4 Aug 2008

Deciphering an old preserves recipe

Teaching food preservation methods combined with science often results in the classical why-questions; why does the recipe tell us to do things this way, and why is that so important (...and is it really that important?)

Autumn means ripe fruit, berries and vegetables, and many of us look forward to harvesting for the coming year. Only a few areas of the world have the benefit of continuous supply of fruit and vegetables (especially we up here in the north), and this has lead to loads of ways to preserve food. Nowadays, however, many of these methods are used for culinary purposes rather than survival. Recently, Martin wrote about cherry jam (I have given a few on his post as well). Also, Hervé This has written about culinary proverbs and old wives tales, "culinary precisions" as he terms them, some being sound advice from a scientific viewpoint, some being directly misleading, and others probably not making that a big difference.

So, I thought this might be a good time to take a recipe for sugar-preserved pears and have a closer look at it, step by step. It is in fact a quite fun exercise.

Traditional sugar-preserved pears «the old fashioned way» (generic recipe)
  1. Heat jar in oven or water by slowly increasing the temperature to above 100 °C and keeping it at that temperature for a certain amount of time (i.e. 15 minutes). Cool.
  2. Peel pears. Cut in half and remove seeds and core
  3. Make syrup from water and sugar by boiling the syrup until the sugar is dissolved. Leave to cool
  4. Fill a jar with the pears. Fill up with syrup, cold or lukewarm, but not hot
  5. Put the lid loosely on, do not tighten
  6. Place jar in a pot with cold water almost up to the rim of the jar. Heat slowly and keep at boiling point for a certain amount of time (i.e. 8 minutes)
  7. Remove jar from pot and
    Either: leave to cool for a certain amount of time (i.e. 15 minutes). Then tighten the lid
    Or: Tighten the lid while boiling hot
  8. Turn jar upside down and
    Either: store upside down
    Or: leave upside down for a certain amount of time (i.e. 15 minutes)
Taking a closer look
Firstly, the main reason for all these operations is to keep the pears edible for a long time, more specific until the next time ripe pears are at hand (a year, usually...). The two main ways that fresh fruit is spoilt are chemical (enzymes and reaction with oxygen from the air) and biological (microorganisms: bacteria, moulds and yeasts). Even though the methods were developed before the discovery of microorganisms and enzymes, the results of these were evidently clear. The source of enzymes is the fruit itself, whereas microorganisms are ubiquitous: the fruit itself, hands, tools, jar and in the air. So a perfectly sterile fruit would over some time be contaminated by just sitting in the air.
Two important facts: microorganisms thrive and multiply at temperatures between 10 and 40 °C, and die at high temperatures. The trick is thus to avoid the 10-40 °C window. Also, microorganisms need water to thrive and multiply.

So, how much of this procedure makes sense from a scientific point of view? I've marked the steps being most «fishy» with a red asterisk:
  1. Heat jar and lid in oven or water by slowly increasing the temperature to above 100 °C and keeping it at that temperature for a certain amount of time (i.e. 15 minutes)
    Makes sense, but cooling leaves the jar and lid ready for infection. However, they're dry and not prone to being infested

  2. Peel pears. Cut in half and remove seeds and core
    No effect other than possibly infecting the pears from hands, tools and surrounding air

  3. Make syrup from water and sugar by boiling the syrup until the sugar is dissolved. Leave to cool
    Sugar has a preserving function due to its dehydrating effect on microorganisms (a later post will deal with this). Sugar won't kill microorganisms, but inhibit growth. If the syrup is thoroughly boiled, microorganisms in the water and sugar are killed. Cooling the syrup is not recommended if it can be avoided (10-40 °C window)

  4. Fill a cold jar with the pears. Fill up with cold or lukewarm, but not hot, syrup
    Warning: we are in the 10-40 °C window. From a microbiological view, the best would be to add hot/boiling syrup. However, this might damage the fruit, such as turning the surface mushy (in the case of plums, the skin would most likely break).

