Note: This bento safety article is one of the first ones I posted here on Just Bento, back in November 2007. I’ve edited it and added some more information, especially since more and more new people are coming to the site. Even if you’re a veteran bento maker, it’s good to go over the basics occasionally!
A traditional portable bento box meal is meant to be eaten eaten at room temperature. It’s typically made in the morning, then held for a few hours until lunchtime, also at room temperature. Millions and millions of Japanese people eat bentos like this (as well as an increasing number of people all around the world). There are some basic, time-tested precautions to take to ensure that your bento box meals will be tasty and safe when you tuck into them.
Using leftover and pre-cooked food reduces the time needed to assemble your bento in the morning. But the longer food has been lying around, even in the refrigerator, it gets less fresh and edible. Re-heating cooked food helps to kill off any micro-organisms that may have started to grow.
The best way to re-heat things in my opinion is in a pan, rather than the microwave, because the surface is where the microorganisms are likely to have formed, and the high heat of a pan will kill those off immediately. This is particularly important with meat, fish and even vegetable proteins. Japanese bento books usually recommend this.
In some cases it can be okay to pack food direct from the refrigerator, such as pre-made salads, instant pickles, and so on. Do use common sense though; packing leftovers from the night before is usually okay, but leftovers from 3 days ago get iffy. When in the slightest doubt, heat it through and cool it down.
You’re probably not going to be carrying a steak tartare to work for lunch. But you might think about sushi. Don’t. Raw fish is not safe unless it’s eaten immediately. (I’ve seen a couple of French bento cookbooks that use raw-fish sushi. They made me shudder, thinking of the people who might get sick eating hours-old raw fish.) If you have leftover sushi from the night before, throw it out or cook the fish if you must. (Yes you might have been ok eating leftover sushi, but you were lucky.)
Sushi that uses cooked or preserved foods (egg, boiled shrimp, broiled eel (unagi), vegetables like kanpyou (dried gourd strips), pickles) is ok, though always use caution. Futomaki, the colorful big pinwheel sushi, usually only uses cooked items so is usually ok. There are also special sushi varieties that are meant to be long-keeping, such as saba zushi (marinated horse mackerel that is pressed onto a log of rice), but you rarely see these outside of Japan. Sushi rice itself keeps fairly well, due to the vinegar and salt that flavors it.
Meat that has been cooked on the surface (e.g. roast beef that’s pink on the inside) is usually ok, but when in doubt pack such foods with an ice pack to keep them cool. Certain meats such as pork and chicken always need to be cooked through when used for bentos (and for other purposes too mostly.)
Undercooked meat is not safe, and neither is undercooked or raw fish. But undercooked or moist protein of any kind should be avoided when the weather is hot. Plain tofu, for example, that hasn’t been cooked through thoroughly, can be unsafe, and should be reserved for the cooler months. Or, use an icepack or other cooling device for safety. (Most tofu recipes on Just Bento will be totally cooked through, so are safe. For simple stir-fries and so on, I prefer using fried tofu over plain tofu because of the lower initial moisture content.)
Conversely, - and this may seem like common sense, but needs to be said - don’t use tofu that is bad in the first place. I have tasted so much bad tofu at various restaurants, and even in people’s homes. If the tofu smells or tastes sour or rotten or ‘off’ in any way, it should be tossed. To keep tofu that’s been opened fresh, totally immerse in fresh water and seal completely. Change the water at least once a day.
Tamagoyaki , the slightly sweet Japanese omelette, is a bento staple. But to be really good it has to be slightly moist in the middle. So, be sure to use fresh eggs, and to cool it down completely before packing into the bento box. (I’ve been expermenting a bit with freezing sliced tamagoyaki recently; I’ll report back soon with the results.)
Home made mayonnaise  is delicious, but should be used with caution, in foods like tuna salad. Commercial mayo has all kinds of preservatives in it that we’d rather not think about, but do keep our tummies happy. (See freezing tuna salad .)
Remember the McDLT? The gimmick with that burger was the two-part styrofoam container, with one side for the lettuce and tomato, and the other for the burger and bun. (Here’s an old commercial for the McDLT  featuring Jason Alexander, who played George Costanza on Seinfeld.) There was logic behind this rather environmentally unfriendly packaging. Any foods that should be kept cool, such as raw fruits and vegetables, should be kept in a separate container from the cooked foods, even if the cooked foods have had a chance to cool down. This will keep veggies fresh and crisp.
Unless an ice pack is specifically called for, all the complete bento lunch ideas as well as the individual component recipes are meant to be safe to eat without refrigeration, if the safety precautions on this page are followed.
In hot weather, it’s safer to pack an ice pack with the cool food. One trick is to freeze a juice carton or a small plastic bottle filled with the beverage of your choice, and to freeze it. Pack that with your lunch box, and it acts as an ice pack and will have defrosted enough to drink by lunch time.
