Safety And PH Measurements Of Sushi Rice In Japanese . - NCCEH
Safety and pH Measurements of Sushi Rice in Japanese Restaurants inBurnaby BC, CanadaCeline Jiyun Lee1, Helen Heacock21 Lead Author, B. Tech Student, School of Health Sciences, British Columbia Institute of Technology, 3700 Willingdon Ave. Burnaby, BC V5G 3H22 Supervisor, School of Health Sciences, British Columbia Institute of Technology, 3700 Willingdon Ave. Burnaby, BC V5G 3H2AbstractBackground and Purpose: The increasing popularity of sushi in Metro Vancouver raises public health concerns overthe consumption of sushi rice being held out of temperature control. Although sushi rice is acidified to control growth ofpathogenic microorganisms, there is no existing documented system to monitor the pH of sushi rice, and pH testing israrely performed by Environmental Health Officers(EHOs)/Public Health Inspectors(PHIs) during routine inspections.The purpose of the study was to measure the pH of sushi rice samples collected from different sushi restaurants inBurnaby, BC and determine whether the pH meets the accepted standard of 4.6 or below.Methods: 30 sushi rice samples were collected from 30 randomly selected sushi restaurants in Burnaby, BritishColumbia. The samples were kept at room temperature and then tested for pH using the Waterproof Palm pH Meter.Results: The mean pH of the samples was 4.09; the median was 4.115; the standard deviation was 0.198; and the rangewas 0.82 with the minimum value of 3.71 and the maximum value of 4.53. 100% (30 out of 30 samples) had the pH lessthan 4.6. The statistical z-test resulted in a p-value of 0.00.Discussion: All of the sushi rice samples had pH values less than 4.6. Therefore, the samples were adequately acidifiedto inhibit the growth of pathogens. The low pH values indicate that the samples are not considered potentially hazardousfood, thus safe to be stored at room temperature for extended periods of time. However, due to the nature of Bacilluscereus that can grow at a pH 4.3 or higher, the target pH of sushi rice is 4.3 or lower.Conclusion: Inadequately acidified sushi rice may pose a health risk if it is stored out of temperature control. The studyshows that sushi rice being consumed by the public in Burnaby, BC is generally safe and has a low public healthconcern. Therefore, EHOs/PHIs can feel assured that sushi rice stored at room temperature is unlikely to cause potentialfoodborne illness.Keywords: sushi, rice, pH, acidity, food safety, Bacillus cereus, Staphylococcus aureus, Burnaby, BCIntroductionSushi, which literally means ‘seasoned rice’, is a type ofJapanese cuisine consisting of acidified rice combined withvarious toppings and fillings, usually raw fish or otheringredients (Bargen, 2011). Since globalization hasintroduced sushi to many countries, it has gained a hugepopularity across the world and has become a part ofCanadian diets today. There are more than 400 sushirestaurants in Metro Vancouver, and the number of sushirestaurants is increasing every year.As much as sushi is favored and consumed by manyCanadians, it possesses potential health risks. Uncooked fishcan be easily contaminated by various pathogens and maycause foodborne illness and other diseases. For example,Anisakiasis is caused by anisakis, a parasitic nematode foundin raw seafood. 90% of all cases of anisakiasis described inthe literature are caused by the consumption of sushi andsashimi (Bucci et al., 2013). Patients generally recover ontheir own, but surgery is often necessary for invasiveanisakiasis that penetrates the intestine, liver and lungs(Sakanari & McKerrow, 1989).Health concerns of eating sushi have been recognizedsince many people are aware of the risk of raw fishconsumption. However, public awareness of risks associatedwith sushi rice is low. Sushi rice is generally kept at roomtemperature or in warm holding unit in most sushi restaurantsas sushi is supposed to be served warm (about 30 C) for theideal taste. The control measure to keep the sushi rice safe atroom temperature is the addition of a vinegar solution toreduce its pH to inhibit growth of harmful bacteria.Environmental Health Officers (EHOs) or Public HealthInspectors (PHIs) have difficulty ensuring safety of sushi ricebecause each sushi chef uses his/her own recipe withdiffering amounts of the vinegar solution, and a pH test ofsushi rice is rarely performed during inspections.The focus of this study was to determine safety of sushirice stored out of temperature control (4 C – 60 C) bycreating a pH database of sushi rice collected from varioussushi restaurants in the City of Burnaby, BC.Literature ReviewWhat is Sushi?History of Sushi: The origin of sushi is believed to befermented fish or meat for the purpose of preservation in thesecond century in Southeast Asia (Sushi Encyclopedia,1
