Food Engineering/Manufacturing/Iced tea


QUESTION: Dear Mr Clark,

I am working on a couple of iced tea recipes. The first uses black tea, lemon and sugar. The second uses rooibos herbal tea, cinnamon, apple juice, sugar and ascorbic acid. Both drinks have a PH of below 4.

As I am working from a domestic kitchen, I can't practically heat the quantity of water needed so am cold-brewing the teas using water at room temperature. The black tea takes about 12 hours, and the rooibos 1 hour to brew fully.

I would like to get an ambient shelf life around the 3-6 month mark, and read online that this will require a pasteurization phase - recommended at 160F for 20 minutes. I would also like to stay away from any artificial preservatives.

I tried this and while it had no effect on the color of the products, which was great, the flavor of both recipes has unfortunately changed. There's a slight 'cooked' taste to them (perhaps not surprisingly!).

I was hoping you could advise me on another course I can take which might affect the flavor less. Is it possible to pasteurize at a lower temperature for a longer period of time, for example?

Thank you for your time, and any suggestions you may have. Merry Christmas!


ANSWER: Christopher,

The 160 F for 20 minutes sounds severe to me. Six minutes might be enough. A lot depends on the pH - the lower the less heat treatment is necessary, so that below 3.3, you might not need any at all. That is pretty acidic to most tastes. You should not be surprised that heating tea affects the flavor, but, as you know, that is the normal way to brew tea. If you are able to heat to 160 F and hold that long, you should be able to hot brew and avoid a separate pasteurization step - brew, mix, and fill hot. To answer your first question, yes longer time at lower temperature can have an equivalent effect, so you could hold at 140 F after filling for, say 12 minutes. That should have less effect on cold-brewed flavor than holding at higher temperature. Both treatments will sterilize bottles and caps, if you invert the bottles so the caps get contacted. Hot filling has the same effect and might be less severe, since you don't have to allow time for heat to penetrate through container and contents.

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Firstly, thank you very much for your detailed and informative reply. It's given me a lot to consider.

Strangely enough the cold brew method seems to make a nicer iced tea, so I will stick to the pasteurization method for now.

I just wanted to double check what you said about steralization in the pasteurization stage. Do the bottles need to be submerged entirely in the hot water for the duration of the pasteurization? Also, at what stage should they be inverted?

Thanks once again for your help, it's much appreciated.

ANSWER: Christopher,

Upon reflection, I realized that the time/temperature recommendation you cited was for treating the filled container and so may be correct. What I said still applies: longer time at lower temperature is equivalent. If you are heating by full immersion, you do not need to invert the bottles; that applies to hot filling, where the hot product sterilizes the bottle and cap. In your case, you are heating from the outside, which is why it takes a relatively long time. There is an understandable difference in flavor between cold-brewed and hot brewed tea. At higher brewing temperature, more bitter phenolics will be extracted. One reason people add cream and sugar is to counter these bitter flavors. Cold brewed tea will taste good with lemon juice and some sweetener. The price is a longer time in processing.

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Thank you for the correction. Am I right in thinking that these times/temperatures relate to the product inside the bottle, rather than the temperature of the water in the water bath? Otherwise I am guessing things like glass thickness and bottle volume would need to be taken into consideration.

Is there somewhere I can find a table/graph showing recommended times/temperatures per PH or is it just a case of trial and error, sending samples to be shelf life tested?

Thanks once agin for your help, this has been a life line for me.

There are tables of suggested times and temperatures as a function of pH, notably in an older book by Irving Pflug. I think they are a bit conservative. The times and temperatures do relate to the contents for exactly the reasons you cite. The key is that pH be below 4.6; then the foods are high acid or acidified and botulism cannot occur. I am away from my own references and so cannot provide specific suggestions, but I believe you can find them on line or at places in UK like Campden or Leatherhead.  

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J. Peter Clark


How various processed foods are made; ways to improve manufacturing; how to make a new food product.


Employment history: Research Engineer, U.S.Agricultural Research Service, Associate Professor Chemical Engineering, Virginia Tech, Director of Research, Continental Baking Company, President, Epstein Process Engineering, Inc., Vice Presdent Technology, Fluor Daniel, Inc., Consultant to the Process Industries

Organizations: American Institute of Chemical Engineers (Fellow) Institute of Food Technologists, American Association of Cereal Chemists, American Association of Candy Technologists, American Society of Agricultural Engineers,

Publications: Several Encyclopedias (Kirk and Othmer, Chemical Technology; Food Science, Food Technology and Nutrition; Wiley Encyclopedia of Food Science and Technology; Encyclopedia of Life Support Systems); five books, two book chapters; numerous journals.

Education: BSChE Notre Dame PhD University of California, Berkeley

Awards: AIChE Food, Pharmaceutical and Bioengineering Division Award 1998

Clients: Major food processing and pharamaceutical companies.

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