Episode 9 Predicting Hydrate Formation Boundaries

Episode 9 Predicting Hydrate Formation Boundaries

(See article links below)

Zachary Cartwright (00:00):
I'm Zachary Cartwright. This is Water in Food.

Mary Galloway (00:02):
If you have a change in solubility, then people aren't uptaking into their body what they're expecting.

Gaylon Campbell (00:11):
And that just stopped the change in water activity essentially. And so it's a beautiful experiment.

Mary Galloway (00:16):
And they see does the water activity change. And when it plateaued and that's what they found, then they could say, "Ah, that's the boundary that we're looking for."

Zach Campbell (00:24):
A little bit of extra knowledge can help upstream.

Zachary Cartwright (00:28):
Welcome to another episode of Water in Food. In this special episode, I'm joined by three guests and colleagues from METER Group. We have Application Specialist, Mary Galloway, Head of Instrumentation, Zach Campbell, and Founder of METER Group, Dr. Gaylon Campbell. Let's have each of you say hello and tell us a little bit about your work at METER, starting with you, Mary.

Mary Galloway (00:49):
Hello. I'm Mary Galloway, I'm an application scientist at METER. I've been here for 10 years. And I would say in a nutshell, what I do is help people understand how moisture is going to affect their products. And I also manage our R&D lab. So we work with instruments from the full development phase and also other instruments and do some fun experiments.

Zachary Cartwright (01:14):
And Mary is also famous for her webinars. And I'll go to you Zach. Tell us a little bit about your role at METER.

Mary Galloway (01:21):
Yeah. Happy to be here. So as Zachary mentioned, my role is Director of Instrumentation. So what that means is I have the privilege of managing our instrumentation portfolio on the food side and includes our water activity meters, new product development and then I get to do some application support sometimes as well.

Zachary Cartwright (01:39):
Dr. Gaylon Campbell.

Gaylon Campbell (01:42):
I also get to work with some of the design stuff. We do both environmental and food instrumentation, and I work in both areas and also some of the applications.

Zachary Cartwright (01:59):
Well, thank you all for being here today. Today, we will be briefly discussing two scientific papers and discussing their applications in both the food and pharma industries. This is a highly technical topic, definitely more than our usual episodes, but it is really critical and we are sure that some of our listeners will find this to be insightful.

Zachary Cartwright (02:19):
The first paper is called Identification of Phase Boundaries in Anhydrate/Hydrate Systems written by scientists at Pfizer and published in the Journal of Pharmaceutical Sciences in 2007. The second paper is called RH-Temperature Phase Diagrams of Hydrate Forming Deliquescent Crystalline Ingredients written by scientists at Purdue University and published in the Journal of Food Chemistry in 2017. Now, hopefully I haven't scared you away with these titles, but one thing I want you to notice in each title is the word hydrate. So my first question today to our guests is what is a hydrate and why is it important? And I'll turn over to you Zach, and you as well Mary.

Zach Campbell (03:02):
Sure. Yeah. So a hydrate is kind of a vague term to be totally honest. It can mean a lot of different things in a lot of different contexts. But today what we're talking about is a crystalline lattice forming hydrate. And the second paper there, RH-Temperature Phase Diagrams by Allan and Mauer, gives a very technical description. But an easy way to think about it is when a crystalline molecule uptakes water and that water is incorporated into the lattice structure, then that can be referred to as a crystalline hydrate in that case.

Mary Galloway (03:35):
Right. The crystal actually binds the water into it. So it becomes part of that molecule and that changes some of its physical properties. And this is caused by humidity and temperature. And that's what we're going to focus on, is finding what those boundaries are. When does that actually happen?

Zach Campbell (03:51):
Yeah. And the interaction between the two isn't necessarily what I would assume to be the case for some of these boundary conditions. So it's an interesting application from that perspective.

Zachary Cartwright (04:01):
And Mary, can you tell us why people in the food industry should be aware of hydrates?