2007). Later on, rice was used to speed up the fermentationprocess. In the sixteenth century, vinegar was beginning tobe added to further reduce the preparation time. This type ofsushi was preferred over the original one and became adelicacy in Japan. From this point onward, fermentation wasnot favored anymore, and a new type of sushi using onlyvinegar and cooked rice began to evolve. Sashimi (slices ofraw fish) was consumed for centuries in Japan, but it was inthe early 1880’s when raw fish and rice were first combined.This acidified rice with raw fish is the sushi widely known tothe world today. During 1970s, sushi was first introduced inNorth America as Japanese businesses started expanding tothe U.S. (Sushi Encyclopedia, 2007).Types of Sushi: There are two main types of sushi sold inBurnaby: nigiri sushi and maki sushi (Figure 1). Literallytranslated, nigiri means “hand-pressed”. Nigiri sushi is small,oval shaped acidified rice with a firmly placed topping, suchas slices of raw fish or other ingredients. Maki sushi is acylindrical shaped roll consisting of acidified rice and fillingssuch as seafood, meat, and vegetables (Bargen, 2011).rice was 0.962 which is far above 0.86. This indicates thatwater activity does not play an important role in inhibitinggrowth of microorganisms in sushi rice.Temperature: Temperature is one of the most importantfactors in safe food handling practices. Temperature valuesfor microbial growth have a range with an optimumtemperature for maximal growth. The temperature rangebetween 4 C and 60 C (40 F and 140 F) is the Danger Zonewhere most bacteria grow (FoodSafe, 2006). If thetemperature is greater than 60 C, most bacteria die. If thetemperature is colder than 4 C, the bacteria stay alive but donot multiply rapidly. Storing sushi rice at room temperaturefor extended hours is clearly temperature abuse unless otherfactors such as pH are controlled to inhibit microbial growth.Acidity (pH): pH is a measure of acid concentration in a foodsample with a range of 0 to 14 (Forsythe, 2010). The pHrange for a microorganism, like the temperature range, has aminimum and a maximum value with an optimum pH.Generally, the optimum pH of most bacteria is 6.8-7.2, andthey cannot survive at pH 4.6 or lower. The pH range ofplain white rice is 6.0-6.7 which falls into the range of theoptimum pH of most bacteria (Forsythe, 2010).Pathogens Associated with Sushi RiceFigure 1. Nigiri (left) and Maki (right)Factors Affecting the Growth and Survival ofMicroorganisms in FoodCooked rice is a potentially hazardous food (PHF) (BCCDC,2006). According to Food Premise Regulation, potentiallyhazardous food is food that is “capable of supporting thegrowth of disease-causing microorganisms or the productionof toxins” (Food Premise Regulation BC, 1999). There aretwo types of factors that affect microbial growth onpotentially hazardous food: intrinsic and extrinsic factors.Intrinsic factors include water activity (Aw), oxygenavailability, acidity (pH), available nutrients, and presenceand identity of natural microbial flora. Extrinsic factorsinclude temperature, relative humidity, atmospherecomposition and packaging (Forsythe, 2010). The factorswhich can be controlled to limit microbial growth in sushirice include acidity (pH), water activity (Aw) andtemperature.Water activity (Aw): Water activity (Aw) is a measure of theavailable water content in a food sample. The Aw iscalculated by the ratio of the water vapour pressure of thesample to that of pure water at the same temperature. Wateractivity ranges in value from 0.0 to 1.0. The Aw of purewater is 1.0 and the value decreases with the addition ofsolutes (Forsythe, 2010). Most microorganisms cannotsurvive in the environment where the Aw is lower than 0.86.Leung (2006) measured the Aw of sushi rice made by therecipe provided by