WHY IS INULIN USED IN ICE CREAM?

Inulin is a naturally occurring carbohydrate widely found in nature in a variety of plants and in some bacteria and fungi. It has always been part of the normal human diet as it naturally occurs in several fruits and vegetables, including leeks, onions, garlic, asparagus, Jerusalem artichokes, bananas, dahlias, yacon, and chicory (1). The average daily consumption has been estimated to be between 3g and 11g in Europe (1) and between 1g and 4g in the USA (2). It is classified as a food or food ingredient, not as an additive, in all EU countries and has GRAS (Generally Recognised as Safe) status in the US (3).

2. DIFFERENT GRADES OF INULIN

The most widely used source of inulin in the food industry is chicory. The production process basically consists of three steps: 1. extraction of the naturally occurring inulin from chicory roots in a manner very similar to the extraction of sucrose from sugar beets; 2. purification to remove impurities; and 3. evaporation and spray drying.

There are several commercial grades of inulin available, their functional attributes being linked, to a substantial extent, to their degree of polymerisation (DP). The most common are: 1. native inulin; 2. short-chain oligofructose; and 3. long-chain high performance (HP) inulin.

2.1 NATIVE INULIN

Native, or standard (ST-inulin), inulin, as it is present in chicory, has a DP ranging from 2 to 65, with an average of about 10 (1). It is available as a white, odourless powder that has a bland, neutral taste without any off-flavour or aftertaste. Native inulin contains 6-10% sugars represented as glucose, fructose, and sucrose and is, therefore, slightly sweet (10% sweetness in comparison to table sugar) (3).

2.2 SHORT-CHAIN OLIGOFRUCTOSE

The partial enzymatic hydrolysis of native inulin produces a short-chain fraction known as oligofructose, a subgroup of inulin. This product has a DP of 2-10, with an average of 4, and is commercially available as a syrup with a dry matter content of 75% (51). It has a sweetness profile similar to that of table sugar with a very clean taste without any lingering effect, and has about 30 – 50% of the sweetness of table sugar (5).

2.2.1 SUGAR REDUCTION

Short-chain oligofructose is used in ice cream production primarily as a low-calorie sugar subsitute. Soukoulis et al.(6) were able to replace 30% of the sucrose in full-fat ice cream with an equivalent amount of oligofructose. The researchers reported smaller ice crystal sizes, increased overrun (the amount of air that is whipped into ice cream), a decrease in air cell size, reduction of iciness, and increased creaminess in the sugar-reduced samples containing oligofructose. The ice cream was, however, reported to be harder than the 100% sucrose control.

2.3 LONG-CHAIN INULIN

By applying physical separation techniques, long-chain, ‘high performance’ (HP) inulin is produced, with an average DP of 25 and a molecular distribution ranging from 11 to 60 5. This product does not contribute sweetness.

2.3.1 FAT REDUCTION

Inulin is an excellent low-calorie fat replacer. When mixed with water or milk, it forms a particle gel network, resulting in a smooth, creamy texture and a fat-like mouthfeel that can be incorporated into foods to replace up to 100% of the fat (7). This gel network is composed of a tori-dimensional network of insoluble sub-micron crystalline inulin particles in water. Large amounts of water are immobilised in this network, giving it its smooth and creamy texture (8). HP inulin with long chain and high molecular weight is the most desirable as a fat replacer, having double the fat-mimetic property of standard inulin with no sweetness contribution (5).

Pintor et al.(9) investigated the reduction of both fat and sugar in ice cream using agave inulin (the DP of which was not published). The researchers found that agave inulin (3%) can be employed to reduce fat from 10% to 7% (30% reduction) and sugar from 15 to 13.2% (12% reduction) in the formulation of low-fat reduced-sugar ice cream. Similarly, El-Nagar et al.(10) were able to reduce the fat content in a yog-ice cream from 10% to 5% (50% reduction) using 5% native inulin (DP 12-13). The researchers found that the sensory attribute ratings for the reduced-fat yog-ice cream containing 5% inulin resembled that of the high fat product.

