Applications of automated direct and indirect methods for food microbiology: Dissertation

Merja Karwoski

Research output: ThesisDissertationCollection of Articles


The microbiological laboratory of the future will have increased automation and instrumentation. More efforts should be focused on the practical and novel applications of improved conventional and new commercial systems. In this thesis two direct and two indirect automated microbiological methods were studied in order to gather more information about their use and applications in the field of food microbiology. The Spiral system Spiral Plater used in this work offered substantial savings over conventional plating, in the recycling of glassware, agar, Petri plates and diluent, in time, space and support labor. The Spiral method could be used for plating when enumerating pure bacterial, yeast and some mold cultures, raw milk and minced beef samples as an alternative to the conventional Aerobic Plate Count (APC) method when plates were counted manually. Microbial pure culture, raw milk (manual or automatic counting) and minced beef (manual counting) results showed that the Spiral method had higher overall recovery when compared with the pour plating method. The Spiral system Laser Colony Scanner connected to the CASBATM Bacterial Enumeration Program could replace manual counting of APC plates when pure bacterial, yeast and some mold cultures were studied. The findings with raw milk, pork loin surface and minced beef samples emphasized the need for careful adjustment of the settings of the automatic colony counter. Another direct method included in this thesis was the Direct Epifluorescent Filter Technique (DEFT). The combined use of semi-automated DEFT and the conventional APC method could identify irradiated (doses ³5 kGy) spices, herbs and seasonings from unirradiated ones. The results had to be confirmed by chemical/physicochemical methods to reveal possible heat treatment or fumigation of samples. The combined DEFT+APC method proved to be a useful screening method for indication of the use of irradiation of spices/herbs/seasonings and it also provided an estimate of the microbiological quality of products prior to and after the irradiation process. The method is easily adaptable as a part of routine microbiological quality control systems. The semi-automated counting equipment used in the DEFT method offered an advantage when many samples were analysed. The DEFT counts could be obtained very quickly and there was no operator fatigue. Two indirect automated microbiological systems included in this thesis were the Bioscreen analysing system and the OmnispecTM 4000 bioactivity monitor system. Both systems measure growth or growth-related microbial processes during incubation of samples. The Bioscreen analysing system turbidometer measured turbidity changes and the OmnispecTM 4000 bioactivity monitor system reflectance colorimeter measured color changes in samples. Both systems offered several advantages that could help food microbiologists provide quality assurance rather than merely quality control. With these automated systems, detection times were shorter than with conventional methods, more frequent readings were possible than by hand with the same number of samples, monitoring the growth of microorganisms in the samples was not time-consuming, data could be further processed, e.g. for statistical analyses. Furthermore, the systems had high capacity and the cost per test due to labor and media was reduced. Both systems were easy to use and had considerable temperature flexibility and both were versatile, providing possibilities for several applications.
Original languageEnglish
QualificationDoctor Degree
Awarding Institution
  • University of Helsinki
  • Niemelä, Seppo, Advisor, External person
  • Salkinoja-Salonen, Mirja, Advisor, External person
Award date14 Oct 1994
Place of PublicationEspoo
Print ISBNs951-38-4632-6
Publication statusPublished - 1994
MoE publication typeG5 Doctoral dissertation (article)


  • microbiology
  • foods
  • microorganisms
  • bacteria
  • counting techniques
  • methods
  • automation


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