SushiLink (2006). The Aw of the sushiMost microorganisms are killed during the rice cookingprocess. However, handling cooked rice is more importantthan the cooking process because cooked rice provides agood environment for pathogen growth. The primarypathogens of concern associated with cooked rice areBacillus cereus and Staphylococcus aureus. In addition,sushi rice can be easily cross-contaminated by otherpathogenic bacteria as well because sushi rice is alwayshandled with sushi chef’s bare hands that touch raw fish andother ingredients at the same time.Bacillus cereus: B. cereus is a spore-forming bacteriumwhich may cause foodborne illness (Labbe and Garcia,2001). Among various Bacillus species found in a widevariety of foods, B. cereus is most commonly associated withfoodborne illness outbreaks. B. cereus produces toxins thatcause illness. There are two recognized types of B. cereusfoodborne illness: diarrheal (watery diarrhea, abdominalcramps and pain) and emetic (nausea and vomiting)(Forsythe, 2010). Diarrheal type of B. cereus is associatedwith meats, milk, vegetables, and fish. B. cereus-producingemetic toxin is found in rice and starchy food products(Forsythe, 2010). Rice can be easily contaminated by B.cereus during growth, harvesting, processing and handling(Haque and Russell, 2005). The spores of B. cereus surviveduring boiling and frying rice and germinate when theenvironment is favorable for growth (Gilbert et al, 1974).Between 1973 and 1985, B. cereus caused 17.8% of the totalbacterial food poisoning in Finland, 0.8% in Scotland, 0.7%in Japan and 2.2% in Canada (Kotiranta et al, 2000).Growth requirements of B. cereus are as follows (Forsythe,2010): Minimal water activity (Aw) is 0.9302
Temperature range is 4 C – 52 C pH range is 4.3-9.3The minimum water activity of B. cereus is lower than theaverage Aw of sushi rice, meaning that sushi rice providesenough moisture to support growth of B. cereus. Theminimum pH that B. cereus can multiply is 4.3, which isslightly lower than general minimum pH for inhibition ofpathogens. This indicates that B. cereus may grow on sushirice if the pH is higher than 4.3.Staphylococcus aureus: S. aureus is a toxin-producingbacterium commonly found on the skin and in the noses andthroats up to 25% of healthy people (CDC, 2006). Most of S.aureus foodborne illness cases are caused by poor hygiene offood handlers and improper food handling practices.Symptoms usually develop within 1 to 6 hours afterconsumption of the contaminated food. Infected individualsexperience nausea, vomiting, abdominal cramps, and oftendiarrhea (Forsythe, 2010). Poor personal hygiene andinappropriate food handling techniques increase the chanceof S. aureus transferred to sushi rice.Growth requirements of S. aureus are as follows (Forsythe,2010): Minimal water activity is 0.83 pH range is 4.0 – 10 Temperature range is 7 C – 48 CS. aureus can grow in the environment with low wateractivity (minimum 0.83) and low pH (minimum 4.0). Thisagain indicates that storing sushi rice at room temperaturewill support the growth of S. aureus and toxin production ifthe pH is higher than 4.0.Other potential pathogenic microorganisms: Although B.cereus and S. aureus are the primary pathogens of concern,sushi rice possesses a high potential for cross-contaminationby other pathogens as well since it involves considerablebare hand contact. Escherichia coli is one of the commonpathogens known to have caused outbreaks associated withsushi restaurants.Escherichia coli is commonly found in the digestive tractof all animals including humans. The presence of E. coli isused as an indicator of fecal contamination (Labbe andGarcia, 2001). Sushi handlers who have poor personalhygiene may transfer E. coli to sushi rice while making sushiwith bare hands. Inadequate pH and temperature abuse willsupport E. coli growth and cause foodborne illness. It wasfound that the E. coli outbreak in Nevada was caused by poorfood-handling practices and infected foodhandlers in sushirestaurants, resulting in 130 reported illnesses (Jain et al.,2008).Sushi Rice PreparationAcidification of rice: Rice is acidified by adding a vinegarsolution to reduce its pH enough to inhibit microbial growth,especially B. cereus and S. aureus. The acetic acid in vinegarlowers the pH of the rice and acts as a bacterial