Modified atmosphere packaging and irradiation preservation of a sorghum porridge and spinach relish meal

Abstract

South Africa is faced with the challenge of providing food security for its entire people. Those particularly in need are people residing in the former homelands and people in the newly developed informal, urban settlements. Their food security needs include: Access to affordable, safe, nutritious foods, which meet their quality demands (e.g. in terms of culture, storability, and convenience); and the means to earn a livelihood. Although a wide variety of traditional South African foods are prepared in the home and enjoyed by a large number of consumers, hardly any of these foods are available commercially. A typical example of one of these traditional South African foods is a sorghum porridge and spinach “morÔgo” meal consumed mainly amongst the black population. This meal requires a long preparation time and has a short shelf-life. The effects of modified atmosphere packaging, irradiation, and selected modified atmosphere-irradiation combination treatments on the microbiological stability and shelflife of a ready-to-eat (RTE) meal consisting of spinach (morÔgo) and sorghum porridge were investigated. The experiment was sub-divided into a preliminary experiment and two phases with different objectives. The objective of the preliminary experiment was to determine the effect of a chlorine (250 mg/l) wash and blanching (77o C for 6 min) in several changes of water, on the microbial count on spinach, i.e. to optimise pre-processing parameters for use in Phases 1 and 2. Washing the spinach in chlorine led to a significant (99.92%) reduction in microbial counts. However, blanching of the spinach following the chlorine treatment did not have an effect on microbial activity. This was probably due to the fact that micro-organisms that survived the Cl2 wash at the concentrations used were possibly resistant to blanching at the processing time and temperature (77o C for 6 min) or that the number of microbes inactivated by blanching following the chlorine wash was insignificant or non-detectable by the methods used. Blanching in more than two changes of water reduced total solids of the spinach significantly. Therefore, it was decided to use only two changes of water for the blanching treatment. Cooking of spinach and sorghum porridge meal for Phases 1 and 2 was done according to a popular consumer recipe. The meal was dished onto a polystyrene tray, inoculated with a Clostridium sporogenes spore suspension, sealed in a full barrier polyethylene bag under the desired modified atmosphere conditions and irradiated using a 60Co source at ambient temperature. The objective of Phase 1 was to determine the effect of two different modified atmosphere packaging gas mixtures (MAP 1: 84.5% N2 + 15.5 % CO2; MAP 2: 82.3% N2 + 15.9% CO2 + 1.8% O2) in combination with irradiation at five different dose levels (2, 4, 6, 8 and 10 kGy) on the inactivation of aerobic mesophilic bacteria (TPC) and C. sporogenes inoculated into the RTE meal, in comparison to a control (0 kGy). The purpose of this phase was to optimise the processing parameters for Phase 2. Initially, it was found that interruptions during the irradiation processing of the RTE meal led to discrepancies in gamma D10-values for C. sporogenes under the different MAP conditions. It was postulated that the duration of these interruptions (up to 14 h) may have been long enough for the microbes to initiate repair of the damaged DNA. After stricter control measures were taken during irradiation processing, more reliable gamma D10-values were obtained. Irradiation reduced C. sporogenes counts and total plate counts (TPC) in the RTE meal significantly, whilst neither MAP conditions had an effect on C. sporogenes counts or TPC. Gamma D10-values for C. sporogenes in the RTE meal were between 2.58 kGy and 2.60 kGy, indicating an effective inactivation rate by irradiation. A target dose of 10 kGy (actual dose 11.52 kGy) resulted in a 4 log10 cycle reduction in C. sporogenes counts. A shelf-stable meal was therefore not produced, as the irradiation dose used was not high enough to obtain a 12 D reduction in C. sporogenes counts. The objective of Phase 2 was to determine the effects of the optimal combination treatment as determined in Phase 1 on the safety and shelf-life of the sorghum porridge and spinach (morÔgo) RTE meal, as measured by C. sporogenes counts and TPC respectively. In Phase 2, a combination of MAP 1 (84.5% N2 + 15.5% CO2) and irradiation at 10 kGy was used in the processing of inoculated RTE meal samples. The irradiation dose of 10 kGy was chosen for use in this phase because the two components of the RTE meal appeared to remain acceptable up to this dose level from a sensory point of view and this irradiation dose reduced inoculated C. sporogenes spores by approximately 4 log10 cycles. MAP 1 (84.5% N2 + 15.5% CO2) was chosen for use in Phase 2 of the research project, as it was postulated that it would minimise the effects of oxidative rancidity in the RTE meal during storage. It was also thought that these MAP conditions (84.5% N2 + 15.5% CO2) would inhibit the proliferation of aerobic mesophilic bacteria (TPC), thus extending shelf-life of the RTE meal during the storage period following irradiation processing. After irradiation, the samples were stored at 5o C and 37o C respectively during which TPC were enumerated on days 1, 3, 5 & 7, and C. sporogenes counts were enumerated on days 1, 3, 5, 7, 9, 11 & 13. Overall, MAP decreased TPC in the RTE meal when compared to the control at both 5o C and 37o C during the storage period. MAP also reduced the growth of C. sporogenes inoculated into the RTE meal at 5o C beyond 5 d of storage but had no effect at 37o C. Initial C. sporogenes and TPC in the RTE meal were significantly reduced by irradiation compared to the control. Storage temperature ultimately determined the rate of growth of TPC and C. sporogenes in the RTE meal samples during the storage period. Growth of both TPC and C. sporogenes was faster at 37o C than at 5o C as this temperature is around the upper limit of the optimum growth temperature range of these mesophilic microorganisms. MAP-irradiation combination processing was found to be synergistic with regard to TPC in the RTE meal stored at 37°C since irradiation inactivated a high percentage of the TPC and MAP kept growth of surviving TPC to a minimum. The shelf-life of the RTE meal at 5o C was as follows: 3 d for the control; 5 d for the MAP alone treatment; at least 7 d for both the irradiation alone as well as the combination treatments. At 37°C, the shelf-life of the RTE meal was: less than 1 d for both the control and the MAP alone treatments; 3 d for the irradiation alone treatment and at least 7 d for the combination treatment. It is possible to produce a safe sorghum porridge and spinach RTE meal with a shelf-life of at least 7 d at 5o C using a combination of irradiation at a target dose of 10 kGy and MAP 1 (84.5% N2 + 15.5% CO2) processing. However, this RTE meal is a low acid food in which C. botulinum can grow and produce toxins under favourable conditions. If the cold-chain is broken during distribution and/or retailing, the safety of the meal would be compromised due to the rapid growth of any surviving bacteria. From a safety point of view, it is recommended that irradiation should not be combined with MAP 1 (84.5% N2 + 15.5% CO2) conditions that favour the growth of C. sporogenes in a full barrier packaging material as it could result in the growth of the anaerobic pathogen C. botulinum. It is also recommended that alternative hurdles to MAP (e.g. use of nitrites and/or aw) be used to extend the shelf-life of the RTE meal and guarantee safety.