2.3.2 DECREASE HARDNESS OF LOW-FAT ICE CREAM

Lowering the fat content in the formulation of low-fat ice cream results in ice cream that is harder. This is because as the amount of fat is reduced, the amount of frozen water increases, resulting in a harder product. Adding oligofructose, native inulin, and long-chain inulin to a low-fat ice cream mix will depress its freezing point (the difference between 0°C (32°F) and the temperature at which water in an ice cream mix first begins to freeze(15)), resulting in a reduction in the amount of frozen water and, consequently, softer texture (50 10). Oligofructose depresses the freezing point of an ice cream mix more than native and long-chain inulin and will, therefore, produce softer ice cream.

2.3.3 REDUCE THE CALORIC VALUE OF ICE CREAM

Oligofructose, standard inulin, and HP inulin are low caloric food ingredients with a lower caloric value compared to fat and sucrose. In 2008, the European Union adopted an energy value of 2 kcal/g for all dietary fibres, including inulin and oligofructose (11 12). Similarly, Health Canada(13) adopted this value and the US FDA has recommended it in its guidance for industry (14). This compares to 9.45 kcal/g for fat and 4 kcal/g for sucrose (15). Oligofructose, standard, and HP inulin can, therefore, be used to produce healthier ice cream with fewer calories by replacing a portion of the fat or sugar.

2.3.4 VISCOSITY ENHANCEMENT

Viscosity can be loosely defined as the thickness of a liquid, with thicker liquids having higher viscosities (honey has a higher viscosity than water for example). In general, as the viscosity of an ice cream mix increases, the perception of creaminess and resistance to melting increases (15). Several studies have shown that inulin addition to an ice cream mix increases its viscosity compared to a control mix without inulin (16 17 18). This is due to the ability of inulin and oligofructose to hydrate and bind water (19). Long-chain HP inulin produces more viscous mixes than oligofructose and native inulin (20 16 21).

When inulin is added to a reduced-fat mix and its viscosity compared to that of a full-fat mix without inulin, however, results appear to be mixed. El-Nagar et al(10) reported a significant increase in the viscosity of low-fat (5%) yog-ice cream fortified with 5%, 7%, and 9% native inulin (DP 12-13) that was greater than the high fat (10% fat) control mix wihtout inulin. Similarly, Acia et al.(22) found that a 50:50 blend of short- and long-chain inulin at 5.5% produced a low-fat dessert (<0.1% fat) that was creamier and thicker, which idicated a higher viscosity, than the control full-fat (2.8% fat) dessert. Akalin et al. (23), however, reported that a reduced-fat (6%) and a low-fat (3%) ice cream with 4% long-chain inulin (DP>20) had a lower viscosity than a 10% fat regular ice cream without inulin.

2.3.5 INCREASE OVERRUN AND MELTING RESISTANCE

Most ice cream studies have demonstrated that inulin substantially enhances overrun and related properties such as foam stabilisation, melting resistance, and shape retention (9 10 23 24). High overrun is related to higher viscosities that promote more efficient air incorporation and the formation of smaller air cells (25 17).

2.3.6 EXTEND SHELF-LIFE

Ice crystal size is a critical factor in the development of smooth and creamy ice cream (26). Smooth and creamy ice cream requires the majority of ice crystals to be small, around 10 to 20 µm in size. If many crystals are larger than this, ice cream will be perceived as being coarse or icy (15 27). During distribution and storage, ice and lactose crystals grow and undergo recrystallisation, which eventually leads to coarse or icy texture. Recrystallisation is defined as “any change in number, size, shape… of crystals [during storage]” (28) and basically involves small crystals disappearing, large crystals growing, and crystals fusing together.

The 3 main types of recrystallisation are isomass, migratory, and accretive recrystallisation (29). Isomass recrystallisation is the change in shape of a crystal without change in mass. Accretion is the joining together of two or more adjacent ice crystals to form a single, larger crystal. Migratory recrystallisation, or Ostwald ripening, involves melting of smaller crystals and movement of the melted liquid to the surface of larger crystals (28 30). At higher temperatures, smaller ice crystals melt partially or completely and when the temperature is lowered again, the water refreezes on the larger crystals (31).

