrun when low-melting milkfat fractions were used in the formulation together with mono- and diglycerides compared to anhydrous milkfat, or high and middle-melting milkfat fractions.

However, while fat destabilisation is an important factor for the stabilisation of the ice-cream's structure, it has been shown that stable ice-creams can also be made when fat destabilisation is limited (i.e. low solvent-extractable fat of <1%), indicating that other factors contribute to the stability of the ice-cream (Koxholt et al. 2001).

Campbell and Pelan (1997) observed an increase in the melt resistance of ice-cream when homogenisation pressure of an ice-cream mix (12% fat, 13% SMP, 15% sucrose) was decreased and attributed this to the inherently less-stable, larger fat droplets which are more susceptible to coalescence and hence contribute more to the fat network. Other workers also found that the meltdown rate of ice-cream (10% milkfat, 11% milk solids non-fat, 10% sucrose, 5% sucrose, 0.5% stabilisers and emulsifiers) was correlated with the size of the fat globule, with meltdown being significantly faster when the fat globule size (D^sub 50, 3^) was less than 0.85 [mu]m (Koxholt et al. 2001). Contrary to these findings, where a lower homogenisation pressure resulted in increased melt resistance, Thomsen and Holstborg (1998) found that the use of different homogenisation pressures on ice-cream mixes (10% hydrogenated coconut oil, 10.8% milk solids non-fat, 12% sucrose, 3% glucose syrup, 0.55% emulsifier and stabiliser) did not affect melting resistance. These authors suggested that an increased number of fat globules can have a role in stabilising the air cell surface, which contributes to melt resistance. The different observed effects of homogenisation on melt resistance may be related to the differences in the formulation and melting properties of the fat used in the various studies.

Apart from its effect on the physical characteristics of ice-cream, the level of fat in the mix influences the sensory properties of the ice-cream. The creaminess and smoothness of chocolate ice-cream increased, but the intensity of the cocoa flavour was reduced, with higher levels of fat in the mix (Prindiville et al. 1999).

Sugars

Sugars contribute to the sensory properties of ice-cream by imparting sweetness to the finished product. Sugars have a functional role, as they contribute to the depression of the freezing point and the development of the viscosity of the ice-cream mix and, hence, help to improve the body and texture of the ice-cream. The type and the amount of sugar used affects the freezing point of an ice-cream mix, as well as the ice crystallisation rate (Wittinger and Smith 1986; Miller-Livney and Hartel 1997).

Hydrolysis of lactose with lactase, or sucrose with invertase, decreased the freezing point of the ice-cream mix, reduced its firmness and increased the sweetness of the product (Lindamood et al. 1989). The increase in sweetness allows a reduction in the concentration of added sugar in the formulation. The organoleptic properties and meltdown characteristics of the ice-cream were not affected when the extent of hydrolysis of the total saccharides was less than ~59%.

However, El-Neshawy et al. (1988) showed that the use of lactose-hydrolysed milk (75% hydrolysis) in place of skim milk resulted in a higher viscosity and better whipping properties of the ice-cream mix, improved resistance of the ice-cream to meltdown, and superior body and texture. The differences in the results of these studies may have been due to the differences in the formulations used.

The lactose, which is present in the MSNF component of the ice-cream mix, can cause a grainy texture if lactose crystallisation is not controlled. Zuczkowa (1970) found that excessive lactose crystallisation could be prevented if the levels of MSNF and lactose did not exceed 12% and 6%, respectively. Lactose crystallisation can be minimised by rapid cooling of the pasteurised ice-cream mix, ageing the mix at ~3[degrees]C with occasional stirring and storing the ice-creams at ~-25[degrees]C.

Stabilisers

Stabilisers are used to build viscosity and their ability to do so is dependent on their inherent hydration properties, as well as on the temperature and time allowed for their hydration (Goff et al. 1994). Stabilisers decrease the bulk diffusion properties of the unfrozen phase, but they do not have a direct effect on ice crystallisation during freezing in a batch freezer and neither do they influence the freezing properties of the ice-cream mix (Flores and Goff 1999a). Stabilisers can influence the storage stability of the ice-cream. They have been used to control ice crystal growth during the storage of frozen desserts. Goff et al. (1993) postulated that, above the glass transition temperature of the unfrozen phase, stabilisers retard the growth of ice crystals.

The effects of stabilisers are dependent on the type and amount used, as well as the other ingredients present and the processing conditions. Stabilisers influence the rate of cooling and the rate of whipping of the ice-cream during freezing and have a marked influence on the body and texture of the ice-cream (Loewenstein and Haddad 1972b).

The inappropriate use of hydrocolloids in an ice-cream formulation can result in defects due to thermodynamic incompatibility between the proteins and hydrocolloids leading to over-stabilisation of ice-cream mixes. The thermodynamic incompatibility between the proteins and hydrocolloids occurs because of the inability of these two biopolymers to co-exist in the same phase beyond critical concentrations. This can result in the microscopic or macroscopic separation of the ice-cream into a protein-rich phase and a hydrocolloid-rich phase. Over-stabilisation due to the use of too-high levels of stabilisers can result in ice-creams that do not melt. The latter can become a source of concern, especially in the formulation of low-fat ice-creams in which fat is replaced by polysaccharides.