inhibitor(Wilson, 2001). Acidified sushi rice is known to be safe atroom temperature for up to 8 hours (University of Florida,2004).White Sushi Rice vs. Brown Sushi Rice: Sushi rice iscommonly made of white rice. Brown rice is not typicallyacidified due to the harder surface coating on the rice whichlimits penetration of acid solutions. Due to this reason, thecooked brown sushi rice must be stored under refrigeration at4 C or below to reduce the chance of foodborne illness(University of Florida, 2004).Legislation and GuidelinesBC Food Premise Regulation does not specify controlmeasures for sushi rice. However, pursuant to section 14(2),“every operator of food premises must ensure that potentiallyhazardous food is stored or displayed at a temperature of notmore than 4 C or not less than 60 C” (BC Food PremisesRegulation, 1999). Acidified rice with pH 4.6 or less is notconsidered a potentially hazardous food as the pH will inhibitthe growth of pathogens (University of Florida, 2004). Thisindicates that sushi rice should be refrigerated unless its pHis lower than 4.6.BC Centre for Disease Control (BCCDC) released a sushisafety handout in 2010. It states that the pH of white sushirice should be less than 4.6 to inhibit bacterial growth. Riceshould be acidified as soon as it is cooked and discarded atthe end of the day (BCCDC, 2010).Health Authorities throughout North America have theirown guidelines on sushi and sushi rice to ensure customersafety. For example, Alberta Health Services requires awritten recipe for sushi rice with the amount of rice andacidification agent added to the rice (Alberta HealthServices, 2011). In the County of San Bernardino, California,if sushi rice is to be held at between 4 C and 60 C, operatorsare required to submit a HACCP plan with a pH test resultsubmitted from an accredited laboratory to the HealthAuthority. Operators also need to measure the pH of theirsushi rice monthly using a pH test strip paper to ensure thepH is lower than 4.6 (Environmental Health Services, SanBernardino, 2008).However, Health Authorities in Metro Vancouver –Vancouver Coastal Health and Fraser Health – do notcurrently have guidelines regarding sushi rice safety.EHOs/PHIs conduct inspections in sushi restaurants, but pHof sushi rice is rarely checked due to the semi-solid nature ofrice that involves complicated on-site measurement orlaboratory testing. This study will provide a good databasefor EHOs/PHIs to assess the general safety of sushi rice inBurnaby and will be a great tool to educate operators on theimportance of pH control of sushi rice.Relevant Previous Research on Sushi RiceThe key factors in testing safety of sushi rice are pH valueand microbial analysis. Mundo et al. (2005) investigated howsushi rice formulation affects pH and water activity (Aw) ofrice and inhibits growth of Bacillus cereus. Twelve differentcommercial sushi rice recipes were used and the formulation3
mainly consists of vinegar, sugar and salt. The study resultsindicate that B. cereus growth is most significantly inhibitedby pH (Mundo et al., 2005). Hence, pH should be accuratelymeasured and monitored to prevent growth of B. cereus insushi rice.Sushi rice with pH 4.6 or lower is known to be safe atroom temperature for extended hours since it is notconsidered a potentially hazardous food any more. Leung(2006), a former BCIT Environmental Health student,conducted an experiment to investigate a correlation betweentotal aerobic bacterial growth and the number of hours thatthe sushi rice is left at room temperature. The results showthat bacterial counts increased in the first 3 hours anddeclined in the next 3 hours (Leung, 2006). Although thehump shape pattern in the third hour needs furtherinvestigation to be explained, the study could not find anyhealth risk of storing sushi rice out of safe temperature zonefor extended hours (up to six hours) as long as the pH of thesushi rice is less than 4.6. While Leung’s study focused onbiological analysis of sushi rice made for the experiments,this study focuses on pH of actual sushi rice samples beingconsumed by the public.Some studies were conducted to investigate the chemicaland biological quality of sushi rice that is actually served tothe consumers. Sushi rice samples collected from 19restaurants in Seattle were tested for pH and