Although several microorganisms have been iso- lat ed from orange juice, few of them cause spoil- age. The high acidity (low pH) of juice lim its the types of microorganism that can grow in the juice. Juice spoilage is caused by mi cro or gan isms that
are able to multiply in juice during its processing and storage (see Ta ble 4.2.)
The growth of microorganisms in orange juice is characterised by fermentation and/or the pro duc tion of off-fl avours that spoil the prod uct. Fermentation may lead to gas formation, which, in turn, results in blown packages.
4.3 Microbiology of orange juice
Acid-tolerant bacteria
Lactic acid bacteria belonging to the genera Lacto ba cil lus and Leuconostoc are the most com mon acid-tolerant bacteria that cause spoil- age of orange juice (see Figure 4.6). Spoil age is characterised by a “buttery” or “but ter milk-like” fl avour resulting from the formation of diacetyl during bacterial growth. Leu con os toc and many species of Lactobacillus also pro duce large amounts of CO2.
Most lactic acid bacteria grow best in orange juice at temperatures between 20 and 37 °C. Their rate of growth is greatly reduced above or
below these temperatures. Growth is very slow at 4 °C in single-strength juice. These bacteria are sen si tive to heat and high osmotic pressure (high juice concentration); no growth is ob served in juice above 45 °Brix.
Lactic acid bacteria pose the biggest prob lem during the processing steps prior to con cen tra tion. After extraction, the juice has to be re frig er at ed
or heat-treated as soon as possible to prevent the build-up of large populations of these bac te ria. It is also important for fruit processors to maintain an effective programme of san i ta tion.
The main species of lactic acid bacteria caus- ing spoilage are Lactobacillus plantarum, Lacto- ba cil lus brevis, Leuconostoc mesenteroides and Leuconostoc dextranicum.
Yeasts
Yeasts (see Figure 4.7) are the most common type of spoilage organism in both single-strength and concentrated orange juice. Spoil age of orange juice by yeasts typically results from an alcoholic fermentation which leads to off-fl avours and CO2
production. Yeasts not ca pa ble of alcoholic fer- mentation may cause tur bid i ty, fl occulation and clumping in juice.
The optimal growth temperature for most yeasts is 20–30 °C. They are more tolerant of cold tem per a tures, high osmotic pressure and lack of nu tri ents than bacteria or moulds. Spe cies of yeasts can easily survive in citrus con cen trates of 58 to 65 °Brix and in frozen juices.
“Yeasts are the most common
type of spoilage organism
”
Saccharomyces cerevisiae, Rhodotorula spp and Zygosaccharomyces spp are the most com mon yeasts present in juice. Saccharomyces cerevisiae is most commonly associated with the spoilage of pasteurised citrus juices. The pres ence of Rhodotorula may be indicative of poor postpas-
teurisation hygiene.
Zygosaccharomyces is an osmophilic yeast, which means that it can survive the high os mot ic pressures and low water activity of con cen trat ed orange juice. It is frequently as so ci at ed with spoilage of concentrates.
Fig. 4.6 Different shapes of lactic acid bacteria.
Fig. 4.7 Yeasts showing budding reproduction. 4.3 Microbiology of orange juice
Moulds
Moulds form colonies of aerial mycelia on the sur- face of juice, and fl occulation or fl oating myc e lia within juice (see Figure 4.8). They can grow under a wide variety of conditions. In gen er al, moulds grow well in acid media and re quire abundant oxygen. Moulds that grow in or ange juice are generally sensitive to heat treat ment and are thus easily destroyed by pas teur i sa tion.
Compared with yeasts and bacteria, moulds have only infrequently been associated with spoil age of orange juice. This is because of their aer o bic (ox y gen-dependent) nature and slow growth rates. However, with the advent of long- term chilled storage of single-strength juice and ex tend ed shelf life of juice in nonaseptic car tons with oxygen barriers, mould growth in cit rus juice has become a more im por tant issue.
Moulds may give rise to concern during juice extraction when fruit-handling and juice room equipment is not kept in hygienic con di tion. When the correct approach to sanitation is neglected, moulds colonise the surfaces of con vey or belts, fruit bins, extractors and other equip ment. These conditions promote the con tam i na tion of juice or the surfaces of containers.
Some moulds that have been isolated from or ange juice are: Aureobasidium pullulans, As pergil lus niger, Botrytis spp, Fusarium spp, Geotrichum spp, Mucor spp, Aspergillus fumigatus, Cladospo- ri um spp and Penicillium spp.
Pathogenic microorganisms
The presence of pathogenic microorganisms in orange juice is rare. The low pH of juice inhib- its growth of pathogens, but long-term survival of some pathogens in refrigerated orange juice is possible. Outbreaks of disease, particularly sal mo nel lo sis, traced back to orange juice have oc curred as a result of consuming unpasteurised orange juice, or due to reconstituted juice be- com ing contaminated before serving.
Diseases attributed to orange juice are main ly caused by incorrect product handling and can be prevented by carrying out approved sanitary pro- cedures, pasteurising the juice, and by preventing product contamination after pas teur i sa tion. Spore-forming microorganisms
Most bacterial spores cannot grow in fruit juic es with a pH below 4.5. Though very rare, thermoresistant acidophilic bacteria have been isolated from shelf-stable juices. In 1992 these strains were classifi ed in a new genus called Alicyclobacillus. Microorganisms belonging to this genus have a preference for thermophilic tem- per a tures with an optimum temperature around 45 °C, and they will most likely not grow below 20 °C. The most likely source of con tam i na tion of fruit juices is fruit con tam i nat ed with soil dur- ing harvesting. Such con tam i na tion is introduced into the manufacturing process through unwashed or poorly washed fruit. A spoilage characteristic of this type of bac te ria has been described as an off-fl avour like “disinfectant” or guaiacol. No gas pro duc tion has been observed.
Fig. 4.8 Example of mould structure.
4.3.2 MICROFLORA OF DIFFERENT