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I've created my own sauce, bagged it and left in the freezer for many days. It is still not frozen stiff (details to follow). I'm struggling to know how to phrase this question but, I'm concerned that since it's not frozen stiff that it's life is therefore reduced.

My freezer is to -18°C.

The ingredients are 300ml water, 100ml white wine vinegar, 2tsp tomato sauce, 2tsp plum sauce, 120g sugar and 2tsp corn flour.

I've Googled freezing white wine vinegar and corn flour to see if these prevent freezing (due to the alcohol in the vinegar and it was just a guess to search for corn flour) but my searches provided no answers.

So, my 2 questions are why does this mixture not freeze and does it affect the life it can remain in the freezer because it's not frozen or is the fact it's still in the very cold environment (-18°C) the thing which means it's safe for a month or two?

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Have you kept sauce in freezer to preserve it for long time or get it frozen like ice? –  Sweet72 Sep 14 '13 at 13:26
    
Just to preserve it –  Dave Sep 14 '13 at 13:56
    
Is that freezer temperature what the freezer claims, or did you measure it with a thermometer? Sugar lowers freezing points, but even sorbet will freeze solid at that temperature. I'd be very surprised if your tomato sauce doesn't actually freeze by -18C. –  Jefromi Sep 14 '13 at 14:57
    
I updated my question with the quantities @Jefromi. Yes, it is what it claims, I will need to buy a thermometer and test it, however, I have many things in my freezer (sauces, foods etc) which freeze solid. It is only this one. –  Dave Sep 16 '13 at 7:10
    
Well with those quantities, yes, it's the sugar keeping it from freezing. But... that doesn't look like tomato sauce, it looks like tomato flavored sugar water. Did you really mean tsp? –  Jefromi Sep 16 '13 at 13:33

2 Answers 2

up vote 8 down vote accepted

Most sauces do freeze solid, but it is the temperature, not the solidity, that matters for food safety.

The hardiest bacteria, temperature-wise, are psychrophiles and can grow in temperatures as low as -15° C. Since they can't grow in temperatures above 10° C, they are very rarely found in food, but if your freezer is actually at -18° C (and you've verified this with a thermometer) then the food is quite safe.

Practically speaking, most of the common foodborne bacteria are mesophiles (Salmonella, E.coli, etc.) and don't grow well below 20° C. It's impossible for any of these to grow at even a "warm" freezer temperature.

Bottom line: Check your freezer temperature (and, if you're really paranoid, the food) with a thermometer to make sure it's working properly, but otherwise, don't worry about it.

P.S. Vinegar lowers the freezing point slightly, but more importantly, sugar causes the other liquids to freeze into much smaller crystals, which may give the appearance of not really being frozen (think of "soft" ice cream). Plum sauce also tends to contain various gums such as xanthan gum as thickeners, which are also used to keep commercial ice cream soft.

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Yes. On average, chemical reactions slow down by a factor of two for each 10°C decrease in temperature. That includes the reactions that might give your sauce an off taste. Also, and partially because of the above factor, most bacteria will not thrive in subfreezing temps.

If I were making a lot of your type of sauce, I'd consider investing in a -70°C freezer, but what you have should be good for at least a few months of home use.

Oh yes, question 1: It's probably the sugar content that's preventing a complete freeze. Think popsicles, or icecream. Lots of things with high solute content are difficult to freeze completely due to freezing point depression.

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You need a very high amount of sugar for noticeable freezing point depression. Ordinarily we'd be talking about maybe 1-2° C unless you're making confectionery. It's not if it freezes but how it freezes that changes. Popsicles and ice cream are still frozen. –  Aaronut Sep 14 '13 at 14:02
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Also, why are you recommending a -70° C freezer for more than a few months of home use? Can you explain that? –  Aaronut Sep 14 '13 at 14:03
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Sugar, and other solutes concentrate during the freezing process, so you get regions of frozen, fairly pure water, and unfrozen regions with most of the salts and proteins etc in them. The result is something more bendy than is usually considered 'frozen'. Much as I'd like to have a -70° at home, that comment was directed more towards anyone thinking of commercializing the sauce. –  Wayfaring Stranger Sep 14 '13 at 15:36
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Do you have a source for this most recent reply? –  Aaronut Sep 14 '13 at 23:14
    
@Aaronut It's pretty standard food science, which I studied last millenia. Here's a couple refs covering some relevent aspects of freezing fodstuffs: Freeze Concentration: books.google.com/… –  Wayfaring Stranger Sep 14 '13 at 23:25

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