microbiologicalanalysis (Adams et al, 1994). All of them had pH levels 4.6or lower, and no fecal coliforms were detected. Bacilluscereus and Staphylococcus aureus were detected in thesamples from 6 restaurants, but the levels were too low to beconsidered a public health concern (Adams et al, 1994).A similar study was done by New South Wales FoodAuthority (2008) which conducted a survey of food handlingpractices and microbiological quality of sushi in Australia.Sushi rice samples were also collected to measure pH, wateractivity, and microbiological quality. It was found that thepH of sushi rice was rarely confirmed after acidificationprocess, resulting in 15% of the samples with a greater pHthan 4.6. Although the microbiological quality of sampleswas generally acceptable, low levels of B. cereus and S.aureus were detected in some samples, indicating a potentialhealth risk if proper acidification does not take place (NSWFood Authority, 2008).This study was based on the principle of the studiesconducted in Seattle and Australia. However,microbiological analysis was not included in the study due tolimited technical resources.Purpose of the Research ProjectThe purpose of this research project was to measure the pHof sushi rice samples collected from different sushirestaurants in Burnaby and determine whether the pH meetsthe satisfactory level – 4.6 or below – as suggested byBCCDC.Methods and MaterialsThe researcher visited 30 Japanese restaurants in the City ofBurnaby and collected an acidified white sushi rice samplefrom each restaurant (Figure 2). The restaurants wererandomly selected – every second sushi restaurant onUrbanspoon (2013). Collected samples were kept at roomtemperature, transported to the researcher’s house andprepared for pH measurement. 15g of each sample was used.Due to the semi-solid nature of rice, each sample was groundusing a mortar and pestle, and distilled water was added toobtain fluidity. Waterproof Palm pH Meter was calibratedwith buffer solutions – 4.00 and 7.00. The pH of each samplewas measured using the Waterproof Palm pH Meter (ModelPH220A).Figure 2. Map of Randomly Selected 30 Japanese Restaurants inBurnaby (Google Map, 2013)Reliability and Validity of MeasuresAccuracy of Equipment: Waterproof Palm pH Meter wascalibrated frequently to get the most accurate readings. Thedevice itself provides accurate and reliable measurements ifit is frequently calibrated and properly used. Themanufacturer’s instructions of Waterproof Palm pH Meterwere strictly followed to increase accuracy of data collected.Measurement Techniques: In addition to the equipment, agood measurement technique is important to obtain accurateresults. The same amount of the samples (15g) was used tomeasure the pH to minimize potential errors. Crosscontamination was prevented by thoroughly washing theelectrode and other apparatus with distilled water after eachuse. The experiment was performed by only one researcherin a consistent fashion.External Environment: Temperature as well as pH wasrecorded to ensure the sushi rice samples are held out oftemperature control (4 C-60 C). The temperature wasmeasured by the Waterproof Palm pH Meter.4
Inclusion and Exclusion CriteriaCollected DataAny acidified white rice stored out of temperature control inJapanese restaurants in Burnaby, BC is eligible for thisexperiment. Other food items, such as non-acidified rice,acidified brown rice, acidified white rice with otheringredients added, and refrigerated acidified white rice, wereexcluded from this study.Table 1 refers to the pH values of sushi rice samplescollected from 30 different Japanese restaurants in Burnaby.Descriptive Statistics: Descriptive statistics of the pH datawere analyzed (Table 2). The results show that the mean ofthe samples is 4.09; the median is 4.115; the standarddeviation is 0.198; and the range is 0.82 with the minimumvalue of 3.71 and the maximum value of 4.53. Among the 30samples, no sample exceeded the pH 4.6.Statistical Results: The results of the z-test (one-sample ttest) of the data were obtained. The p-value is 0.000 (p 0.05).The null hypothesis was rejected at α 0.05. The power forthe null hypothesis is 1.000 at both α 0.05 and α 0.01. Thedata is normally distributed according to Skewness, Kurtosisand Omnibus Normality tests.Interpretation: The normally distributed data confirms that aparametric statistical test was appropriate. Since the p-valueis 0.000, the null hypothesis was rejected at α 0.05, and theresearcher concluded that there is a statistically significantdifference between the mean pH of the sushi rice samplescollected and the standard value, 4.6. The alternativehypothesis was therefore not rejected, indicating that the pHof the sushi rice samples is less than 4.6.Pilot StudyA pilot study was conducted to examine feasibility of anapproach and identify modifications needed in the design ofthe larger hypothesis testing study (Leon et al., 2011). 