Effect of Pre-Processing Steps, Nitrite and Irradiation Combination Preservation of a Ready-to-Eat Spinach Relish and Sorghum Porridge Meal


The effects of pre-processing steps (washing, blanching and cooking) and combination preservation of irradiation (10 kGy) and nitrite (0, 50, 100, 150 and 200 mg·kg–1) on the survival of Clostridium sporogenes spores in a ready-to-eat (RTE) spinach relish and sorghum porridge meal were investigated. Chlorine wash (250 mg–1) reduced the C. sporogenes counts in spinach by 1.6 log cycles. Blanching following the chlorine wash caused no significant decrease in the spore counts in spinach. On the other hand, cooking significantly reduced the counts in the porridge by about 1.7 log cycles. In both components of the meal, there was a significant decrease in the Clostridia counts with increased sodium nitrite levels. However, the counts increased in the sorghum porridge component after 12 d of storage at 10℃. Cooking alone significantly reduced the final nitrite levels in both components of the meal. In both components of the meal, nitrite in combination with irradiation reduced the C. sporogenes counts to less than 10 cfu/g. A safe RTE spinach relish and sorghum porridge meal could be expected when a pre-processing, followed by a combination treatment of at least 50 mg·kg–1 sodium nitrite and a target dose of 10 kGy is applied.

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R. Shilangale, "Effect of Pre-Processing Steps, Nitrite and Irradiation Combination Preservation of a Ready-to-Eat Spinach Relish and Sorghum Porridge Meal," Food and Nutrition Sciences, Vol. 3 No. 7, 2012, pp. 873-878. doi: 10.4236/fns.2012.37116.

1. Introduction

In South Africa, most of the African people in formal and informal urban areas depend on commercially produced food products. It has also been shown that significant numbers of these people prefer traditional meals. However, traditional foods are not commercially produced, have a short shelf life and are laborious to prepare.

Few studies have been done on the preservation of RTE foods such as spinach morôgo and sorghum porridge. However, the survival and growth of pathogens in RTE meals is possible. Previous study in RTE spinach and sorghum porridge meal produced by a combination of Modified Atmosphere Packaging (84.5% N2 + 15.5% CO2) with irradiation (10 kGy) found the possible growth of Clostridium botulinum if the meal is temperature abused [1].

From the microbiological point of view, food irradiation alone has two potential advantages, first it reduces the number of pathogens and so increases food safety, and secondly it extends the shelf life of the product by reducing the number of spoilage organisms [2]. However, one of the major concerns about food irradiation is that insufficient doses of radiation might serve as a mutagenic catalyst that could create radiation-resistant strains, in the same way that certain strains of microbes have developed resistance to antibiotics [3]. On the other hand, the formation of benzine (and its delivetives) and alkylcyclobutanones (ACBs) has become a food safety concern because of the potential of ACBs to induce DNA damage which have been observed under experimental conditions [4]. So far, regulatory and health organizations recommend that food irradiation below 10 kGy as safe level [4]. However, irradiation alone at 10 kGy dosage does not render food free from pathogenic bacteria.

Hence, there is a need to try other methods or a combination of methods to ensure the safety of RTE meals and avoid the use of higher irradiation dosage. A combination preservation methods such as sodium nitrite in combination with irradiation could be considered. Sodium nitrite has been found to be effective in preventing outgrowth of C. botulinum in temperature abused cured meat products. A considerable amount of research has been done relating to the use of nitrites in perishable cured meat such as bacon, sausages, vieners and canned hams [5] but very little research has been done on ready-to-eat (RTE) meals.

The objective of this study was to investigate the effects of the pre-processing steps and nitrite-irradiation combination processing on the safety of a RTE sorghum porridge and spinach morôgo meal.

2. Materials and Methods

2.1. Effects of Pre-Processing Steps on C. sporogenes Spores

Pre-processing treatments were followed in order to reduce the spore counts in the raw materials prior to further processing. Figure 1 gives the summary of the process description.

2.2. Preparation of a RTE Sorghum Porridge and Spinach Relish

Sorghum porridge and spinach based relish meals were prepared following the method adapted from Duodu et al. (Figure 2) [6]. Samples of the product were taken immediately before and after irradiation and the Clostridia

Figure 1. Pre-processing steps of spinach morôgo and sorghum porridge meal.

