The Microbiological Quality of Cooked Rice from Restaurants and Take-Away Premises in the United Kingdom

877 Journal of Food Protection, Vol. 6, No. 8, 999, Pages 877 88 Copyright, International Association of Milk, Food and Environmental Sanitarians The Microbiological Quality of Cooked Rice from Restaurants and Take-Away Premises in the United Kingdom G. L. NICHOLS, * C. L. LITTLE, V. MITHANI, AND J. DE LOUVOIS Environmental Surveillance Unit, Public Health Laboratory Service, Communicable Disease Surveillance Centre, 6 Colindale Avenue, London NW9 EQ, UK; and Food Hygiene Laboratory, Public Health Laboratory Service, Central Public Health Laboratory, 6 Colindale Avenue, London NW9 HT, UK MS 99-: Received 9 January 999/Accepted March 999 ABSTRACT The microbiological quality of,6 samples of cooked rice from restaurants and take-away premises in the United Kingdom was examined, including ready-to-eat rice purchased at point-of-sale and rice that was stored precooked for reheating on demand. The majority of point-of-sale cooked rice samples (,8 of,97; 9%) were of acceptable microbiological quality, but (%) samples were of unacceptable quality (Bacillus spp. and, 0 CFU/g; Escherichia coli, 0 CFU/g), indicating a potential risk to health. The prevalence of Bacillus spp.,, and was significantly greater in precooked stored rice than in point-of-sale cooked rice (P 0.00 to 0.000). Bacillus spp. ( 0 CFU/g), ( 0 CFU/g), and ( 0 CFU/g) were present in 7%, %, and 9% of precooked stored samples, respectively, compared to %, 0.%, and %, respectively in point-of-sale samples. Although final heating at the point of sale reduces the levels of microorganisms present in rice it will not inactivate the emetic toxin if present. Rice from Indian premises was of poorer microbiological quality than that from Chinese and other premises. Although most point-of-sale cooked rice samples (9%) were of an acceptable microbiological quality, evidence from this study indicates that the microbiological quality of cooked rice sold from certain outlets in the UK is of concern. Cooked rice was first recognized as a cause of food poisoning through contamination with Bacillus cereus in 97 (). can give rise to two distinct forms of foodborne disease, the emetic and diarrheal syndromes. Outbreaks attributed to emetic food poisoning in England and Wales have frequently implicated boiled or fried rice from Chinese restaurant and take-away outlets as the vehicle (6, ). Between 99 and 996 in England and Wales there were 9 outbreaks of food poisoning attributed to cooked rice contaminated with or other Bacillus species, with 6 people affected (Public Health Laboratory Service [PHLS] Communicable Disease Surveillance Centre [CDSC], unpublished data). Many outbreaks of a similar nature have occurred in Canada, Finland, the Netherlands, Japan, and the USA (, 0, 6, 8, 0, ). Incidents of rice-associated food poisoning in England and Wales have been linked to the practice of preparing rice in bulk in advance, in anticipation of subsequent need. The traditional practice in Chinese establishments involves boiling a large quantity of rice that is then kept at room temperature before reheating. There has been resistance to the refrigeration of boiled rice because this, apparently, has an adverse effect on the finished product (). Uncooked rice frequently contains spores of that can survive boiling. If cooked rice is subsequently stored at room temperature for long enough, the heat-resistant spores will germinate, proliferate, and may produce emetic toxin in the * Address for correspondence. Tel: 08-00 6868; Fax: 08-90 9907; E-mail: gnichols@phls.nhs.uk. product (8, 9, ). Reheating the rice prior to serving will not inactivate the emetic toxin and render the product safe. Levels of in foods incriminated in incidents of the emetic form of food poisoning have ranged from.0 0 to.0 0 0 organisms per gram (). It is only when large numbers of are present or when toxin is produced that becomes a hazard. Cell multiplication during inadequate cooling of cooked rice is the greatest problem. The Local Authorities Co-ordinating Body on Food and Trading Standards and the PHLS implement a program of coordinated food surveillance projects through the local authorities Food Liaison Groups. This program is designed as a bottom-up exercise in which foods of concern are identified by Environmental Heath Officers or PHLS microbiologists, and surveys are organized nationally on those foods about which there is widespread concern. The purpose of one of these studies was to establish the microbiologial quality of cooked rice from restaurants and takeaway catering outlets in the UK in relation to levels of aerobic plate count (APC), Escherichia coli, Bacillus spp., and. In addition, the study sought to determine whether the practice of cooking rice and storing it prior to final heating produced a significant risk to health through growth by to high numbers, and to establish whether the final cooking process produced rice of an acceptable microbiological quality. MATERIALS AND METHODS Cooked rice collection. Samples were collected by local authority Environmental Health Officers from restaurants, take-away

