Keeping Quality
Chemical Preservatives in soft drinks
Dr Mark Wareing discussing the use of chemical preservatives in soft drinks
Microbial growth in soft drinks can produce off-flavours and odours, clouding and ‘inflation’ of flexible packaging from carbon dioxide released by fermentation. In the worst case, customer health could be affected. Preservatives can inhibit the growth of yeasts, moulds and many types of bacteria, when used in conjunction with other hurdles to microbial growth such as heat treatment, added acid (low pH), reduced water activity (sugar addition) and oxygen removal (e.g. via carbonation or nitrogen flushing). The quality and processing of raw materials should minimise the number of microbes introduced into soft drinks and if any micro-organisms are expected to survive into the final packaging, preservatives are added to prevent this small number from proliferating; rather than act as a sterilant for poor quality ingredients. Preservatives are required in drinks, which have either not been aseptically processed/packed, or require storage in a refrigerator after opening (dilutable squashes and cordials). Other chemicals and processes with a stabilising and sanitizing action may be used in the treatment of mains water and the preparation of raw materials for soft drinks.
Preservatives in soft drinks
| E Number | Related salts | Maximum level (ppm) | |
| Sorbic acid | E200 | E201-E203 | 300 (250)(1) |
| Benzoic acid | E210 | E211-E213 | 150 |
| DMDC | E242 | N/A | 250 |
| SO2 | E220 | E221-E228 | 20 |
Table 1 Chemical preservatives permitted in flavoured, non-dairy drinks (see EU 1129/2011) (1) Lower level applies when sorbate/benzoate used in combination.
The three primary preservatives in flavoured soft drinks in Europe are potassium sorbate, sodium benzoate and dimethyl di-carbonate (DMDC) – with Sulphur Dioxide (SO2) playing a secondary role as a carryover preservative from other ingredients.
Sodium benzoate and potassium sorbate are cold water-soluble and must be dissolved prior to acidifying the drink, since at low pH they are converted to their less soluble acid forms (benzoic acid and sorbic acid) and the salts will not dissolve in acid/fruit juice. The sorbate and benzoate salts should be as dilute as possible when they are finally acidified by mixing with fruit juice and/or a concentrated solution of the appropriate acid (e.g. citric or malic). A typical soft drink pH is 2.7-3.7. Above pH 4.6, ambient-stable drinks should be UHT-treated and aseptically packed. EU legislation defines the permitted use-levels as sorbic acid and benzoic acid, from which can be calculated from the added sodium benzoate (84% benzoic acid) and potassium sorbate (75% sorbic acid). Sorbate and benzoate work well together, in particular in very low pH drinks where preservative solubility can become more of an issue. Less sorbic acid is permitted when the sorbate and benzoate are used together (see Table 1). Sodium benzoate has better solubility than potassium sorbate in high solids cordials and squashes. Some preservative breakdown can occur during processing. The use of sodium benzoate in soft drinks is often avoided in the presence of vitamin C, which can potentially convert the benzoate salt into benzene.
DMDC is very effective and is metred intothe product immediately prior to filling, using specialist equipment. Since DMDC breaks down rapidly after addition, it is not practical to add earlier in the process or indeed check its level after packing. DMDC presents certain challenges as far a maintaining a safe production environment is concerned. DMDC is sometimes used with potassium sorbate.
SO2 is an allergen and is only permitted in beverages at low levels – as carryover from raw materials such as fruit concentrates and purees. SO2 does not have to be declared if it is present at up to 10ppm in the final drink. Higher levels must be declared on the label up to a maximum permitted level of 20ppm. After packing off, some of the SO2 will diffuse out of the drink into the headspace, where it can still fulfill a useful function of preventing mould growth on the surface of the drink; in particular in none carbonated drinks.
Natural preservatives: Benzoic acid occurs naturally (with vitamin C) in many types of berry fruits – sometimes at higher levels than permitted in soft drinks under EU additives legislation. Sorbic acid was also first identified as a naturally-occurring compound in plants. There are other naturally-occurring materials claimed to have preservative effects – such as saponins and flavanoids.
Preservative-enhancers
Acid is required to activate the preservative. The choice of acidulant has a significant contribution to the final flavour. Citric acid is suited to lemon and orange flavours, whilst malic acid works well with red berries and apple. Phosphoric acid is compatible with cola flavours – the maximum permitted use-levels of phosphoric acid (and phosphates) are based on the P2O5 content and the application (a lower maximum level is specified for sports drinks). Acidulants are sometimes used with corresponding acid salts to form a buffer system – to achieve a better control of pH and a more intense flavour note. An example would be citric acid with tri-sodium citrate.
Other additives can have a preservative-like effect in that they can increase shelf-life. Vitamin C as an anti-oxidant can stabilise colour and other vitamins (e.g. vitamin E). Sequestrants such as sodium citrate and sodium phosphates can bind traces of heavy metals like iron, to inhibit oxidation and also enhance the effectiveness of preservatives. Carbonation (typically 1.5-5g/l) not only produces a more refreshing drink but also lowers pH, moderates sweetness and the carbon dioxide has a preservative effect, since yeast fermentation is inhibited by carbon dioxide and the lack of oxygen inhibits mould growth. A lack of oxygen also inhibits undesirable oxidation reactions, which can produce off-flavours and colours.
General processing and packaging
A combination of raw material quality controls (including water treatment), heating (e.g. flash pasteurisation of fruit syrups) – in conjunction with strict GMP and plant cleanliness – contribute to a low initial bio burden in raw materials. Filtration and/or reverse osmosis membrane technology can remove cryptosporidium, bacteria, viruses, pesticides and organic off-flavours in mains water. Water softening can reduce the formation of haze in clear drinks, reduce alkalinity and remove destabilising metals salts such as iron, which also encourage microbial growth and the formation of off-flavours. Plastic packaging is permeable to oxygen, which can limit the shelf-life of some carbonated drinks, compared to equivalent canned drinks. Transparent plastic packaging can result in the degradation of ingredients such as vitamin C over shelf-life. This is exacerbated in the presence of oxygen and therefore carbonation or nitrogen flushing can extend shelf-life of vitamin C in clear bottles.
Chlorine, ozone, hydrogen peroxide, peracetic acid and UV light have been used in initial sanitizing steps for process water or in preparing sterile packing. Chlorination works best at pH 3-5. Chlorine residues must be removed by filtration to optimise flavour and protect fragile down-stream filtration membranes.