Keep food fresh by wrapping it

[Tomás Ó hÉilidhe-3]

This has always annoyed me because I don't understand it.

Let's say you open a pack of biscuits, but before you put it back into
the cupboard you wrap it in "cling film" to keep them fresh (I think you
Americans call it "saran wrap").

Now when you wrap something in cling film, you're doing either one of
two things:

1) Making it air-tight
2) Not making it air-tight (you're restricting the air flow into it)

If something were to be wrapped air-tight, then I could see how the air
within it would eventually become exhausted (I think bacteria take
oxygen from it or something like that), and so the decay process would
become retarded as soon all the oxygen's depleted. I could understand this.

However, when you normally wrap a packet of biscuits, you're not nearly
making it air-tight. I'm very surprised that wrapping it even has an
observable effect on the air flow into it (considering it's sitting in a
cupboard and there's no draft), so I'm even more surprised that it
actually has an effect on whether the biscuits will be soft when you go
to take them from the cupboard the next day.

Is there something I'm missing here? Does the cling film really have
such a huge effect on the airflow into the biscuits?

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[Marc Nicholas-2]

CO2 not O2 my friend!

-marc

On Wed, Aug 27, 2008 at 3:49 AM, Tomás Ó hÉilidhe <toe_list@...>wrote:

>
> This has always annoyed me because I don't understand it.
>
> Let's say you open a pack of biscuits, but before you put it back into
> the cupboard you wrap it in "cling film" to keep them fresh (I think you
> Americans call it "saran wrap").
>
> Now when you wrap something in cling film, you're doing either one of
> two things:
>
> 1) Making it air-tight
> 2) Not making it air-tight (you're restricting the air flow into it)
>
> If something were to be wrapped air-tight, then I could see how the air
> within it would eventually become exhausted (I think bacteria take
> oxygen from it or something like that), and so the decay process would
> become retarded as soon all the oxygen's depleted. I could understand this.
>
> However, when you normally wrap a packet of biscuits, you're not nearly
> making it air-tight. I'm very surprised that wrapping it even has an
> observable effect on the air flow into it (considering it's sitting in a
> cupboard and there's no draft), so I'm even more surprised that it
> actually has an effect on whether the biscuits will be soft when you go
> to take them from the cupboard the next day.
>
> Is there something I'm missing here? Does the cling film really have
> such a huge effect on the airflow into the biscuits?
>
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[M. Adam Davis-2]

The issue with "stale" products left out in the air is largely two fold:

1. Evaporation - the products are best enjoyed when their moisture
content is at a certain (designed) level.  When sealed properly the
packaging maintains a certain moisture level once the humidity of the
air in the package settles.  You can still get the same effect with a
sealed package by cooling it off so the water condenses on the inside
of the packaging, robbing the moisture fromt eh food (and then
spurring mold growth when the water makes contact with the food item)
This also results in "freezer burn".  Note that the reverse can happen
- crunchy foods absorb too much moisture, and become chewy or soft.

2. Odor absorbtion - the food picks up odors from the air surrounding
it, so the taste will change over time.  You may be used to the
particular scent of your house, but your cupboards and such have a
slightly different scent which will impose itself on any unsealed
food.

These two issues can be largely alleviated by partially sealing the
package - it's not necessary to completely seal it if you're going to
eat it within a week or two, but yes, over longer periods of time a
package that is not air tight will have the same problems as a package
left completely open does after a few hours/days.

There are other factors, such as the chemical reactions and breakdown
you mention (which affect some products, such as cut fruit, more than
others, such as bread with preservatives).  But these two seem to be
the overriding factors for most baked goods.

