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Old 03-02-2008, 11:49 AM
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Are you supposed to leave onigiri (rice balls) out?

So, I think Japanese people just leave their onigiri out on the counter, right? Because I'm not sure if I should refrigerate mine or not. But I've read somewhere there's a kind of food poisoning you get from old rice, and I just don't know - but I don't want them to dry out in the fridge. So what's the traditional way to do it?

Also, how long are they supposed to last? I did several, with umeboshi and some other stuff. Stuck olives in a few.
Old 03-02-2008, 01:50 PM
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Rice left out can cause food poisoning. Bacillus cereus spores can survive conventional cooking. Once cooked rice should be kept hot (>140f) or refrigerated (<45f).

Are you using Japonica Rice for your onigiri? I have read that it is less prone to hardening when chilled compared to other kinds of rice.
Old 03-02-2008, 02:25 PM
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Yes, I am, and yes, they didn't get as hard as I expected, but Himself would rather they be left out.
Old 03-02-2008, 02:32 PM
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Traditionally, onigiri is eaten with in a few hours of when it's made, so at home it's not refrigerated. I was told off for putting some made in the morning in the refrigerator before lunch, since the rice will get hard.

If you buy onigiri at the combini (convenience store), they are stored in a cooler (often a glass display case), but not a true refrigerator.
Old 03-03-2008, 12:59 AM
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This isn't onigiri and I don't have access to the entire article, but according to this white rice cakes are good for at least one day at room temperature. It does not seem to be too far of a stretch to say that the safe lifetime for plain steamed rice would be similar.

The act of cooking the rice kills almost all the active B. cereus and most but not all of the spores. Improper cooking and further contamination of the rice after cooking is a possible concern and maybe played a part in the case of food poisoning described in the link from my previous post.

As far as common usage goes, I think most people make rice fresh in the morning and put them in unrefrigerated lunches for consumption later in the day. Keeping your cooked rice in an airtight container is also probably a prudent thing to do.

Prepared and handled properly 6 hours unrefrigerated before eating seems like a safe bet to me. From a food service point of view it is probably better to err on the side of caution and assume that proper preparation and handling will not always happen.


Quote:
"Survival and Growth of Foodborne Pathogens during Cooking and Storage of Oriental-Style Rice Cakes" Journal of Food Protection, Volume 69, Number 12 (2006): 3037-3042.

Fresh cooked rice cakes for retail sale are typically held at room temperature because refrigeration dramatically reduces their quality. Room temperature, high water activity, and a pH of >4.6 provided an environment conducive to pathogen growth. To date, no studies have been published regarding survival and growth of foodborne pathogens in fresh cooked rice cakes. This study was undertaken to investigate the effect of steam cooking on foodborne pathogens and their subsequent growth in five varieties of rice cakes made from flours of regular rice, sweet rice, white rice, tapioca, and mung bean. Bacillus cereus spores were detected in white rice, tapioca, and mung bean samples. The rice cake flours were inoculated with non– spore-forming foodborne pathogens (Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, Listeria monocytogenes, and Staphylococcus aureus) or spore-forming bacteria (Bacillus cereus) and steam cooked (100°C) for 30 min. Steam cooking significantly reduced (>6 log CFU/g) non–spore-forming foodborne pathogens in all samples and inactivated spores of B. cereus by 1 to 2 log CFU/g. Although spores of B. cereus survived steam cooking and germinated during 3 days of storage at room temperature, populations in most rice cakes remained below 106 CFU/g, which is the threshold for producing toxin. Rice cakes made from mung bean flour supported growth and germination of B. cereus spores above that critical level. In mung bean rice cakes, enterotoxin production was detected by the second day, when B cereus cell populations reached about 6.9 log CFU/g. The toxin concentration increased with storage time. However, our results suggest that rapid growth of total mesophilic microorganisms by more than 7 to 8 log CFU/ml during the first day of storage produced off flavors and spoilage before B. cereus was able to grow enough to produce toxins. Therefore, steam-cooked rice cakes made from a variety of flours including mung bean flour are safe for sale for up to 1 day after storage at room temperature and are free of B. cereus toxins.
Old 03-03-2008, 02:19 AM
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Here's a graph from a Japanese site on which discusses this. The red line represents the danger level of b. cereus. Plain onigiri without anything additives reach the danger level at 9 hours. Those which have shiroko (milt) added are good for more than 18 hours.

These were tested at 30 deg C. I would expect that a chilled environment, such as Cerowyn describes, it would last longer. However, one needs to remember that the onigiri have been stored at this temperature for an unknown number of hours, so grabbing some onigiri at the konbini and taking them along as for long hikes isn't recommended.

Its OK if you make your own, and keeping them cool.
Old 03-03-2008, 05:01 PM
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Here in Hawaii, we eat a lot of rice. How long one can refrigerate rice depends on the quality of the rice. The super premium rice from Japan, which is usually marketed as sushi rice can withstand a couple of days in the fridge. We generally leave sushi, onigiri or rice in general in a cool place to prevent spoilage or hardening in the fridge. Plain rice can always be microwaved to restore it if it hardens in the fridge, although steaming gives a better texture. Since most onigiri is filled with something or wrapping in nori, steaming or microwaving is not recommended. In a pinch, I have microwaved something that was refrigerated, but then I left it to cool down.
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