3 sushirice samples were randomly collected from 3 differentJapanese restaurants in Burnaby, and the pH of each samplewas measured by the Waterproof Palm pH Meter. The resultsof the pilot study were evaluated to confirm that the materials,equipment and experimental procedure are capable ofmeasuring pH of sushi rice samples.ResultsThe obtained pH data of 30 sushi rice samples underwent astatistical test to analyze statistical significance. The obtaineddata are numeric and continuous. Numeric continuous data isa measurement on a continuum, such as temperature and pH(Heacock & Sidhu, 2013a).Inferential StatisticsZ-test was performed to compare the pH of the samples topH 4.6, the maximum pH of sushi rice suggested by BCCentre of Disease Control (BCCDC, 2010). Z-test comparesthe mean of the population to a specific value (Heacock &Sidhu, 2013b). The hypotheses of this study are as follows:Null hypothesis (Ho): µ 4.6Alternative hypothesis (Ha): µ 4.6The null hypothesis predicts that the mean of the pH of sushirice in Burnaby is greater than 4.6. The alternativehypothesis predicts that the mean of the pH data is equal orless than 4.6.Probability, p 0.05 (or 5%), was used as a significancelevel to evaluate statistical significance. If p 0.05, theresearcher concludes that there is a significant differencebetween the mean of the data and the standard value, 4.6, andrejects the null hypothesis. If p 0.05, the researcherconcludes that the results are not statistically significant atthe 5% level, thus does not reject the null hypothesis.Microsoft Excel 2013 and NCSS 9 were used to conduct astatistical z-test. The obtained data was arranged in a tableusing MS Excel 2013 and then transferred to NCSS. Z-test isequivalent to ‘One-Sample T-Test’ in NCSS (Hintze, 2013).Instructions of running the z-test (or one-sample t-test) wereprovided in the NCSS manual (Hintze, 2013).Table 1. pH values of the sushi rice samples collected from 30different Japanese restaurants in BurnabySampleRestaurantspHTemp ( C)1Kilala Sushi3.8319.52Kokoro Sushi3.8221.23Sushi S3.821.84Yo Sushi3.7121.15Sushi Town3.822.96Osaka Sushi3.91237Hong Sushi3.9221.98Black Dragon4.09219Nao Sushi4.0420.910Fresh Box Sushi4.1621.811Narita Sushi3.8620.712Sushi Garden Metrotown4.2821.213Tang Tang Sushi4.2421.414Osaka Island4.0722.315Kamamarui4.4822.616Sushi Garden Brentwood4.242317Asakusa Sushi4.0922.618Yakko4.1922.819Akira4.3621.920Sushi &4.052221Gaya Sushi4.1522.222Kato Sushi4.1822.423Sushi Gen4.0222.124LA Sushi4.2321.925Sushi California4.532226Okoman Sushi4.1622.427Kita Sushi4.1922.728Sushi Oyama4.1422.829Little Toko's Sushi4.1122.530Sushi Kaku4.1222.65
Table 2. Descriptive Statistics of pH of the sushi rice samplesMean4.09Median4.115Mode3.8Standard 30Calculated by Microsoft Excel 2013 (MS Excel, 2013)DiscussionA common method carried out by EHOs/PHIs to ensure thesafety of sushi rice is to educate operators to discardtemperature-abused sushi rice after 2 hours since it has beenmade. Generally, potentially hazardous food is consideredsafe if it is consumed in 2 hours because it does not allowsufficient time for the pathogenic growth that causesfoodborne illness. If sushi rice has pH 4.6 or below, then it issafe to be stored at room temperature for up to 8 hours(University of Florida, 2004).As of December 2013, 56 Japanese restaurants areoperating in Burnaby. The 30 collected samples representabout 57% of the total restaurants. The average pH value ofthe samples was 4.09, ranged from 3.71 to 4.53. The fact thatall of the samples randomly collected for the experiment hadthe pH value less than 4.6 indicates that the samples weresufficiently acidified to inhibit the growth of pathogens,especially Bacillus cereus and