Mary Galloway (04:06):
Yeah. There are quite a few ingredients, especially, that can fall into this category. Some that they list in this paper are glucose, lactose, which we actually hear quite a bit of grief about lactose, maltose, trehalose, citric acid, malic acid, sorbitol, and then thiamine chloride, which is like a form of B1 vitamin, which is not just B1, but there are other functional groups like that, that can have trouble with forming a hydrate. Some of the examples I can think of right now is if you form a hydrate, like you raise the temperature, a hydrate forms, you don't know it, you dehydrate it, you now are releasing that moisture that was incorporated in that crystal is now released into your formula or whatever. And now you've got this extra moisture that might be causing caking and clumping and issues like that.

Mary Galloway (04:57):
It also changes the solubility and the stability of an ingredient. So if we talk about functional foods in that regard, that's a big problem. And also vitamins, like I was mentioning before with a specific one that they mentioned, which is a B1 vitamin. Also, if you're having a formulation where you specifically are weighing out solids for something, let's say vitamins again, we use that, it just works for a lot of ways, and you think, okay, well I have this percent of a vitamin in my formula and you've formed a hydrate, well, water weighs, right? So you actually don't have as much of that functional ingredient or that vitamin let's say, as you thought you did because of the hydrate because of the water that's in there.

Mary Galloway (05:40):
And this can also affect the shelf life. If you've got these issues that are happening, if you hit over above these points that we're going to talk about and you're forming these hydrates, it changes the shelf life. So if you're able to know where that point is, keep the temperature and the RH down, it keeps it nice and stable and you can extend your shelf life. So there's a lot of key points with knowing about the hydrates and some of these ingredients.

Zachary Cartwright (06:05):
Thanks, Mary. And what about hydrates in the pharma industry? Why is this important, Dr. Campbell?

Gaylon Campbell (06:10):
A lot of the same things apply there that Mary talked about. For the API, the active ingredient in pharmaceuticals to work right, it has to be soluble. The research that's done on it is done assuming that its molecular structure doesn't change and if it does, then that affects its efficacy. And so it's important to know the temperature and humidity boundaries at which hydrate will form and to make sure that the work that's done on it is either in the anhydrous state or the hydrated state, but not some in one and some of the other.

Zach Campbell (07:23):
That information, knowledge of how that's going to occur can help drive choices as it pertains. How is this packaged and distributed? How is it encapsulated? What excipients are used? In a suspension, what media is used to suspend it? Because either of those materials, for instance, the excipient could form a hydrate or it could form anhydrous material. And so by expelling that water, it could impact the API as well. So a little bit of extra knowledge can help upstream.

Zachary Cartwright (07:54):
And I just want to clarify this. It sounds like both in food and pharma, if you're getting a hydrate formation, that might mean that what's on the label doesn't represent what's in the product itself. Is this true?

Mary Galloway (08:05):
Yeah. I think you need to know what's happening. For food, like when we talked about functional ingredients and the vitamins and when we're specifically shooting for a concentration and we have that on the label, that's definitely going to be a problem. For the other issues, that can happen like the caking and clumping and the solubility and that kind of stuff. That's also an issue in the formulation. Although, when we're talking about the API as well, like in the pharma industry and also these functional foods, if you have a change in solubility, then people aren't uptaking into their body what they're expecting. And that's a problem too.

Zachary Cartwright (08:47):
All right. Thank you, Mary. And now that we know a little bit about the different applications, let's come back to these papers. So I'll start with you, Dr. Campbell. What was the goal of the Pfizer paper and how does this compare to the goals in the Purdue paper?

Gaylon Campbell (09:04):
The goal in both papers is to determine the phase boundaries in terms of temperature and humidity, at which the hydrate forms. And in the Purdue paper, the boundary where the crystal deliquesces, or you get the change from the crystalline state to the solution phase. So if we think of a phase diagram, the phase diagram, in this case, is a plot of on the y-axis relative humidity or water activity, and on the x-axis, temperature. Then the lines on that graph will show the boundaries where the different phases can exist. So you can have a crystalline anhydrous phase for the material that you're interested in. You can have a phase where the hydrate has formed, and then you can have a solution phase. And if once we know the location of the boundaries in that diagram, then we can always predict whether we'll have a hydrate or an anhydrous form of the material or a solution.

Zachary Cartwright (10:47):
And you're pointing out these diagrams. What instruments are required to make these diagrams? Maybe you can speak about that Zach.

