Microbiology of soft drinks and fruit juices

P. Wareing and R.R. Davenport

11.1 Introduction

Soft drinks and fruit juices represent an important market within the food industry. The increasing variety of products being released at a bewildering rate has altered the potential for spoilage problems. Soft drinks are generally nutrient-poor media that are spoiled by relatively few organisms - usually yeasts, and a few acid-tolerant bacteria and fungi. Carbonation shifts the spoilage flora to those organisms tolerant of carbon dioxide. Soft drinks enhanced by the addition of low levels of fruit juice tend to exhibit similar spoilage flora to fruit juices. The use of ever more exotic raw ingredients may lead to the discovery of unusual spoilage organisms in the future. Yeasts in general, and Zygosaccharomyces bailii in particular, remain the key spoilage organisms because of their overall physiology and resistance to organic acid preservatives (Stratford et al., 2000).

Microbial problems within soft drinks and fruit juices can be divided into two groups:

(1) growth in, and deterioration of, the product by general organisms to produce spoilage;

(2) growth in, or contamination of, the product by pathogens to produce food poisoning.

There have been relatively few instances of food poisoning in fruit juices or soft drinks, but microbial spoilage is very common. The previous edition of this book contained an excellent review and practical guide to the identification of spoilage problems in the soft drink industry by the late Professor Davenport (1998). The present review will therefore keep key parts of the previous text, enhancing it with recent data, and simplifying some of the information. For example, some new information on acid-tolerant bacteria such as Alicyclobacillus acidoterrestris, and data on the pathogens Escherichia coli O157:H7 and Salmonella spp. within fruit juices, merit examination, as do some novel processing methods.

11.2 Composition of soft drinks and fruit juices in relation to spoilage

There is a bewildering variety of soft drink and fruit juices for sale, and many methods for their manufacture. Soft drinks can be non-carbonated, carbonated, with or without added fruit juice, often with the addition of organic acid preservatives. They can be filled on standard or clean fill lines. Fruit juices, fruit juice concentrates and fruit nectars may be fresh, unpasteurised and clean filled, or pasteurised, then hot, aseptic, or clean filled (Stratford et al., 2000; Stratford and James, 2003). Recent technology using ultra-high pressure has been used to produce 'cold pasteurised' fruit juices. These have the advantage of a fresh juice mouthfeel, but with destruction of pathogens and the majority of spoilage agents, enhancing the shelf life of an essentially fresh product (Mermelstein, 1999; Zook et al., 1999).

Simple soft drinks such as orangeade and lemonade are too acidic for the growth of most organisms, so that spoilage is generally by carbonation-resistant species such as Dekkera anomala (Stratford and James, 2003). Yeasts usually require a carbon source such as a hexose sugar, a nitrogen source such as amino acids or ammonium salts, simple salts (phosphate, sulphate, potassium and magnesium ions), trace minerals and vitamins. Some yeasts have particular sugar requirements; for example, Z.bailii and Z.rouxii cannot utilise sucrose (Pitt & Hocking, 1997; Stratford et al., 2000).

Sugars have a protective effect on the heat resistance of yeasts and bacteria; this is an important consideration at higher concentrations of sugar. Soft drinks are often nitrogen poor and thus the addition of fruit juice greatly enhances the potential for spoilage. Some yeasts, for example Dekkera bruxellensis, can use nitrate. Phosphate levels are often low, trace minerals satisfactory, particularly in hard water areas. The low pH value of soft drinks and fruit juices, pH 2.5-3.8 (Table 11.1), inhibits most bacteria, but leaves yeasts unaffected. In soft drinks

Table 11.1 Examples of fruit and vegetable juice pH and risk organisms

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