If you’re carrying a bento meal that’s meant to be consumed more than a few hours later — e.g., a bento packed in the morning meant to be eaten in the evening for a night class — use an ice pack to be on the safe side, or use the foods that keep food fresher longer listed below. (An umeboshi onigiri (rice ball) will even keep for a couple of days.)
Some people assume that using a thermal lunch container such as Zojirushi’s Mr. Bento  will guarantee food safety. Don’t fall into that trap! Do remember that a thermal lunch container maintains heat at a certain level for longer than a regular lunch or bento box. So, if you pack a lukewarm, moist lunch, your food will become a great cosy breeding ground for the nasties. Be sure to follow the manufacturer’s instructions for these thermal containers, and pack either piping hot or thoroughly cooled food in them.
Plain rice can actually go bad pretty fast, since it’s so moist. It can also become inedible quite fast if it goes a bit dry and hard, especially in the desert-like atmosphere of a typical refrigerator. This is why a rice cooker with a timer function  is a very useful thing to have.
If you do cook rice the day before, make sure it’s wrapped up completely before storing in the refrigerator. If you want to keep rice longer, it’s best to freeze it . You can carry the frozen rice as-is in a block (especially in hot weather), or heat it through completely in the microwave or by steaming, and cool down completely.
If you live in a very warm and humid climate, you may want to consider carrying rice in the form of sushi rice or as well-salted onigiri only in the summer - or with umeboshi (see below).
Traditionally, umeboshi (pickled plum)  has been used in bentos and as onigiri filling. The traditional hinomaru bento, a bento box filled with white rice sporting one, lone umeboshi in the middle, is a very good keeper. (Incidentally, the logo for Just Bento is inspired by the hinomaru bento.) Umeboshi may have antimicrobial qualities, as do (supposedly) shiso leaves.
Fresh, nontoxic green leaves used as dividers and wrappers are also supposed to have antibacterial qualities. These are not edible. The most popular one is fresh bamboo leaves. (Dried bamboo leaves are also used as as wrappers, but don’t have the same antimicrobial qualties as fresh.) Banana leaves, used in South-East Asian cuisine, may have similar properties.
Wasabi and ginger may also help to keep things fresher. Try using wasabi to flavor vegetables (example: broccoli with wasabi sauce ), or tucking some pickled ginger in the corner of your bento.
Salt is a time tested preserver, so the salt you put on the surface of onigiri  is not just for flavor - it’s to keep the rice fresher longer.
Sugar can also act as a preservative (think of jams and preserves), though you need to use it in some quantity.
Vinegar is also a preserver. Salty or vinegary foods keep longer than foods with little seasoning. Sushi rice keeps better than plain rice because of the vinegar, salt and sugar.
Some bento boxes, such as this one on the J-List/JBox site , have an anti-bacterial silver ion coating, which is supposed to keep your food safer for longer. You can also find antibacterial bento sheets designed to be placed on top of the food. However, using these products doesn’t mean that you shouldn’t follow the safety precautions listed on this page! Personally, I don’t use these products (though I do have the bento box linked to, just because it’s a big black one that’s perfect for The Guy’s bentos) because I have a feeling they just give a false sense of security.
A no brainer perhaps. But don’t try to re-use an unwashed bento box or Mr. Bento dish! Wash and dry everything thoroughly before you use them again. The same goes for any eating utensils, such as chopsticks, forks and spoons, and picks, and all the cooking implements you use to prepare your bento. As pointed out in the comments, don’t forget to wash the gaskets and edges of the lids and so forth too.
Did you know that you can contaminate food by tasting it? Example: if you dip a spoon into a pot of soup, taste from that spoon, then put the spoon back in to the pot, you’ve added whatever germs are in your mouth to the soup. This may not be a big deal if you’re eating the food right away, but if you’re going to be carrying it around for a few hours you want to be extra cautious. If you do need to taste while cooking, don’t use the utensil you put into your mouth back into the food.
Don’t forget about your hands either! Don’t use your fingers to pick up food and put it in your mouth while you’re making onigiri for example. And of course, you should wash your hands thoroughly before handling any food.
If you are a budding cute bento or charaben artist, be careful of over-handling food that you are bringing for bento, especially in hot weather. Try to avoid using your hands as much as possible. If you need to practice your decorative bento skills, you may want to consider eating the results immediately. There’s no rule that says rice shaped like Totoro has to be only be eaten for bento! Skilled charaben artists use chopsticks, toothpicks and tweezers to avoid over-handling the food.
My aim is not to scare you off of making bento lunches by any means. But it’s better to take just a few commonsense precautions. Bentos should make you healthier, not sick!
I’ve noticed that several kyaraben/charaben (cute bento) oriented Japanese bento blogs are scaling back on the intricacy of their bentos recently. It gets very warm and humid in the summer throughout most of Japan, so food safety is a big issue. Complicated charaben require a lot of handling of the food, which should be avoided when the weather is warm.
I have already put together a comprehensive list of bento safety tips , but here are some top summer bento safety tips.