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Type: Theses
Title: A FR 13 microbiological global risk model: demonstrated for pasteurization of raw milk with viable Mycobacterium avium subsp. paratuberculosis
Author: Chandrakash, Saravanan
Issue Date: 2017
School/Discipline: School of Chemical Engineering
Abstract: Steady-state unit-operations are used globally in chemical engineering processing. Importantly however, there are naturally occurring (random) fluctuations in parameter values about a ‘set’ mean. These are not sufficient to be considered transient and a random change in one is often off-set by a change in another - with the result that the output remains seemingly steady. Significantly, traditional chemical engineering does not address these random fluctuations explicitly. Davey and co-workers (e.g. Abdul Halim and Davey, 2015; Zou and Davey, 2016) have shown that these natural fluctuations can combine and accumulate in one direction and leverage unexpected and surprise behaviour across a ‘failure - not failure’ boundary. Their hypothesis they titled Fr 13 (Friday 13th) to underscore the surprise element of the failure event. Their probabilistic Fr 13 framework has been usefully applied to a number of 1-step unit-operations including failure in: UV irradiation for potable water (Abdul-Halim and Davey, 2015; 2016); thermal efficiency of a commercial coal-fired boiler (Davey, 2015), metals pitting (Davey et al., 2016), and; failure to remove whey protein deposits in Clean-In-Place milk processing (Davey et al., 2015). A significant advantage is that the framework can be used in quantitative ‘second-tier’ studies (Abdul-Halim and Davey, 2016) to improve design and safety of unit-operations. A limitation however is that the framework had been applied to only 1-step (single) unit-operations until very recently when Zou and Davey (2016) demonstrated its applicability to integrated 2-step membranes processing. Generally however, it is not known if there is any benefit in developing the framework as a useful tool for integrated, greater multi-step unit-operations and its possible combination (Davey et al., 2013) with existing software to enhance design capability. Davey (2011) had suggested these integrated multi-step analyses be termed ‘global’ models. A research program is therefore undertaken with the aim to advance the Fr 13 framework to gain unique insight into how naturally occurring fluctuations in apparent steady-state plant parameters can be transmitted and impact in progressively complex (in the context of ‘integrated’ not ‘complicated’) multi-step processes, and to assess the framework as a new design tool. A logical and stepwise approach is implemented as a research strategy. Because foods processing is globally the largest manufacturing sector, and within it, pasteurization is the most widely used unit-operation, a typical 3-step pasteurization unit operation, consisting of individual 1) heat-up, 2) holding and 3) cool-down, unit-operations is selected as a prudent and stringent test of the Fr 13 risk thesis to multi-step unit-operations. An initial assessment, based on typical commercial pasteurization equipment for raw milk (plate heat exchangers and an external-coil holding tube) is synthesized for the first time (Chandrakash et al., 2015; 2014; Davey et al., 2014) and a generalized method of notation for the Fr 13 risk framework is developed to unambiguously identify particular unit-operations in integrated multi-step processes. Failure is defined in terms of not meeting a globally used Regulatory combination of temperature (T) - time (t) (72 °C, 15 s). Results revealed that pasteurization of raw milk is vulnerable to failure in 12.5 % of all cases over the long-term as a result of with-in system fluctuations in flows, and thermal conditions. If each simulation is (reasonably) considered a daily process this translated to some 46 failures each year with a 2 % design tolerance 2 to meet the required T - t pasteurization criteria. Results highlighted that apparent steady-state pasteurization is actually a combination of successful and (potential) failed operations. This insight could not be obtained from existing traditional risk and hazard approaches, with or without sensitivity analyses. A drawback soon acknowledged however, is that this equipment model did not explicitly address the reduction in unwanted levels of survival of potential contaminant microorganisms in the treated milk. To overcome this, a microbiological global risk model is developed for the first time for the 3-step pasteurization. The logarithmic reduction of viable Mycobacterium avium subsp. paratuberculosis (MAP), a common bacterial contaminant and pathogen, is selected as an indicator of efficacy of process, and an inactivation model is then synthesized (Chandrakash and Davey, 2017 a). Results showed that for a design Regulatory reduction of log₁₀ = 5.5 in viable MAP the 3-step pasteurization is vulnerable to failure in 5.75 % of cases with a 2 % design tolerance averaged over the long term. This equated to ~ 21 failures with viable MAP each year based on a daily operation. To further test applicability of the risk framework to multi-step processing, a fourth integrated step, the storage of the pasteurized milk, is added for the first time (Chandrakash and Davey, 2017 b). A justification is that this simulated commercial practice more closely. Results of simulation of this 4-step model showed that with a design tolerance of 2 % for a Regulatory design reduction of log₁₀ = 5.5 in viable MAP on heat-up to 72 °C with 15 s holding in commercial plate equipment, there would be no further failures i.e. the rate of vulnerability to failure in a 4-step microbiological model for pasteurizing and storing milk remained 5.75 %, averaged over the long term. Results from investigative second-tier studies with the new 4-step Fr 13 model to improve design and safety, revealed vulnerability to microbiological failure can be readily mitigated by installing precise safety-integrity-level (SIL) mass flow control on the raw milk in existing plant to ensure a holding time of ≥ 15 s. It is concluded the Fr 13 framework appears generalizable to integrated multi-step steady-state processes without methodological problems and an advance over current existing risk/hazard methodologies. If properly developed, it is believed that this novel framework could be adopted as a new design tool for steady-state processing at both design and synthesis stages. Research findings will aid a detailed understanding of factors that contribute to failures, and to increased confidence in steady-state unit-operations processing. This research work is original and not incremental work. Findings will be of direct interest to risk analysts, milk processors and manufacturers of pasteurizer equipment.
Advisor: Davey, Kenneth Richard
O'Neill, Brian Kevin
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering, 2017.
Keywords: Fr risk assessment
integrated Fr 13 risk
pasteurizations of raw milk
pasteurizer failure
global microbiologal risk model
Mycobacterium avium subsp. paratublerculosis
Provenance: This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at:
DOI: 10.4225/55/5ac58c692ee64
Appears in Collections:Research Theses

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