-Adam

On 8/27/08, Tomás Ó hÉilidhe <toe_list@...> wrote:

>
> This has always annoyed me because I don't understand it.
>
> Let's say you open a pack of biscuits, but before you put it back into
> the cupboard you wrap it in "cling film" to keep them fresh (I think you
> Americans call it "saran wrap").
>
> Now when you wrap something in cling film, you're doing either one of
> two things:
>
> 1) Making it air-tight
> 2) Not making it air-tight (you're restricting the air flow into it)
>
> If something were to be wrapped air-tight, then I could see how the air
> within it would eventually become exhausted (I think bacteria take
> oxygen from it or something like that), and so the decay process would
> become retarded as soon all the oxygen's depleted. I could understand this.
>
> However, when you normally wrap a packet of biscuits, you're not nearly
> making it air-tight. I'm very surprised that wrapping it even has an
> observable effect on the air flow into it (considering it's sitting in a
> cupboard and there's no draft), so I'm even more surprised that it
> actually has an effect on whether the biscuits will be soft when you go
> to take them from the cupboard the next day.
>
> Is there something I'm missing here? Does the cling film really have
> such a huge effect on the airflow into the biscuits?
>
> --
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> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>

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[Tomás Ó hÉilidhe-3]

Thanks for the explanation Adam.

What I'm surprised at though is that the cling film has such a
pronounced effect on air flow. If the food was sitting outside exposed
to the wind, I could see how cling film might have a
more-than-non-negligible effect on the air flow -- but when I think of a
pack of biscuits sitting in a cupboard, I just find it hard to fathom
that the cling film can really make a non-negligible difference to the
air flow seeing as how the there's very little air flow to begin with!

Ah... maybe I just have to accept that the cling film really does have a
massive effect on air flow.


M. Adam Davis wrote:
> These two issues can be largely alleviated by partially sealing the
> package - it's not necessary to completely seal it if you're going to
> eat it within a week or two, but yes, over longer periods of time a
> package that is not air tight will have the same problems as a package
> left completely open does after a few hours/days.

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[Peter Loron]

I think it's more that the scale is small...not much oxygen is needed  
to cause flavor to spoil, and not much moisture needs to leave the  
bread before it becomes a rock.

-Pete

On Aug 27, 2008, at 3:59 PM, Tomás Ó hÉilidhe wrote:

>
> Thanks for the explanation Adam.
>
> What I'm surprised at though is that the cling film has such a
> pronounced effect on air flow. If the food was sitting outside exposed
> to the wind, I could see how cling film might have a
> more-than-non-negligible effect on the air flow -- but when I think  
> of a
> pack of biscuits sitting in a cupboard, I just find it hard to fathom
> that the cling film can really make a non-negligible difference to the
> air flow seeing as how the there's very little air flow to begin with!
>
> Ah... maybe I just have to accept that the cling film really does  
> have a
> massive effect on air flow.
>
>
> M. Adam Davis wrote:
>> These two issues can be largely alleviated by partially sealing the
>> package - it's not necessary to completely seal it if you're going to
>> eat it within a week or two, but yes, over longer periods of time a
>> package that is not air tight will have the same problems as a  
>> package
>> left completely open does after a few hours/days.
>
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[Roger, in Bangkok]

Hi Tomás--

May I suggest that your focus on air flow is incorrect?

Look instead at air volume.  Isn't the pack of biscuits basically trying to
hydrate the surrounding air space?  How many ml2 airspace do you suppose
there is in total within the confines of the cling film?  Compare that with
how many l2 of air space there is within the cupboard.  Won't the airspace
within the cling film saturate with moisture quickly ... will the relatively
huge air space within the cupboard ever saturate from such a small quantity
of biscuits?

Regards/Roger, in Bangkok

On Thu, Aug 28, 2008 at 5:59 AM, Tomás Ó hÉilidhe <toe_list@...>wrote:

>
> Thanks for the explanation Adam.
>
> What I'm surprised at though is that the cling film has such a
> pronounced effect on air flow. If the food was sitting outside exposed
> to the wind, I could see how cling film might have a
> more-than-non-negligible effect on the air flow -- but when I think of a
> pack of biscuits sitting in a cupboard, I just find it hard to fathom
> that the cling film can really make a non-negligible difference to the
> air flow seeing as how the there's very little air flow to begin with!
>
> Ah... maybe I just have to accept that the cling film really does have a
> massive effect on air flow.
>
>
> ...
>

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[Apptech]

> May I suggest that your focus on air flow is incorrect?