878 NICHOLS ET AL. J. Food Prot., Vol. 6, No. 8 TABLE. Microbiological results from,6 precooked and point-of-sale ready-to-eat samples of rice Rice sample ND a 0 b 0 to 0 0 to 0 0 to 0 0 to 0 6 0 6 to 0 7 0 7 Precooked (n,897) Bacillus spp. f Point-of-sale (n,97) Bacillus spp. f 9 c,78 d,60 d,66 e 7 6 6 80 0 7 69 7 9 7 7 8 0,8,9,8,9 9 8 0 0 9 0 Not specified (n 9) g Bacillus spp. f 67 89 77 87 6 7 8 7 6 a ND, not detected. b CFU/g. c Lower limit of detection 00 CFU/g. d Lower limit of detection 00 CFU/g. e Lower limit of detection CFU/g. f All Bacillus spp. excluding. g Cooking status of collected rice was not specified. catering premises, cafes and public houses serving both traditional and ethnic cuisine, throughout the UK during April and May 99. Samples consisted of precooked rice awaiting reheating or frying that were collected from the food preparation area, and reheated (ready-to-eat) rice collected at the point of sale, as if being purchased by the customer. In order to prevent bias, resulting from samples being heated more thoroughly than usual prior to service, the point-of-sale samples were purchased before the precooked rice samples. Samples were collected during the normal working hours of the premises. No advance warning of the visit was given. Rice samples collected included basmati, brown, and wild rice. Rice salads, rice dishes, rice newly boiled from a dry state, and rice puddings were all specifically excluded from the study. Rice samples were collected in accordance with the Food Safety Act of 990 (Code of Practice No. 7) (). Samples were delivered to the laboratory in an insulated cool-box for examination on the same day or were refrigerated and examined the following morning. Information on how the precooked rice was stored, its temperature and length of time it was stored for was also recorded. Microbiological examination. APCs were determined according to British Standard (BS) 76: Part () by spiral or surface plating (7). Plates were incubated at 0 C for 8 h. Isolation and enumeration of was determined by BS 76: Part () but used selective agar (Oxoid, Basingstoke, UK) as the isolation and enumeration medium (7). Isolates of B. cereus and Bacillus spp. from samples where the level exceeded 0 CFU/g were further characterized and typed by the Food Hygiene Laboratory, Central Public Health Laboratory. Enumeration of was determined by a - -tube most-probable-number method based on BS 76: Part 8 (). Minerals-modified glutamate broth (Oxoid) and brilliant green bile broth (Oxoid) were used as the culture medium in place of lauryl sulfate tryptose broth and EC broth, respectively (7). Statistical analysis. The chi-square ( ) test was performed using Minitab. RESULTS Precooked and point-of-sale rice. A total of,6 samples of cooked rice were examined, of which 7% were point-of-sale (ready-to-eat) samples and 6% were precooked stored samples. For 7% of samples the cooking status of the rice was not recorded. The APC exceeded 0 6 CFU/g in % point-of-sale rice samples, and Bacillus spp. and at or greater than 0 CFU/g were detected in % and 0.% samples, respectively. One percent of point-of-sale samples contained at 0 CFU/g or more (Table ). Point-of-sale rice samples with an APC 0 6 CFU/g were significantly (P 0.00) more commonly contaminated with (%), Bacillus spp. (%), and (%) than those with lower APCs (%, %, and %, respectively). Significantly more samples of precooked stored rice had a higher level of contamination with Bacillus spp., B. cereus ( 0 CFU/g), or ( 0 CFU/g) compared with point-of-sale cooked rice (Table ). Bacillus spp. ( 0 CFU/g) were present in 7% precooked rice samples compared to % (P 0.000) in point-of-sale samples, with ( 0 CFU/g) isolated from % precooked rice and 0.% point-of-sale samples (P 000). ( 0 CFU/g) was present in 9% precooked rice compared to % in point-of-sale samples (P 0.000). Microbiological quality of point-of-sale cooked rice. Based on the Microbiological Guidelines for ready-to-eat foods sampled at the point-of-sale (7), 9% of point-of-sale

