Orange Serum Agar

The low pH of fruit juices makes citrus fruit products susceptible to spoilage by yeasts, molds and the bacteria Lactobacillus and Leuconostoc. Hays investigated spoilage in frozen concentrated orange juice. He found that an agar medium containing orange serum was superior to others in isolating the microorganisms responsible for spoilage. Orange Serum Agar is specially indicated for growing the lactic acid microflora that spoil citric products, such as Lactobacillus, Leuconostoc and molds.

Casein peptone is a source of nitrogen, vitamins, minerals and amino acids essential for growth. Yeast extract is a source of vitamins, particularly the B-group. Glucose is the fermentable carbohydrate providing carbon and energy. Monopotassium phosphate acts as a buffer. Bacteriological agar is the solidifying agent. The low pH of citric juices and products limits the growth of microorganisms to acid-tolerant pathogens.

Typical formula in g/L, adjusted and/or supplemented as required to meet performance criteria.

Preparation

Suspend 50 grams of the medium in one liter of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Sterilize in autoclave at 121 °C for 15 minutes. DO NOT OVERHEAT.

Instructions for use

Pour plate method:

1. Deposit 1 mL of the initial suspension and/or diluted sample in an empty Petri dish.
2. Add 18-20 mL of agar cooled to 45-50 °C in each Petri dish and mix gently by moving the plate.
3. Incubate at 30 °C for 2-5 days.
4. Report the CFU/mL of test material.

• Hays G.L. (1951), Proc. Florida State Hort. Soc., 94th Ann.
• Murdock D.I. and Brokaw C.H. (1958), Food Tech., 12. 573-576.
• American Public Health Association (1976), Compendium of Methods for the Microbiological Examination of Foods, APHA Inc. Washington DC.