真空低溫料理（Sous Vide）一定要真空么？

如果只是放在塑料袋裡之後放到水浴里行不行，如果用錫紙包裹後放到恆溫的暖箱或者烤箱里行不行，真空在料理的過程中起到了怎樣的作用？是必須要的么？

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題主的理解有誤，Sous Vide 低溫烹飪雖然把食物裝進袋子里，偶爾抽走部分空氣（不是必須），但絕不是「真空」，而是較低氣壓而已。「真空」是錯誤的理解。

食物經過Sous Vide料理後有兩種結果：

1.馬上被吃掉,這種情況是否抽走部分空氣，影響並不是很大。你自己在家拿個溫度計用簡單的塑料袋也能做Sous Vide，只是沒那麼好控制而已。

2.儲存起來，日後再使用，又叫Cook-Chill。這種方式對於需要事先準備較大批量原材料的專業廚房裡也蠻長見。為了避免儲存過程中食物變質，通常會抽走空氣。在不同溫度下，能保存1周到6個月。

另外，Sous Vide所用的袋子有很多不同的型號和質地，適用於不同型號的封裝機器（僅僅密封不抽出空氣，有些抽出部分空氣，有些能抽出大部分空氣，當然價格貴很多）同時也影響最終出品食物的質感。

用非常強力的封裝機器加上緊貼的袋子煮出來的肉類會更干一些，原因在於強力的封裝機瞬間不僅把大部分空氣，甚至部分食物里的水分汽化拉出來，造成細胞膜破裂，汁液流失。這種結果有兩種方式避免：1.在食物溫度很低是封裝。2.沒必要的情況下，不要把袋子里的氣壓降得過低。

對於植物來說，真空包裝能產生擠壓效果Vacuum compression，由於細胞結構的不同，植物細胞里有儲存水和部分空氣的Vacuoles（空泡）。在包裝中會被壓破，最終導致植物看起來萎縮了許多，這也是許多廚師喜歡用的技巧，但動物細胞不會。

用來做Sous Vide料理的設備，不僅僅是水浴，較常用的有如下幾種，各有優劣：

1.非循環水浴

便宜，容易清洗，但溫度控制較差，有冷、熱點。

2.循環水浴

較貴，精確溫度控制，大部分專業廚房用這個。功率通常不超過1800W，因此容量往往不超過30L

3.水蒸氣烤爐

較貴，容量比水浴大得多，但料理小份食材時不如水浴有效率。在高溫區表現較好，濕度可控。

4.Combi Oven像Rational之類的

非常貴！（8萬以上），容量非常大，可精確控制溫度、濕度和實物的核心溫度，也能達到數百度高溫，但是在60°C以下溫控穩定性比水蒸氣烤爐和水浴差。

從穩定性來看，普通烤箱實在不是一個很好的Sous Vide工具。

比起傳統的烹飪方式，Sous Vide 有如下四大優勢：

1.Control控制

Sous Vide最適合用來料理海鮮和肉類，由於蛋白質對溫度的變化極其敏感，完美的質感往往僅在非常窄的溫度範圍內出現，稍不小心就煮過了或里外受熱不均（即使熄火後，食物的內部還會因為餘熱而繼續升溫）。Sous Vide 能對溫度進行精確控制，菜鳥不需要多年的經驗和練習也能做到完美，這是最大的優勢。

2.Consistency 穩定度

比起人為的操作，Sous Vide 具有高度的可複製性，每份出品的重量、大小、調味品等都能在裝袋的準備過程中精確量化，確保了餐廳出品的高度穩定。

3.Quality 品質提升

Sous Vide 料理的密封袋有幾大好處：1.袋中的氧氣非常有限，避免了烹飪過程氧化造成食物變色（如牛肉氧化變灰色，蘋果等水果變褐色）或味道改變。2.由於處於密封，食物內外都處於高濕度狀態，出品往往更軟嫩多汁。

4.Safety 食品安全

由於外面有層膜保護，在食物儲存過程中能更好地避免交叉感染。

Sous Vide之後偶爾也會表面用火槍噴、炙烤或丟沸水裡快速泡一下（Blanching），主要目的有二：

1.殺菌，避免食物表面在處理過程中可能發生的交叉感染。

2.發生梅納反應，增加口感和風味的變化，讓表面更香。

From Modernist Cuisine

Sous Vide烹飪全教程

http://www.chefsteps.com/classes/sous-vide-cooking#/what-is-sous-vide

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不一定需要抽真空。

抽真空的目的有兩個

空氣的傳熱非常慢，塑料膜和食材之間如果有氣泡，這部分熟的就比其他部分慢

如果水浴的溫度比較高，需要防止一些氧化（比如脂肪）

但這兩條都可以用水壓來完成，就是把塑料袋包住食材，然後浸到水裡。小心把氣泡都弄出來。

塑料袋裡有一些油或者水的時候會更容易，少許的液體會把氣泡壓到上面，等壓出封口的時候封住就好了。

這個辦法，因為裡面的殘留空氣不是低壓抽出去的，所以分壓仍然差不多是大氣壓。但因為殘留量足夠小，所以其實完全滿足上面的兩個要求。

烤箱其實也可以做低溫料理，只是溫度控制做不到水浴那麼精確。因為空氣的熱容太小，即便是Viking這樣貴到離譜的烤箱也不行。所以雞蛋是不要指望用烤箱來精確控溫了，但拿來烤個雞啊，牛排啊什麼的都可以。