J. Food Prot., Vol. 6, No. 8 MICROBIOLOGICAL QUALITY OF COOKED RICE 879 TABLE. Samples of point-of-sale rice categorized according to the published Microbiological Guidelines Microbiological quality (CFU/g) Criterion a Satisfactory Borderline limit of acceptability Unsatisfactory Unacceptable/ potentially hazardous and B. subtilis group c 0 6 0 0 6 0 7 0 0 0 7 0 0 NA b 0 0 0 0 0 0 0 Point-of-sale rice, n (% of total) d,7 (87%) (7%) 0 (%) (%) a For key to classification see Gilbert et al. (7). b NA, not applicable. c B. subtilis, B. Richeniformis, and B. pumilus when present in large numbers are also unsatisfactory or unacceptable/potentially hazardous. d Total n,97. rice samples were of acceptable microbiological quality. Overall, % of point-of-sale rice samples were of unsatisfactory microbiological quality, and % were of unacceptable microbiological quality (Table ). Unacceptable results were most frequently due to counts of or Bacillus spp. at or greater than 0 CFU/g, or E.coli at 0 CFU/g or more (Table ). Bacillus spp. and isolated from precooked (stored) and point-of-sale rice samples. Bacillus spp., including, at counts of 0 CFU/g or more were isolated from 87 samples of cooked rice (Table ). More than one isolate was present in % of samples. These organisms were further characterized and typed. Most isolates were identified as (%) or B. subtilis (%) (Table ). An attempt to type 88 of the 89 isolates of B. cereus produced 7 different H serotypes (Table ). However, the majority of the isolates (%) were nontypable. Serotypes commonly associated with emetic food poisoning (types,,, and 8) made up only % of the total. Storage of precooked rice. The time between initial cooking and serving at the point of sale was recorded for TABLE. Bacillus spp. and isolated at 0 CFU/g or more from cooked rice Bacillus species B. amyloliquefaciens B. firmus B. licheniformis B. mycoides B. polymyxa B. pumilus B. subtilis B. thuringiensis Bacillus spp. not identified Total Number of isolates 89 7 60 69 87 9% of precooked stored rice samples. High counts ( 0 CFU/g) of Bacillus spp. and occurred less frequently in rice sampled within 8 h of the initial cooking process compared to samples stored for more than h (Fig. ). The number of precooked rice samples with high TABLE. Serotypes of isolated at 0 CFU/g or more from cooked rice serotype iii lecithinase negative V 8 6 7 7V 7B iii 8 0 6 A AA a C G H iii HC Ii Nontypable Total a AA, autoagglutination. Number of isolates 6 0 7 88

880 NICHOLS ET AL. J. Food Prot., Vol. 6, No. 8 FIGURE. Microbiological quality of precooked rice in relation to storage time. q, APC 0 6 CFU/g;, 0 CFU/g; r, Bacillus spp. 0 CFU/g;, 0 CFU/g. counts of both ( 0 CFU/g) and APC ( 0 6 CFU/ g) was greater in samples stored for between 8 and h but was lower in those kept for 8 h (Fig. ). None of the samples of precooked rice stored frozen, in a steamer or rice cooker, or in plastic bags contained high levels of Bacillus spp., ( 0 CFU/g), or ( 0 CFU/g), whereas 0% of samples stored in open pans etc. did. The level of contamination of precooked rice samples stored at ambient temperature with Bacillus spp. and was directly related to the length of storage. 0 CFU/g was found among % of samples stored for less than 8 h compared to % if stored for more than h at ambient temperature. This relationship regarding contamination and length of storage was not evident in precooked rice stored at refrigeration temperature. Cuisine type. Rice samples from a wide range of cuisine types were examined, of which most were Chinese (%) or Indian (%). The majority of samples were collected from take-away premises (%) or restaurants (%), with 6% from public houses, % from cafes, and % from other establishments. For 6% the origin was not specified. Based on published Microbiological Guidelines (7) significantly fewer point-of-sale cooked rice samples from Chinese establishments were of unacceptable microbiological quality (%) compared with those from Indian establishments (8%) (P 0.000). The majority of unsatisfactory or unacceptable samples of point-of-sale rice from Indian outlets contained high levels of Bacillus spp. ( 0 CFU/g). This was associated more with pilau rice (9%) than with other types of rice (%). Indian rice, particularly pilau rice, also had proportionally more samples that contained high APCs and had Bacillus spp. present (Fig. ). ( 0 CFU/g) was also present more frequently in Indian (%) than in Chinese rice at point of sale (%). DISCUSSION This study has shown that most (9%) point-of-sale rice samples collected from restaurants and take-away establishments in the UK were of acceptable microbiological quality. However, % were classified as unsatisfactory using published PHLS guidelines (7), suggesting that the preparation and storage practices of these rice samples should be improved. One per cent of point-of-sale rice samples were unacceptable, or a potential risk to public health. The presence of high APC, Bacillus spp., and counts may indicate postcooking contamination of the product in open containers or that the length of time and temperature control in storage facilities was inadequate to prevent bacterial growth. The microbiological quality of cooked rice in this study as judged by APC and the presence of and/or Bacillus spp. was higher than that found in the previous PHLS survey of cooked rice from Chinese and Indian establishments in England and Wales in 976 (). However, the present study found that significantly more rice samples from Indian than from Chinese establishments were of unacceptable microbiological quality compared to that in 976 and probably reflects poorer hygiene practices in these premises during handling and storage of the rice following cooking. Contamination may also have been introduced from spices added to rice, as these are often heavily contaminated with Bacillus spores (, 9). The PHLS Working Party on Microbiological Specifications for Foods () previously reported that the degree of contamination in rice with and other Bacillus