Figure 2. Flow diagram of preparation and processing of RTE spinach morôgo and sorghum porridge meal.

counts were performed on day 1, 6 and 12 after irradiation. The time intervals used for Clostridia count were chosen based on a previous study by Obilana [1]. Storage temperature of 10˚C was chosen in order to determine the safety of the RTE food in case of temperature abuse during storage. Although samples were irradiated at a target dose of 10 kGy, samples received irradiation doses of 13.8, 10.4, 10.0 and 12.3 kGy at a dose rate of 1.4 kGy/h for the first to the fourth replicates respectively.

2.3. Determination of Residual Sodium Nitrite

Residual sodium nitrite levels in RTE sorghum porridge and spinach relish were determined using the Association of Analytical Communities (AOAC) Official Method of Analysis [7].

2.4. Microbiological Analysis

2.4.1. Sampling and Preparation of Dilutions

Tenfold serial dilutions of the samples were made by aseptically transferring 20 g of sample into a sterile stomacher bag containing 180 ml of sterile 0.1% peptone water. Samples were homogenised for 0.5 to 1 min using a Stomacher 400 laboratory blender (Seward Laboratory UAC House, Britain). Further dilutions (10–2, 10–3, 10–4, 10–5 and 10–6) were made by transferring 1 ml of successive serial dilutions into McCartney bottles containing 9 ml of sterile peptone water.

2.4.2. Enumeration of C. sporogenes

C. sporogenes spore suspension (isolates Cl3, Cl5 and Cl10) was enumerated as described by Anellis et al. [8]. Cells were not heat shocked because the preparation of samples involved a cooking process whereby the temperature and time used were enough to activate spore germination. Peptone P (Merck, Wadeville, South Africa) was used to prepare Tryptic Yeast Thioglycolate agar instead of Thiotone (BBL).

2.5. Statistic Analysis

The analysis of variance (ANOVA) was carried out using a Statistica Version 5.0 from the Microsoft Corporation. The least significant difference test (LSD-test) was used to determine whether a difference existed between means of treatments. All comparisons were done at a level of 5% significance.

3. Results

The effects of pre-processing steps on the survival of inoculated C. sporogenes spores in the spinach and sorghum porridge components are shown in Table 1. The analysis of variance revealed that there was a significant (p < 0.05) decrease of 1.6 log10cfu/g in the spores survived in spinach washed with 250 mg·l–1 NaOCl. No significant decrease of spores was observed after subsequent blanching in two water changes (at 77˚C for 7 min).

Table 1. Effect of pre-processing of spinach and sorghum porridge on the survial of C. sporogenes (log10cfu/g).

The Clostridia counts after cooking were significantly higher than those after blanching. With sorghum porridge, a significant (p < 0.05) decrease of 1.7 log10cfu/g in the number of C. sporogenes spores was observed after cooking.

Table 2 shows the effect of different sodium nitrite levels on the survival of the inoculated C. sporogenes spores and their subsequent growth in the spinach component of the meal stored at 10˚C for 12 d period. Statistical analysis showed that there was a significant decrease (p < 0.05) in the C. sporogenes counts with increased sodium nitrite concentration. However, there was no significant (p < 0.05) change in the spore counts over time.

Conflicts of Interest

The authors declare no conflicts of interest.