Heston就拿烤箱烤過24小時的雞（60度）和牛排（50度）。但我感覺牛排還好，表面噴槍燒過比較容易無菌。烤雞光開水燙一下恐怕不安全，禽類的細菌可能在肉里。

補充一下Nathan Myhrvold在哈佛課上關於sous vide的那節。

NATHAN MYHRVOLD: So to understand sous-vide, we should first talk about

a conventional way to cook.

Let"s say you"re going to cook a steak in a pan.

In that case, the pan is probably around 300 to 400 degrees Fahrenheit.

And that 300 to 400 degrees Fahrenheit is way hotter than you want

the steak to be.

Suppose you want the steak to be medium rare.

Medium rare for a steak is going to be about 125 to 130 degrees.

So you have a 300 or 400 degree heat source, and you"re trying to cook the

center of the steak to be say, 130 degrees.

Well, that means that in the steak there"s going to be a thermal gradient

in temperature, where it"s 300 or 400 where the steak is contacting the pan,

and then the temperature will fall and you"re trying to get the

center to be 130.

Now you can do it, but that requires very precise timing.

If you"re off by even a few seconds, you"ll overshoot your 130 or you"ll

undershoot it and you"ll have food that isn"t properly done.

The temperature gradient means that you"ll have a thick band of gray meat

where the steak touched the pan.

That meat will be overcooked.

It won"t be your desired medium rare.

It will be well done.

In fact, in a typical steak that you cook in a pan, something like 25% to

35% of the steak will be overcooked.

It will be above the temperature that you"re asking for.

Well, what"s the alternative?

The alternative with sous-vide cooking is to take a steak and cook it in

water that is 131 degrees, 132 degrees.

So only a couple degrees above the temperature that you"ve desired.

Now that means that there"s a very low temperature gradient.

It"s effectively the same doneness from one edge of the steak to the

other edge.

Now there"s a catch.

And the catch is it takes a long time for the heat to soak through

the steak that way.

So whereas you might cook a steak for five minutes on each side in a frying

pan, when you put it sous-vide, it might take 25 to 45 minutes for the

heat to slowly leak all the way through.

Now the other problem is that we"re used to having the outsides of our

steaks seared with the wonderful brown colors and flavors that come from the

Maillard reaction.

So typically when you do sous-vide cooking, you"re using one method--

sous-vide--

to cook the interior.

You"re using a very different method to finish the outside.

I like to finish the outside of a steak with a blowtorch, but you can

also use a super hot pan or several other techniques.

So the key idea is cooking with precision.

Now precision gives you a tremendous set of benefits.

Salmon is a great example.

In cooking salmon, you can go from cooking at maybe 110 degrees--

that"s actually the temperature of a pretty warm hot tub--

up to much higher.

Every temperature that you cook it at will have a different texture, a

different color, different experience for people to have.

If you cook it at 110 degrees, typically the salmon

will not change color.

It"ll look dead raw.

Even though it will have a texture that"s cooked, it"ll look raw.

If you move it up to maybe 113 degrees, 115 degrees, something in

that range, it"ll now look cooked.

To my mind, it"s perfectly done at that stage.

Now if you want outside of your salmon and the skin to be crispy, then of

course, you can use this other cooking technique to do that.

One of the great misconceptions in sous-vide is that it"s

about vacuum packing.

Another great misconception is that vacuum packing is dangerous because

it"s in plastic.

So the very first people who experimented with sous-vide were in a

Swedish hospital system.

They wanted to cook in a central kitchen and ship the cooked entrees

out to a set of six or seven hospitals.

It was a great idea.

And to make that work, they used a new technology of vacuum packing, which

helped seal the food in, keep oxygen off, keep it sterile, and keep it

fresh longer.

Vacuum packing has all those properties and it"s very convenient.

So they started off this idea, hey, let"s vacuum

pack as part of sous-vide.

Well today, chefs still vacuum pack, but you don"t have to.

Actually, any sous-vide technique will work if you seal the food in a canning

jar, Mason jar, or a Kerr, Ball, any of those canning jars

will work just fine.

It takes a little bit longer for the heat to soak through the thicker glass

than it does the thin plastic, but they certainly work.

You don"t even need to have a cover.

If you have a low temperature steam oven, such as CVap, or a combi oven,

in those you can cook directly at the low temperature.

So the food can be uncovered.

