Preservation of Sugar Feedstock for Bioethanol/Animal Feed

[mellotango]

Hi,

I've been doing some research on how to preserve sugar based feedstock such as sap, syrup, juice, etc to be used later for bioethanol production or animal feed. Sugar feedstock derived from sugar crops spoil very quickly due to presence of airborne yeast and bacteria. We have looked at pasteurizing but feel that using chemicals such as sodium benzoate (for animal feed) or formic acid (for bioethanol production) is the most convenient way. My question is upon successfully preserving the sugar contents, and when it is ready to be processed into bioethanol again, how are you supposed to re-activate the yeast bacteria? Since the sugar is already spiked with formic acid or sodium benzoate, and any new yeast added in won't be able to survive anyway. How do you reverse the process?

Thanks in advance.

 

[chemisttree]

concentrate the syrup first. Add the benzoate only at the rate needed to stop fungi... it will be very low. When you dilute, the benzoate is below the action level and things move right along. Be sure to acidify the syrup/benzoate to a pH below 4.5 since it works best at that level. Acidify with citric acid or phosphoric acid.

 

[mellotango]

Hi Chemisttree,

I'm still a little bit unclear as to the content of your useful reply, sorry for the confusion. So you mean to say that sodium benzoate won't work on its own to preserve the sap, but need to add citric acid to decrease that ph so that the sodium benzoate will be to get to work to stop the yeast fungi?

And to reverse the process, we need to increase the ph, so that sodium benzoate will stop working, and instead, yeast will be able to start working again to ferment the sap?

Also, what I have got hold of now is sodium metabisulphite, which is also a known preservative. Do I need to add citric acid to the sap syrup together with this preservative to get the ph down also?

 

[jim mcnamara]

Bacteria are more osmotically sensitive than are fungi - meaning bacteria do not "like" very concentrated sugar solutions, fungi can tolerate them. Fruit preserves are an example - like grape jelly. Bacteria do not grow well in concentrated sugar solutions, fungi can.

Chemistree is telling you to make "preserves" (like grape jelly), then add citric acid to the preserves to inhibit fungal growth. Fungi generally do not thrive in acid conditions. By doing both of these you get very limited microbial degradation. Which is why commercial grape jelly is resistant to microbes.

To "undo" it, which is the reverse of your original question, raise the pH and add water.
Sodium bicarbonate will raise the pH. Water dilutes sugar reducing the osmotic pressure. As the concentration of sugars goes down, bacteria take over, in part because you have reduced sodium benzoate concentration, -- that is not the main reason - reduced sugar concentration is the driving force.

 

[chemisttree]

Yes, Jim. Part of what you wrote was what I was suggesting. That part about the concentrate and acid alone being able to preserve the sap is a bit beyond what I was imagining. My suggestion to concentrate the sap was mostly to give the OP a straightforward method to eliminate the fungistatic property of sodium benzoate... dilution to a level below it's action level. The use of citric or phosphoric acid increases the effectiveness of the sodium benzoate by converting it to its active (fungistatically speaking) form as benzoic acid. Doing this allows the user to use it at much lower levels and making simple dilution an effective way to remove the fungistat's activity. The use of either phosphoric or citrate was to both lower pH and leave behind essential nutrients for the yeast after dilution and pH adjustment reestablished favorable growing conditions.

 

[chemisttree]

Also, what I have got hold of now is sodium metabisulphite, which is also a known preservative. Do I need to add citric acid to the sap syrup together with this preservative to get the ph down also?

Bisulfite is mot a great preservative against unwanted fermentation. Yeast can tolerate it somewhat. They actually produce it during fermentation.

 

[chemisttree]

Uhhhh, mot=not.

 

[mellotango]

Thx for the replies. I found this info...

"One chemical concern when handling DOW FILMTEC elements is the preservative used with wet tested elements. The preservative solution in wet elements is either 1% sodium metabisulfite in water or 1% sodium metabisulfite and 2.4% citric acid in water."

So I'm assuming this is the proportion more or less I should experiment with. Would using sodium metabisulphite not be effective as sodium benzoate? I already committed to buying a bag of sodium metabisulphite :-( Now I just have to get the citric acid and a ph meter. Should I add more?

I have seen some exporters of sap using sodium metabisulphite. In any case, I just need to make a few litres of unconcentrated sap and concentrated sap as samples for a potential buyer. And these samples need to fly overseas and last at least a few weeks or months in order for them to do some sap analysis.

 

[chemisttree]

At higher levels, metabisulfate will work as a preservative. Not too sure how to neutralize it though. 1% is a lot of bisulfite, by the way. It's a reducing agent so peroxide would remove it as would any strong oxidant. Don't know if O2 would work. Interesting to try it. Acid makes it's preservative effect stronger as does concentration as Jim has explained.

Storing a couple of months implies pasteurization... possibly refrigeration for shipment followed by freezer storage longer term. Certainly the case for the raw sap. The concentrate should be the safer option. Think of it as you might orange juice. Fresh squeezed juice spoils faster than the frozen concentrate.

If you are shipping overseas, you might find that concentrating solves many problems and saves a lot of money on shipping.