Migratory recrystallisation is influenced greatly by the rate at which the water molecules diffuse, or move, to the larger ice crystal surface, which is known as diffusion kinetics. The diffusion, or movement, of the water is largely dependent on the viscosity of the serum phase: as the viscosity of the unfrozen serum phase increases, the diffusion of water molecules decreases, thus retarding ice crystal growth (32). Research suggests that inulin and oligofructose may extend the shelf life of ice cream through their capacity to retain and bind water, as well their ability to enhance the viscosity of the unfrozen serum phase, thus decreasing the diffusion of water molecules and retarding ice crystal growth (33 18). Long-chain high DP inulin exhibits better shelf life extension potential compared to oligofructose (34).

3. HEALTH-PROMOTING PROPERTIES

In addition to the functional properties listed above, inulin and oligofructose have several important health-promoting properties. These include a prebiotic effect, suitability for diabetics, a reduction in the risk of diarrhea, constipation, colon and breast cancer, osteoporosis, and heart disease, immunomodulatory effects, regulation of serum cholesterol and triglyceride levels, improvement of calcium absorption, reduced plasma glucose levels, and anti-inflammatory and anti-cariogenic properties (45).

3.1 PREBIOTIC

The term ‘prebiotic’ was introduced by Gibson and Roberfroid in 1995 to describe ‘a nondigestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon, and thus improves the host’s health’ (35).

The colon is colonised by a complex ecosystem of bacteria. Some strains have potentially harmful effects, such as the production of toxins and carcinogens, whereas others are considered to provide a health-promoting function. Nourishing beneficial bacteria, such as Lactobacilli and Bifidobacteria, with inulin stimulates their growth and surprises the viability of potentially harmful bacteria, such as Escherichia coli, Campylobacteri jejuni, Enterobacterium spp. Salmonella enteritis, and others (45), thereby improving the health of the host.

3.1.1 SHORT-CHAIN AND LONG-CHAIN BLEND

Several nutritional studies (43 44) recommend the use of blends of short- and long-chain inulin to maximise fermentative and prebiotic effects because they are selectively metabolised in different portions of the large intestine (short-chain inulin in the proximal colon and long-chain inulin in more distal colonic regions). A 50:50 blend of short- and long-chain inulin also enhances calcium absorption and bone mineralisation in children (46) and proves effective in reducing the amount of gas produced while increasing or maintaining its prebiotic effect (47).

3.1.2 WHAT IS THE RECOMMENDED DAILY INTAKE?

Although there is no daily recommended amount for prebiotics, at least 4 g per day, but preferably 8 g per day, of inulin is needed to significantly increase bifidobacteria in the human gut (48 49).

3.2 USE BY DIABETICS

Due to the non-digestability of inulin and oligofructose, they are suitable for consumption by diabetics. Researchers found no influence on serum glucose, no stimulation of insulin secretion and no influence on glucagon secretion (36 37).

4. NEGATIVE EFFECTS OF OVERCONSUMPTION

In general, there are no safety concerns with the ingestion of inulin or oligofructose, but excessive intake may cause undesirable side effects, including flatulance, laxation, and abdominal discomfort. Slower fermenting compounds are more easily tolerated than faster ones. Long-chain inulin is, therefore, better tolerated than short-chain oligofructose since it is fermented at a rate that is about 50% lower than that of oligofructose (38 39).

Havenaar(40) reviewed studies in humans and concluded that in adults, up to 20 g per day of inulin with an average DP of 9 does not cause serious adverse side effects, except mild to moderate discomfort such as flatulance in some individuals. The tolerance of moderate doses (5g to 10 g per day) of oligofructose was confirmed in 2 randomised controlled trials (41 42).

5. SUMMARY

Inulin is a natural component of several fruits and vegetables, including leeks, onions, garlic, asparagus, Jerusalem artichokes, bananas, dahlias, yacon, and chicory. Several different commercial grades are available, the most commonly used being oligofructose, native inulin, and long-chain inulin. Oligofructose has a sweetness profile similar to that of table sugar and is about 30% – 50% as sweet. It is used in ice cream primarily as a low-calorie sugar substitute with research showing that 30% of the sucrose can be replaced with an equivalent amount of oligofructose. Native inulin has 10% of the sweetness of table sugar and is used primarily as a fat replacer in the formulation of low- or reduced-fat ice cream; 5% native inulin has been successfully used to reduce the fat content of a types of ice cream by 50%. Long-chain HP inulin is the most desirable as a fat replacer. It does not contribute any sweetness and has double the fat-mimetic property of native inulin.