It doesn"t need to be sealed at all.

Vacuum sealing is still part of sous-vide for many chefs because it"s

a very convenient thing to do for other reasons.

It"s a great way to store food.

It doesn"t degrade.

It doesn"t oxidize.

Probably the single biggest thing that makes food go bad is oxidation.

Now food will still go bad if it"s vacuum packed.

So you have to refrigerate it.

And you have to follow food safety procedures to make sure that when it"s

hot, it"s hot and it"s cold, it"s cold and it doesn"t spend much time in the

tepid danger zone in the middle.

But vacuum packing for sous-vide is really only a convenience.

A nice convenience, but it"s logically a separate thing.

But then that brings up another question.

People say but, what about cooking in plastic?

Isn"t there some terrible thing that"s going to happen, that weird chemicals

will leach out of that plastic and go into your food?

Well in this case, there"s a lot of evidence that that"s not the case.

For more than 20 years, people have made laboratory equipment using a

plastic called polyethylene.

It"s common in almost any biology or chemistry lab.

There"s tons of polyethylene containers.

Polyethylene is very well characterized in terms of what leaches

out of it, what doesn"t.

If it didn"t, you couldn"t use it in these critical laboratory

applications.

Now it happens, polyethylene is also the plastic used in Ziploc bags and

Saran wrap and in sous-vide bags.

So I don"t think that there is any rational fear that the polyethylene

that"s used in sous-vide bags has any health consequence.

There are some plastics that do have an issue.

PVC based plastics can have some chemicals leach out of them.

It"s very well known.

Yet they still are sold in some places for food wraps.

Somewhere on the thing it usually says don"t use with heat or something else.

But if you go to a big warehouse store and you buy a big thing of clear

cooking wrap, look carefully.

That"s almost certainly PVC.

So if you do care about plastic leaching into your food, avoid PVC.

But polyethylene has really been very well characterized.

The process of sous-vide is very different than traditional processes.

Instead of cooking with this high heat source very dramatically for a short

period of time where timing is everything, you cook for a long period

of time at low temperature.

The timing issue goes away.

You have to allow enough time so that the heat will

transfer through the food.

But if you"re 10 minutes early or you"re 10 minutes late, it really

makes no difference.

In fact, for most sous-vide processes, being hours late

doesn"t matter at all.

So it"s the ultimate convenience way to cook.

It"s also the best way to cook so you get repeatable results every time.

Instead of having to judge is that steak done, is that steak done, the

steaks are all done to exactly the same temperature every time.

Why?

Because the water bath we use has a digital thermostat in it.

And that digital thermostat will regulate the temperature to within one

degree typically, which means, for all intents and purposes, it"s perfect.

Now there"s some people who look at this with horror and

they say, oh my god.

Doesn"t sous-vide take the soul out of cooking?

And I say, well you know, for me--

I can only speak for myself here--

I don"t think there"s a lot of soul in being a thermostat.

So yeah, I"m using a digital thermostat to regulate the temperature

instead of me watching the steak and poking it.

And as a result, it"ll be perfect.

Whereas if it was on the stove, sometimes I"d overcook it, sometimes

I"d undercook it.

And I couldn"t do other things in the kitchen.

To me, there"s no soul in overcooking something.

There"s no great benefit in saying, oh yeah, I served it more done than Frank

would like and way too rare for Roxanne.

I mean, that kind of mistake, that isn"t about soul.

And that isn"t about creativity.

Meanwhile, the rest of cooking is full of creativity and opportunities to

have unique combinations of flavors and textures create something that"s

just amazing.

So I completely reject the idea that sous-vide takes

the soul out of cooking.

It"s the kind of Luddite Stockholm syndrome sort of thing that some

people get attached to.

But I don"t get it.

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工業級真空泵銷售狗路過。食品級真空設備，一般都是無油乾式真空泵。由於食品袋是錫紙之類的包裝負壓不會太高，接近工業級的絕對真空不可能發生，因為紙袋的強度是達不到的，而且裡面的吃食。。。。真不敢想像口味能夠再差成什麼樣。

所以我一直覺得，這樣的食品，撐死只能說是被抽干。和家裡那吸塵器吸真空袋包著的羽絨被差不多意思。

如果你要放烤箱里加熱，盡量要把外包裝拿掉啊

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一定要真空，不過這裡的真空不是工業上規定的什麼什麼嚴格的真空。低溫慢煮的時候，食物周圍不被一層空氣包裹就好，至於這個怎麼界定，肉眼看不出來就好了。

真空的主要目的是方便傳熱。如果空氣包裹住了食物則相當與給食物穿了一層防熱服，由此導致傳熱速率會變慢。如果是低溫慢煮時間非常長還好，如果時間較短，則會出現食物沒有煮熟的情況。我自己操作的時候就因為空氣沒有抽好而失敗過，當時煮的是牛排，我那個口袋有問題，到時間拿出來，生的。

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