Microbiological Testing Using Nutrient Pad Sets for Quality Assurance of Beverages

Inspection of final product is no longer sufficient to meet beverage quality assurance requirements. Rather, manufacturers must also inspect incoming raw materials and perform in-process quality control tests throughout the entire production process.

Microbiological and aseptic testing plays a major role in quality assurance. For example, in the soft drink industry, microbiological and hygienic quality (including biological stability) are key criteria for quality assessment. Large quantities can be ruined by just a few microbes. Significant advances have been made to reduce the risk of microbial contamination, but escalated production and the demand for longer product shelf life now necessitate more sophisticated testing procedures.

Membrane filtration

Figure 1 – The Sartorius Microsart e.motion dispenser automatically releases membrane filters from their sterile packaging at the touch of a button or via hands-free operation. Dispensing is triggered when the optical sensor detects approaching tweezers.

The membrane filtration method provides quantitative and reproducible trace contamination detection and operates efficiently and economically under routine conditions. Membrane filtration concentrates microorganisms from relatively large samples on the surface of the membrane filter of appropriate pore size (see Figure 1). Microorganisms in the test sample are retained on the filter surface. Growth inhibitors can be removed by flushing the membrane with, e.g., sterile sodium chloride solution after filtration. Microbes are then cultured on a nutrient pad or an agar culture medium.

Nutrient pad sets (NPS) are an attractive alternative to agar plates. A nutrient pad set is a sterile dehydrated culture media in cellulose pad with a suitable membrane filter. It simplifies many microbiological testing procedures and eliminates time-consuming, labor-intensive culture media preparation steps, such as sterilization and cleaning. Furthermore, it is not necessary to cool the NPS during transport or storage.

Once they are moistened with 3.0–3.5 mL of sterile and demineralized (or distilled) water, the pads are ready for immediate use. The level of moisture is optimal when an excess ring of water surrounding the pad is visible.

The standard nutrient pad set from Sartorius contains 100 sterile nutrient pads, each of which is individually inserted in a petri dish and sterilized (see Figure 2). Ten petri dishes are sealed in an aluminum bag to protect formula constituents from humidity and temperature fluctuations during transport and storage, and to ensure product quality throughout the 24-month shelf life (Figure 3).

Figure 2 – Standard nutrient pad set from Sartorius.


Figure 3 – Nutrient pad set packaging protects contents during transport and storage and ensures product quality and long shelf life.

 

 

 

 

 

 

 

 

 

All nutrient pad set types are supplied with the appropriate membrane filters (pore size and color), which are also presterilized and individually packaged. The membrane filters are customized to meet the requirements of microbial detection, and are supplied in 47- or 50-mm diameters.

Figure 4 – Wild yeasts from lager beer.

Thirty types of NPS are available and can be used for the detection and enumeration of microorganisms in beer (see Figure 4), fruit juice, soft drinks, wine and mineral and bottled water. In addition to the European Drinking Water Directive, the sets comply with a variety of other international regulations and recommendations, including international pharmacopoeias; DIN and ISO standards; Standard Methods for the Examination of Water and Wastewater and Foods (APHA water and APHA food); brewery guidelines, i.e., Journal of the Institute of Brewing, European Brewery Convention; and food industry recommendations, i.e., International Federation of Fruit Juice Producers.

In comparison to agar plates, nutrient pad sets do not require time-consuming preparation. The sterile water used to wet the nutrient pads can be modified by additives. During manufacturing quality assurance, the media are compared with the corresponding agar medium’s growth-promoting properties to establish consistent quality and reproducible results.

Juliane Pfützenreuter is product manager for microbiological quality control products, Sartorius Lab Instruments GmbH & Co. KG, Weender Landstr. 94-108, 37075 Goettingen, Germany; tel.: +49 551 308 0; fax: +49 551-308-3289; U.S. contact: Sartorius North America; Bohemia, N.Y.; tel.: 631-254-4249; e-mail: [email protected]; www.sartorius.com

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