Staphylococcus aureus, at thetemperature above 4 C. In other words, the sushi ricesamples were not considered potentially hazardous food, thussafe to be stored out of temperature control for extendedhours. The pH values are slightly lower than the values fromother previous research that identified some samples with theunacceptable pH values. The study conducted in Australia byNSW Food Authority showed that 15% of sushi rice sampleshad a pH value greater than 4.6, having an average pH of 5.3and a maximum level of 6.8 (NSW Food Authority, 2008). Asimilar study conducted in Seattle had an average pH valueof 4.3, ranged from 3.9 to 4.6 (Adams et al., 1994). Inanother study reported by a former BCIT student, Leung(2006) used a commercial sushi rice recipe to make sushi rice,and the pH was found to be 4.2, which is greater than theaverage pH value of the sushi rice samples collected inBurnaby. 23 out of 30 samples had a pH of less than 4.2.Therefore, the researcher is confident to conclude that sushirice being consumed by the public in Burnaby is generallysafe as all of the samples met the standard value of 4.6 or lessas suggested by BCCDC.However, the standard pH value of 4.6 to determinewhether a food is a potentially hazardous food or not doesnot completely eliminate the possibility of all pathogenicgrowth. The pathogens of concern with sushi rice, such as B.cererus and S. aureus, can grow in a wide range of pH. Theminimum pH that B. cereus can grow is 4.3 (Forsythe, 2010).This means that B. cereus may slowly grow in high-acidenvironment even if the pH is less than 4.6. Improper coolingof cooked rice prior to acidification provides the environmentfor the growth of B. cereus. 3 samples of sushi rice in thisstudy had the pH greater than 4.3. Although the pH of all ofthe three samples was less than 4.6, both B. cereus and S.aureus may still potentially grow. S. aureus may grow whenthe pH is 4.0 or higher (Forsythe, 2010). 22 samples out of30 had pH of 4.0 or greater. This indicates that the majorityof the samples may allow the growth of S. aureus at roomtemperature. Considerable bare hand contact when handlingsushi rice and poor hygiene of food handlers increase thechance of introducing S. aureus to sushi rice. However, therisk can be reduced by frequent and proper hand washing andproper food handling techniques.Food is preserved by various controlling techniques thatlimit microbial growth. Hurdle technology is a commonmethod to preserve food by using multiple techniquessimultaneously to increase the overall effectiveness (Leistner& Gorris, 1995). For example, a food product is acidified tolower the pH and then refrigerated to inhibit microbialgrowth. However, the only control measure for sushi rice isacidification. High water activity (Aw) of sushi rice andtemperature abuse provides an optimal environment formicrobial growth. This emphasizes the importance ofadequate acidification of sushi rice as there is no other hurdle.In this case, the pH value greater than 4.3 may not besufficient to limit the growth of B. cereus even if it is lessthan the standard value of 4.6.B. cereus is commonly associated with cooked rice. Itproduces spores that may not be destroyed by cooking. Thespores will germinate when the conditions are met, and thegerminated B. cereus will start to multiply (Labbe and Garcia,2001). Considering the notable foodborne illness history of B.cereus, the safe limit of pH of sushi rice should be 4.3 or less.This means that the three samples with the pH greater than4.3 need further acidification to lower the pH in order toensure the safety of the sushi rice.It is important to note that other ingredients of sushi affectthe overall pH of the finished sushi. Even with a low pH ofsushi rice, other ingredients may increase the total pH of thesushi products. For example, the ingredients of a Californiaroll, such as avocado, cucumber and imitation crab meat,have a pH greater than acid
was 0.82 with the minimum value of 3.71 and the maximum value of 4.53. 100% (30 out of 30 samples) had the pH less than 4.6. The statistical z-test resulted in a p-value of 0.00. Discussion: All of the sushi rice samples had pH values less than 4.6. Therefore, the samples were adequately acidified to inhibit the growth of pathogens.
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