Emulsifiers

The main emulsifying components in the ice-cream mix are the proteins and the low-molecular-weight emulsifiers. They influence the structure of the interfacial membrane in the ice-cream mix during processing and the final ice-cream. The effects of emulsifiers are dependent on the type and amount used in the formulation and the processing conditions. Much work has been done to understand the role of emulsifiers in ice-cream (Goff et al. 1987; Goff et al. 1989; Baer et al. 1999; Goff et al. 1999; Goff and Spagnuolo 2001).

The characteristics of the interfaces in the ice-cream mix and of the final ice-cream are determined by the ratio of emulsifiers (protein and low-molecular-weight surfactants) to fat. However, the properties of the interface change during the many processing stages of the ice-cream as the ice-cream mix is homogenised, pasteurised and aged at 4[degrees]C before freezing and churning. The proteins adsorbed on the fat globule during homogenisation may be displaced by low-molecular-weight surfactants in the ice-cream mix during the cooling and ageing of the ice-cream mix (Goff and Jordan 1989). This occurs because the low-molecular-weight surfactants can more effectively lower the interfacial tension, but the rate of the protein displacement during ice-cream processing also controls the composition and properties of the interface in the final ice-cream.

Pelan et al. (1997) used model emulsions to study the effects of milk proteins and small-molecule surfactants on surface coverage and emulsion stability, and their relationship to the physical properties of ice-cream. Protein displacement was greater with use of a water-soluble surfactant (Tween 60) than with an oil-soluble surfactant (a monoglyceride). Protein displacement is generally accompanied by increased fat destabilisation, the development of smaller air cells and improved melt resistance.

Low-molecular-weight emulsifiers affect the solid fat content of the ice-cream mix. Saturated monoglycerides and, to a lesser extent, unsaturated monoglycerides increase the amount of solid fat in an emulsion, hence increasing the crystalline fat that is required for destabilisation and partial coalescence of fat globules, which, in turn, are essential for the development of the ice-cream structure (Krog 1997; Hartel et al. 1998).

Storage conditions for ice-cream

Inappropriate storage conditions of ice-cream can result in the development of a range of defects in ice-cream, including re-crystallisation of ice, lactose crystallisation and a collapse of the ice-cream structure.

The stability of an ice-cream during storage is dependent on the storage temperature and the extent of thermal fluctuations experienced by the ice-cream (Donhowe and Hartel 1996).

Storage of ice-cream at temperatures of ~-16[degrees]C causes re-crystallisation in ice-cream. Re-crystallisation of ice-cream and growth of ice crystals can be avoided by storage at lower temperatures (~-30[degrees]C), which are nearer to the glass transition temperature of the unfrozen phase in ice-cream. Storage of ice-cream below its glass transition temperature ensures maximum stability (Flores and Goff 1999b). This is because, at the glass transition temperature, the unfrozen phase of ice-cream changes from a viscous liquid into a glass and there is a significant increase in viscosity. The high viscosity of the phase inhibits the mobility of components, reduces melting of components, and retards Ostwald ripening of ice crystals and coalescence of air cells. Hagiwara and Hartel (1996) observed an inverse relationship between the re-crystallisation rates and the amount of frozen water.

Acceptability of ice-cream

The physical, functional and sensory properties of ice-cream influence the consumer's perception and acceptance of an ice-cream. A desirable ice-cream has good flavour, body and texture, colour, and melting characteristics and should be of good microbial quality (Rothwell 1985; Marshall and Arbuckle 1996). Apart from defects in microbial quality that arise because of improper processing and poor-quality raw material, the physical attributes of ice-cream are governed by the formulation of the ice-cream mix, the processing conditions during ice-cream manufacture and the storage conditions of the finished product.

Flavour is one of the more important attributes of ice-cream for the consumer. The pasteurisation of the ice-cream mix causes loss of volatile flavours and governs the extent of interactions between components of the mix. Homogenisation and freezing affect the flavour through their effect on the size of the fat globules, which, in turn, governs the mouthfeel and the flavour-release properties of the ice-cream (Lipsch 1986).

Conclusion

Ice-cream is expected to remain a popular product among consumers. Both ice-cream manufacturers and suppliers of ingredients for the ice-cream industry need to keep abreast of developments in ice-cream formulations and processing. For the dairy industry, this means the continued development of cost-effective dairy-based ingredients that offer improved convenience and enhanced functionality in ice-cream.

As the ice-cream market grows, and as more large manufacturers of ice-cream move towards the use of powdered dairy ingredients in place of fresh dairy ingredients, because of ease of handling and storage, there is likely to be increased interest in examining the functionality and consistency of dried dairy ingredients in ice-cream.

Further research will be required to determine how factors such as the composition of the milk and whey ingredient, the history of the milk or whey before dehydration, and the effects of the storage conditions of dried dairy ingredients impact on their suitability for use in a range of ice-cream formulations.

Acknowledgements

The support of the Dairy Research and Development Corporation for research in the area of dairy ingredients is gratefully acknowledged.

Sidebar

Aust. J. Dairy Technol. 58, 21-25

References

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