Zach Campbell (10:56):
Sure. Yeah. So the actual formation of these boundary diagrams is fairly complicated. No one instrument is going to build the entire thing out. So the paper by Allan and Mauer, used actually two or three different methods to build these out. For that transition from the anhydrous to the hydrate, what they were doing is mixing binary solvent solutions of different alcohols in water. And the reason they were doing that is it allowed them to basically control the humidity in solution, which is important from this perspective because as we mentioned, the entire conversion from an anhydrous material to a hydrate has some time component encompassed within that.

Zach Campbell (11:42):
On top of that, if you're doing it in a vapor phase, so if you're subjecting they were using glucose, citric acid, and one other I believe, if you subject those to a humid environment, then that's going to add in additional time for that to equilibrate. To the point where the timescale of the experiment extends into perhaps even the year range. So by controlling the water activities of the solvents, they're able to introduce that crystalline hydrate or anhydrous material, and then measure whether there's any change to the crystal lattice. And if there is, then it's assumed that there was, it is unstable at that point. And so to verify that I believe they were using IR crystallography of some sort.

Mary Galloway (12:29):
That's how they were checking to see which form was in there. It's pretty neat stuff. Basically, you specifically build a water activity solution. They used ethanol in that paper and water together, and then they just put that stuff in there, like the crystal, the hydrate or an anhydrous solution. And they put it right in there. And it worked so much faster because it's in direct contact with that solution. And so then they monitor the water activity and they see does the water activity change. And when it plateaued, and that's what they found, then they could say, "Ah, that's the boundary that we're looking for." And they did it at various temperatures too. And they did a whole kind of scan.

Mary Galloway (13:07):
But yeah, they reference 195 days and they had no change. In another paper, they reference that they had done something like that for a year where they just did it this old way, where you just hold it over a specific humidity with a salt solution, and they'd see a 3% change in weight over one year. That's a lifetime of experiments to try to get that kind of data. But to do this and it would take a few hours, I think, three to seven days maybe depending on what we're forming, that's huge. That's doable, right?

Zachary Cartwright (13:42):
So how are they speeding up that process if some of these experiments are taking a year's time? Is there a faster way to get the same results?

Zach Campbell (13:50):
So that particular method they were using, where they introduce it into the aqueous solution, helps speed that up dramatically, where it becomes, I wouldn't call it instantaneous, but much, much faster at that point. I don't remember how long they were leaving them in equilibrium for, I think overnight, at least.

Gaylon Campbell (14:08):
It was, Mary said three to seven days.

Zach Campbell (14:11):
Three to seven days.

Gaylon Campbell (14:12):
So they'd just make up the crystal that they were interested in with the alcohol-water solutions and they knew the water activity of those solutions by measuring it with the TDL water activity meter. And they just put those along with a little stir bar inside of one of the sample cups, and then they'd seal that up with parafilm, set it on the stirrer, and let it stir for a few days. And then they'd measure the water activity at the end. And so below the water activity at which hydrate started to form, the water activity of the stuff that they put in at the beginning, they got the same water activity at the end when they opened it up. But when the hydrate started to form, why that took up a lot of the water, went into a crystalline state with the molecules they had put it in there, and that just stopped the change in water activity essentially.

Gaylon Campbell (15:29):
And so it's a beautiful experiment where it can just plot the water activity at the start versus the water activity at the end. Just increased linearly, and then when hydrate started to form, it went flat. And so just at the intersection of those two lines, they knew that was the water activity that they needed for that hydrate boundary. And like Zach said, by doing it in solution, that way they could get it go from maybe a few years for an experiment down to a few days for an experiment, just because the exchange is so much more rapid in solution than it is in through the gas phase.

Zach Campbell (16:18):
Yeah. It is a really simple, and like you said, a beautiful method for taking something that was originally difficult to measure and simplifying it. And it matches up pretty well with how similar processes are done at scale in industry. A benefit of using a volatile is that you can remove it from that. Once you form the hydrate or form the anhydrous, then you can remove that solvent. And that's why they were using the TDL as opposed to a different water activity meter, is it allowed them to measure various different solvents, even though they ended up using ethanol across the board.