[1] A. O. Obilana, “Modified Atmosphere Packaging and Irradiation Preservation of Sorghum Porridge and Spinach Relish Meal,” M.Sc. Dissertation, University of Pretoria, Pretoria, 1998.
[2] D. J. Olson, “Irradiation of Food: A Publication of the Institute of Food Techologist’s Expert Panel on Food Safety and Nutrition,” Food Technology, Vol. 52 No. 1, 1998, pp. 56-62.
[3] J. E. Prejean, “Food Irradiation: Why Aren’t We Using It?” J.D. Dessertation,. Havard Law School, Harvard, 2001.
[4] J. S. Smith and S. Pillai, “Irradiation and Food Safety,” Food Safety, Vol. 58 No. 11, 2004, pp. 48-55.
[5] L. N. Christiansen, “Factors Influencing Botulinal Inhibition by Nitrite,” Food Technology, Vol. 10, 1980, pp. 237-239.
[6] K. G. Duodu, A. Minnaar and J. R. N. Taylor, “Effect of Cooking and Irradiation on the Labile Vitamins and Antinutrient Content of a Traditional African Sorghum Porridge and Spinach Relish,” Food Chemistry, Vol. 66, No. 1, 1999, pp. 21-27. doi:10.1016/S0308-8146(98)00070-3
[7] Association of Official Analytical Chemists, “Official Methods of Analysis of the Association of Official Analytical Chemists,” Method 39.1.21., Washington DC, 1995.
[8] A. Anellis, E. Shattuck, D. B. Rowley, E. W. Ross, D. N. Whaley and V. R. Dowell, “Low-Temperature Irradiation of Beef and Methods for Evaluation of a Radappertization Process,” Applied Microbiology, Vol. 30, No. 5, 1975, pp. 811-820.
[9] “General Standard for Irradiated Foods Codex Standards 106-1983,” 2003. http://www.codexalimentarius.net/download/standards/16/CXS_106e.pdf
[10] W. B. Hugo and A. D. Russell, “Type of Antimicrobial Agents,” In: A. D. Russell, W. B. Hugo and G. A. J. Ayliffe, Eds., Principles and Practice of Disinfection, Preservation and Sterilization, Blackwell Scientific Publication, Oxford, 1982, p. 8.
[11] A. Anderson, U. Ronner and P. Granum, “What Problems Does the Food Industry Have with Spore-Forming Pathogens, Bacillus cereus and Clostridium perfrigens?” International Journal of Food Microbiology, Vol. 28, No. 2, 1995, pp. 145-155.
[12] F. M. Driessen, “Importance of Bacillus cereus in Fermeted Milks and Processed Non-Fermented Dairy Foods,” Bulletin of the International Dairy Federation, Vol. 287, 1992, pp. 11-15.
[13] M. W. Peck, B. M. Lund, D. A. Fairbairn, A. S. Kaspersson and P. C. Underland, “Effect of Heat Treatment on Survival of and Growth from Spores of No-Proteolytic Clostridium botulinum at Refrigeration Temperatures,” Applied and Environmental Microbiology, Vol. 61, No. 5, 1995, pp. 1780-1785.
[14] J. M. Jay, “Modern Food Microbiology,” Chapman & Hall, New York, 1992.
[15] R. B. Tompkin, “Nitrate and Nitrite,” In: P. M. Davidson and A. L. Brasnen, Eds., Antimicrobials in Foods, Marcel Dekker, New York, 1998, pp. 205-256.
[16] T. M. Abo Bakr, S. M. El-Iraqui and M. H. Huissen, “Nitrate and Nitrite Contents of Some Fresh and Processed Egyptian Vegetables,” Food Chemistry, Vol. 19, No. 4, 1986, pp. 265-275. doi:10.1016/0308-8146(86)90050-6
[17] F. Yang, E. Troncy, M. Francoeur, B. Vinet, P. Vinay, G. Czaika and G. Blaise, “Effects of Reducing Reagents and Temperature on Conversion of Nitrite and Nitrate to Nitric Oxide and Detection of NO by Chemiluminescence,” Clinical Chemistry, Vol. 43, No. 4, 1997, pp. 657-662.
[18] N. L. Mondy, S. K. Koushik and L. B. Munshi, “Irradiation and Packaging Affect the Nitrate-Nitrogen Concentrations of Potatoes,” Journal of Food Science, Vol. 57, No. 6, 1992, pp. 1357-1358. doi:10.1111/j.1365-2621.1992.tb06856.x
[19] B. R. Thakur and R. K. Singh, “Food Irradiation—Chemistry and Applications,” Food Review International, Vol. 10, No. 4, 1994, pp. 437-473. doi:10.1080/87559129409541012
[20] N. Nygaard and K. M. Lie, “Inactivation of Clostridium Sporogenes Spores in Fish By-Products by a New Processing Method,” 2011. http://www.nofima.no/filearchive/Rapport%2010-2011.pdf
[21] H. Pivinick, M. A. Johnston, C. Thacker and R. Loynes, “Effect of Nitrite on Destruction and Germination of Clostridium botulinum and Putrefactive Anaerobes 3679 and 3679 h in Meat and in Buffer,” Canadian Institute of Food Technology Journal, Vol. 3, No. 3, 1970, p. 103.
[22] J. F. Diehl, “Safety of Irradiated Foods,” Marcel Dekker, Inc., New York, 1990.

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