Scheduling of manufacturing systems under dual-resource constraints using genetic algorithmsOriginal Research ArticleJournal of Manufacturing SystemsScheduling belongs to the special class of NP-hard problems for which no polynomial time algorithm has been found. Therefore, a schedule that is the best possible near-optimal solution is often acceptable. This paper presents a scheduling approach, based on Genetic Algorithms (GAs), developed to address the scheduling problem in manufacturing systems constrained by both machines and workers. This genetic algorithm utilizes a new chromosome representation, which takes into account machine and worker assignments to jobs. A set of experiments for determining the best staffing level and machine and worker assignment to jobs was performed. A study was conducted using dispatching rules with various performance measures for two types of shop characteristics: (i) dual-resource (machines and workers) constrained, and (ii) single-resource constrained (machines only). An example is used for illustration and comparison. The resulting scheduling methodology is capable of determining the best staffing level and dispatching rules for the chosen performance measure in both single and dual-resource constrained shops. Decisions to adopt the prescribed staffing strategy to improve the primary performance measures such as mean flow time, mean tardiness, and mean waiting time must be balanced by managers against the potential increase in direct cost. The developed scheduling approach and formulation proved to be very useful for optimizing production performance under the realistic conditions imposed by both machine and worker availability constraints. Such a tool should be used to define a priori the best dispatching rules and schedules for a given set of production requirements and objectives.Microbial contamination of food refrigeration equipmentOriginal Research ArticleJournal of Food EngineeringRefrigeration systems in chilled rooms in 15 plants processing a variety of foods were studied. These included plants processing raw meat and salads, Chinese ready meals, dairy products, slicing and packing of cooked meats and catering establishments. An initial survey of total numbers of microbes at a total of 891 sites on evaporators, drip trays and chilled room walls was followed up with a more detailed examination of 336 sites with high counts, selecting for Listeria spp., coliforms, enterococci, Staphylococcus aureus and Bacillus cereus. Temperatures (particularly air on and air off, maximum and near defrost heaters) relative humidity, airflow, layout and cleaning regimes were surveyed. In general, no correlation could be found between any of the physical measurements and the numbers and types of bacteria detected. Maximum mean temperatures in the chilled rooms varied from 1 to +16.9 C and few chilled units were regularly cleaned. Twenty five percent of sites examined had more than 105 colony-forming units per cm2, although, very few pathogens or faecal indicator bacteria were detected. Listeria spp. were not found and coliforms were found only once, in low numbers. Low numbers of S. aureus or B. cereus were present in 9 of the 15 plants, B. cereus was found on evaporators and associated drip trays in two catering plants and two plants processing cooked meat. Enterococci and S. aureus were found most frequently in a raw red meat slaughterhouse (always in low numbers). In general, microbial contamination was lower in rooms where wrapped rather than unwrapped products were stored. The type of product also affected the degree of contamination, with raw red meat and poultry or dry ingredients giving highest counts, and raw vegetables and cooked products lowest. The work demonstrated that bacteria were present on evaporator cooling coils in all factory cold rooms visited. Although evaporator-cleaning procedures were carried out in some factories as part of routine maintenance these were not shown to be effective at maintaining low levels of bacteria on evaporators. To maintain evaporator hygiene it is suggested that more regular cleaning procedures, possibly by means of automated cleansing systems, should be considered.Article Outline1. Introduction2. Materials and methods 2.1. Physical measurements2.2. Microbiological sampling3. Results 3.1. Total viable counts (TVC)3.2. Specific bacteria3.3. Effect of temperature, relative humidity and air velocity3.4. Cleaning procedures4. DiscussionAcknowledgementsReferencesRDF production plants: I Design and costsOriginal Research ArticleApplied Thermal EngineeringMunicipal solid waste (MSW) management calls for the integration of different recovery, recycling and disposal technologies. Among these possible options, MSW may be treated in order to obtain a fuel to be sold to third party users or directly utilized to generate electricity provided it is of sufficient quality to be employed i