Plate Loop Method for Determining Total Viable Count of Orange Juice1

The plate loop count as described in Standard Methods for the Examination of Dairy Products, 13th Edition, was investigated as a simplified technique for determining total viable counts in single strength orange juice. This pro cedure requires the use of standardized loops (0.01 and 0.001) for making serial dilutions in stead of pipets and dilution bottles. Data have been presented to show there is no practical difference in results between the stand ard plate count and plate loop count procedures for determining total viable population of orange juice. The plate loop count can be used as a re liable alternate to the standard plate count method. It can also be carried out without flam ing the loop between samples. The plate loop count saves time and equipment, but requires analysts to be specially trained in order to main tain this degree of accuracy. The total viable microbial populations in citrus juices and concentrates are determined by an agar plate method commonly referred to as the standard plate count procedure (SPC). This method is used by some citrus plants to determine the microbial population of citrus juices in various stages of the processing operation, while others use it only to determine the number of viable organisms in their finished products. The standard plate count is time-consuming and requires a considerable amount of equipment—pipets and dilution bottles. Donnelly et al (2) found that a 0.001 ml cali brated loop used in connection with the oval tube method compared closely with the standard plate count in examination of raw milk. Thompson et al (4) investigated the use of the loop method for determining viable counts of raw milk. Their technique employed a 0.001 ml calibrated loop attached to a continuous volume syringe for rinsing the sample into a standard petri dish prior to pouring with agar. The results of their method compared closely with the standard plate count performed simultaneously on the same iThe authors wish to thank William J. Hepburn, Florida Citrus Mutual, for his statistical analysis of the data. Fig. 1. Calibrated loops 0.001 ml (L) and 0.01 ml (R) used for measuring sample. MURDOCK ET AL: PLATE LOOP COUNT METHOD 295 Fig. 2. The 0.01 ml measuring and transfer instrument as sembled and ready for use. sample. The plate loop method is now recognized as an alternate procedure for determining the total microbial population of milk. As far as is known, it has never been used by the citrus in dustry for orange juice. The application of this technique was investi gated as a simplified method for determining total viable counts in single strength orange juice.