J. Food Prot., Vol. 6, No. 8 MICROBIOLOGICAL QUALITY OF COOKED RICE 88 FIGURE. Relationship between the APC and the presence of Bacillus spp. in cooked rice of different cuisine. q, Indian pilau rice;, Indian boiled rice;, Chinese fried rice; r, Chinese boiled rice. spp. was directly related to the temperature of storage and length of time the rice was kept before serving. Evidence from the present study also indicates that precooked rice should not be stored at ambient temperature for more than h as this would allow heat-resistant spores of present in precooked stored rice to germinate and increase to high levels. Although the final heating step at the point of sale would reduce the levels of microorganisms present in rice, it would not inactivate the emetic toxin if present. Therefore, it must be assumed that high counts of in precooked rice represent a risk to health. Reheating rice before serving will not protect the consumers from heat-stable toxins that were produced previously. Data on the direct relationship between the length of storage at ambient temperature and the degree of contamination with Bacillus spp. and in cooked rice has been used in conducting hazard analyses of restaurants and takeaway establishments (). Evidence from England and Wales and other countries indicates that food poisoning attributed to enterotoxigenic continues to occur from consumption of cooked rice. During 99 to 996 in England and Wales, there were reported outbreaks where 8 people were affected after consuming rice purchased from predominantly Chinese or Indian take-aways or restaurants (CDSC, unpublished). Other Bacillus species such as B. licheniformis, B. subtilis, and B. pumilus have also been implicated in foodborne illness associated with consumption of rice dishes in England and Wales (CDSC, unpublished). Rice, and also spices, of good microbiological quality should be used when cooking rice dishes to reduce the risk of food poisoning from consuming this product. Temperature and storage conditions under which rice is prepared, cooked, and served in restaurants and take-aways is also still an area of concern. As large numbers of enterotoxigenic are required to cause food poisoning ( 0 CFU/g), it is therefore essential that control measures should be directed at preventing the germination and outgrowth of spores in cooked rice. Control measures should include rapid cooling and ideally refrigeration of the cooked rice in closed containers. The rice should not be stored for long periods or produced in bulk quantities. ACKNOWLEDGMENTS The authors thank the staff in Environmental Health Departments throughout the UK who collected the samples for this study and the staff in both PHLS and non-phls laboratories who performed the microbiological analyses. The authors also thank Clare Cunningham at the Local Authorities Coordinating Body on Food and Trading Standards for coordinating the participation of Environmental Health Officers, Janet Norcup for processing the data, and to Professor Richard Gilbert formerly of the Food Hygiene Laboratory for his support and advice. REFERENCES. Beckers, H. J. 976. The bacteriological quality of boiled and fried rice from Chinese restaurants in relation to Bacillus cereus food poisoning. Arch. Lebensmittelhyg. 7:66 68.. British Standards Institution. BS 76, Part. 99. Colony count at 0 C (surface plate technique). BSI, London.. British Standards Institution. BS 76, Part 8. 99. Enumeration of presumptive Escherichia coli. Most probable number technique. BSI, London.. British Standards Institution. BS 76, Part. 988. Enumeration of Bacillus cereus. BSI, London.. Bryan, F. L., C. A. Bartleson, and N. Christopherson. 98. Hazard analyses, in reference to Bacillus cereus, of boiled and fried rice in Cantonese-style restaurants. J. Food Prot. :00. 6. Evans, M. R., C. L. Riley, and C. D. Ribeiro. 99. Fried rice from the take-away. Communicable Dis. Rep. Rev. :R00. 7. Gilbert, R. J., J. de Louvois, T. Donovan, W. L. Hooper, G. Nichols, R. N. Peel, C. D. Ribeiro, and D. Roberts. 996. Microbiological guidelines for some ready-to-eat foods sampled at the point of sale an expert opinion from the Public Health Laboratory Service (PHLS). PHLS Microbiol. Dig. :.

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