Look instead at air volume.  Isn't the pack of biscuits
basically trying to
hydrate the surrounding air space?  How many ml2 airspace do
you suppose
there is in total within the confines of the cling film?
Compare that with
how many l2 of air space there is within the cupboard.  ...

"Brownian motion"  [Gargoyle knows] aka diffusion will allow
the transportation of molecules at substantial velocity
through air etc in surprisingly short times. A dissimilar
gas mix will attempt to establish an equilibrium based on
the partial pressures of the gases involved.

Useful / interesting experiment: Acquaint yourself (if not
already acquainted with the smell of "Methylated Spirits"
that has been denatures with Pyridine. Partially fill a
container with said spirits and cap securely with a quick
release cap. (Cork good - screw cap OK). Have an assistant
(optionally beautiful) stand at the far side of a largish
[draft | draught]less room with minimal odo[u]rs present.
Have said assistant shake the capped bottle well to get the
internal sides wet and air inside well saturated then uncap
the spirits and note how long it is before you note the
characteristic smell. You are liable to be very very very
surprised at the result.


        Russell

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[Alan B. Pearce]

>I could see how cling film might have a more-than-non-negligible
>effect on the air flow -- but when I think of a
>pack of biscuits sitting in a cupboard,

It is not the air flow, the cling film is keeping the moisture out that
makes the biscuits go soft.

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[Tomás Ó hÉilidhe-3]

Alan B. Pearce wrote:
>> I could see how cling film might have a more-than-non-negligible
>> effect on the air flow -- but when I think of a
>> pack of biscuits sitting in a cupboard,
>>    
>
> It is not the air flow, the cling film is keeping the moisture out that
> makes the biscuits go soft.


But isn't the moisture in the air? Isn't the moisture flowing through
the folds of the cling film just like the air is?

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[Dave Joyce]

G'day all,
I see it as being an impervious layer to keep the "baddies" out and to stop
the food from spoiling (eg the biscuits, or bikkies here in Australia), thus
keeping the food "fresher" for longer.

----- Original Message -----
From: "Alan B. Pearce" <A.B.Pearce@...>
To: "Microcontroller discussion list - Public." <piclist@...>
Sent: Thursday, August 28, 2008 5:25 PM
Subject: Re: [OT] Keep food fresh by wrapping it

> >I could see how cling film might have a more-than-non-negligible
>>effect on the air flow -- but when I think of a
>>pack of biscuits sitting in a cupboard,
>
> It is not the air flow, the cling film is keeping the moisture out that
> makes the biscuits go soft.
>
> --
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[Apptech]

>>> I could see how cling film might have a
>>> more-than-non-negligible
>>> effect on the air flow -- but when I think of a
>>> pack of biscuits sitting in a cupboard,

>> It is not the air flow, the cling film is keeping the
>> moisture out that
>> makes the biscuits go soft.

> But isn't the moisture in the air? Isn't the moisture
> flowing through
> the folds of the cling film just like the air is?

When I use or see cling film for food it is applied in a way
to aim at air tightness - ie few or no holes of any great
size. As a barrier to diffusion of water vapour it should be
much more effective than air alone.


            Russell


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[M. Adam Davis-2]

On 8/28/08, Tomás Ó hÉilidhe <toe_list@...> wrote:
> But isn't the moisture in the air? Isn't the moisture flowing through
> the folds of the cling film just like the air is?

Given that there is no pressure difference between the wrapped product
and the exterior, then there is very little air movement.

So the only way to carry the moisture back and forth is air movement
cause by pressure changes, and regular brownian motion.

The cross section of the little gaps presented in folds of seran wrap
should be very small (ie, the better you wrap it, the smaller this
cross section) so the brownian motion would have very little space to
move moisture in our out of the package.

-Adam

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