  5. Put the lid loosely on, do not tighten
    Makes sense. During heating, the contents expand and trapped and dissolved air is expelled (gas solubility is lower at higher temperature). The air above the fruit expands when heated and needs to go somewhere

  6. * Place jar in a pot with cold water almost up to the rim of the jar. Heat slowly and keep at boiling point for a certain amount of time (i.e. 8 minutes)
    We are in the 10-40 °C window for quite some time during slow heating. The historic reason is most likely that old glass types had tensions/stress that would result in the jar cracking from shock heating or cooling. Modern glass production methods solve this by i.e. annealing. My experiences with ordinary jam jars is that they manage shock heating and cooling quite well. If the fruit survives, rapid heating is preferable.

  7. * Remove jar from pot and either leave to cool for a certain amount of time (i.e. 15 minutes). Then tighten the lid, or tighten the lid while boiling hot Leaving the jar open at this point is certainly not a good idea. Cooling results in the air above the fruit contracting, sucking in air from the surroundings (see note). Even though microorganisms might die when entering, thermally stable spores might enter which might develop at a later point. Tightening the lid right away is by far preferable. If you use modern jars with aluminium lids that pop down due to reduced pressure inside the jar, this should happen when the jar cools (if you buy a jar of jam and the lid doesn't pop when you open it, return it and get another). If the lid pops up before or during storage, consume immediately («oh, what a disappointment» ;)).

  8. Turn jar upside down and either store upside down or leave upside down for a certain amount of time (i.e. 15 minutes)
    Makes sense. Getting the lid sterile is always a difficult task. Turning the jar lets the hot contents come in contact with the lid, sterilising it getting rid of a majority of the microorganisms present. One might speculate whether keeping the lid seals moist might also be a reason (why wine is stored lying), but a biologist friend of mine meant that the atmosphere above the fruit would result in a moist enough atmosphere for that purpose, and that the sterilising effect of killing microorganisms is the point here. If that is true, it should not make a difference which way the jars are stored.

Remember that operations conducted in a kitchen are far from sterile procedures. For this reason, many such recipes rightfully ask for two and sometimes three actions with apparently the same purpose.

Finally, while hard cheeses with unwanted mould often are safe to eat if the mould is removed by cutting away a layer of the cheese, stored preserves such as jams, sugar preserved fruit and syrups should be discarded if the seals are broken or visible mould is seen. In the cheese, the microorganisms cannot travel due to the dry and solid structure, whereas diffusion is jams etc. occurs rather easily and the whole jar might very well be contaminated even though the visible mould is removed.

What might be learned/taught
Loads of microbiology. However, since I'm primarily a chemist, I won't venture too far into this. To me, taken that microorganisms are everywhere and that they die when they are heated, the most important things to teach would be (again, from the top of my head):
  • temperature-volume relationships of gases and gas-liquid relationships (popping lids, see note below)
  • critical thinking, don't always believe what you read
  • working systematically, always questioning why should I do this? (see Five cardinal rules in cooking)
In fact, this experiment/recipe might be among the few cases where one can test and predict something rather complicated based on a very limited amount of knowledge, but are often stated as among the most important treats in enquiry based science teaching (quite a paradox, really). Such situations are scarce, and I'm thrilled every time I stumble upon one.

Erik


Note: Heat expanding air is easily illustrated by putting a blown-up balloon into the freezer; it contracts. Take it out, and it expands back to (almost, at least) original size. However, when the jar contents are boiling, the headspace will in fact be filled by mostly steam from the preserves. When this water condenses (in the closed jar), the volume of the steam (now liquid water) is reduced by a factor of ca. 1300(!), resulting in a considerably lowered pressure.
video


Addendum (6. August)
some statements about sterilisation are modified as common canning does not result in sterile product (hence the need for preservatives such as sugar, salt, acid/vinegar etc.). This way of canning/preserving more resembles high pasteurization which is common in some milk products.

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