Mary Galloway (16:52):
Yeah, they picked ethanol I believe because it gave them less side reactions or anything like that.

Zach Campbell (16:56):

Mary Galloway (16:57):
I think it just really worked well with what they were testing. But they did it at different temperatures too, and they scanned. But the other interesting thing I thought was it could go both ways. You can start with the anhydrous and then kick the water activity up. But they also did it in the other direction, which was they started with the hydrate and went down and they could confirm that same point going either direction. And they also used a DVS method too, which is where basically you're holding those samples at a specific community and then watching how that changes, the weight. And then they use that to help validate. I think they call it the saturated water activity method. So they looked at it a few different ways. And then, they had the x-ray diffraction, which also would go and say, okay, because when they hit that boundary, it's quite interesting. Both of those states are in equilibrium, meaning that there's a hydrate and an anhydrous form in there, and then they would check that with the x-ray diffraction. It was all nifty.

Zachary Cartwright (17:59):
There were a couple of acronyms that were in there. I just want to point them out. One of them was TDL. This is a type of water activity meter. And in all fairness METER does produce this meter. But why is this specific instrument important for this project and what does TDL stand for?

Zach Campbell (18:16):
Yeah, so the TDL stands for tunable diode laser. And specifically for this application, because they were using alcohol-based solvents to control the water activity, no other instrument would work for that, or would have some serious pitfalls they'd have to work around as well. So that was the critical reason why they chose that instrument.

Zachary Cartwright (18:36):
And Mary, you mentioned DVS. Can you tell us what that stands for and how that information is collected?

Mary Galloway (18:42):
Yeah. So there are other instruments that basically will do an isotherm where it's a static vapor sorption isotherm. So they're holding a sample at a specific humidity and then tracking the weight, and then that can be related to moisture content. And we also have an instrument that does this very thing called the VSA, or vapor sorption analyzer, that is very useful for this stuff. And one of the things as we read in this paper, and they didn't use ours which is fine.

Gaylon Campbell (19:16):
Ours would've done it.

Mary Galloway (19:18):
Yeah, ours would have definitely done it. But I was wondering too, we have another patented method that only our VSA does, which is the dynamic dew point isotherm, which basically is just pushing moisture over the sample. And we're tracking the weight as well, but it's not allowed to equilibrate. Right?

Mary Galloway (19:36):
So we're just forcing these real-time reactions to happen. And when we do that, when we do those with salts and things that we have and sugars, those kinds of things, because they also are crystalline, we absolutely see the deliquescence point, that point where it goes right from solid right into a liquid phase. It's so clear when we do it that way. And also we can also see these waters of hydration form. So my thought was, "I wonder if they used our instrument, if they could have also gotten some real data that way?" It's a really fascinating trend when you look at it that way when these waters of hydration are added and then basically the water activity jumps down. It's kind of the similar process that we're talking about here when they form the hydrate, because it's removed from that solution. So it's the same kind of idea. So it's in there and then it's held on. And so the water activity drops, but the weight doesn't change. Anyways, it's fascinating stuff. I'll be quiet now.

Zachary Cartwright (20:30):
You're just full of excitement, I can see. If they would have used the DDI method, the dynamic dewpoint isotherm, could they get results even faster?

Mary Galloway (20:40):
Yes, I think so. Because when we were talking about when we do those types of tests we're doing, a specific flow rate, and crystalline structures are a little tricky because you don't want to speed it up too fast. So we have to slow the reaction down. But it does happen a lot faster than what we've talked about here, where we're talking months or years for that vapor because we're constantly introducing more vapor, if that makes sense. There's where when you do a desiccated chamber, a chamber that has saturated salts below it, that there's limited, it'll continue to release moisture to a certain point because it has that capability because that's just built into the salt. But then the desiccators themselves, if they're not sealed well, there's just more problems with that. I think so, it would be an interesting idea to try actually. But I know for a fact that we can definitely see those phenomena happen that we talked about.

Zachary Cartwright (21:34):
And then Dr. Campbell, is there anything in these papers that is missing? What would you like to see in a follow-up experiment?

Gaylon Campbell (21:44):
Some of the kinds of things that Mary just mentioned. I would love to do, repeat parts of these experiments with our VSA that has the dynamic isotherm scan and see if we can identify these changes associated with hydrate formation. I'd kind of like to try the other thing with the TDL and slurries in those cups too, just to see if I could make that work the way they made it work, such a beautiful experiment.

Zachary Cartwright (22:39):
Well, is there anything that we've missed today? So we've talked about the papers, we've defined the hydrates and talked about their applications. We've gone over methodology and instruments. Is there anything else that you guys would like to add today to this podcast?

Zach Campbell (22:54):
I think that we covered some of the big picture stuff. The papers themselves are quite in depth and both authors, all authors did a great job of going through the methodology. So I would implore those who are interested to take a look at the source papers.

Zachary Cartwright (23:11):
Well hopefully this podcast hasn't been too technical for you listeners and hopefully you have a better understanding of what hydrates are, how they're analyzed and why they are important to both the food and pharmaceutical industries. I'd like to thank each of my guests today for their time and for their expertise.

I'm Zachary Cartwright, this is Water in Food. Find this podcast on Apple iTunes, Spotify, or wherever you listen to podcasts.

Article Links:
Matthew Allan, Lisa J. Mauer,
RH-temperature phase diagrams of hydrate forming deliquescent crystalline ingredients,
Food Chemistry,
Volume 236,
Pages 21-31,
ISSN 0308-8146,
Abstract: Several common deliquescent crystalline food ingredients (including glucose and citric acid) are capable of forming crystal hydrate structures. The propensity of such crystals to hydrate/dehydrate or deliquesce is dependent on the environmental temperature and relative humidity (RH). As an anhydrous crystal converts to a crystal hydrate, water molecules internalize into the crystal structure resulting in different physical properties. Deliquescence is a solid-to-solution phase transformation. RH-temperature phase diagrams of the food ingredients alpha-d-glucose and citric acid, along with sodium sulfate, were produced using established and newly developed methods. Each phase diagram included hydrate and anhydrate deliquescence boundaries, the anhydrate-hydrate phase boundary, and the peritectic temperature (above which the hydrate was no longer stable). This is the first report of RH-temperature phase diagrams of glucose and citric acid, information which is beneficial for selecting storage and processing conditions to promote or avoid hydrate formation or loss and/or deliquescence.
Keywords: Crystalline ingredients; Phase transformations; Anhydratehydrate transitions; Deliquescence; Phase diagrams

Joseph F. Krzyzaniak, Glenn R. Williams, Nina Ni,
Identification of Phase Boundaries in Anhydrate/Hydrate Systems,
Journal of Pharmaceutical Sciences,
Volume 96, Issue 5,
Pages 1270-1281,
ISSN 0022-3549,
Abstract: ABSTRACT
Near-infrared spectroscopy was used to monitor the phase conversion for two solvatomorphs of caffeine, an anhydrous form and a nonstoichiometric hydrate, as a function of time, temperature, and relative humidity. The transformation kinetics between these caffeine forms was determined to increase with temperature. The rate of conversion was also determined to be dependent on the difference between the observed relative humidity and the equilibrium water activity of the anhydrate/hydrate system, that is, phase boundary. Near the phase boundary, minimal conversion between the anhydrous and hydrated forms of caffeine was detected. Using this kinetic data, the phase boundary for these forms was determined to be approximately 67% RH at 10°C, 74.5% RH at 25°C, and 86% RH at 40°C. At each specified temperature, anhydrous caffeine is the thermodynamically stable form below this relative humidity and the hydrate is stable above. The phase boundary data were then fitted using a second order polynomial to determine the stability relationship between anhydrous caffeine and its hydrate at additional temperatures. This approach can be used to rapidly determine the stability relationship for solvatomorphs as well as the relative kinetics of their interconversion. Both of these factors are critical in selecting the development form, designing appropriate stability studies, and developing robust conditions for the preparation and packaging of the API and formulated drug product.
Keywords: polymorphism; hydrate; phase boundary; equilibrium water activity; near infrared spectroscopy; caffeine; solvatomorphism

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