The standard analysis of gel point applies a criterion developed by Winter & Chambon (HH Winter, F Chambon - Journal of rheology, 1986): in a linear viscoelastic rheology test, at the gel point the elastic (G') and loss (G'') moduli of a material have the same exponential dependence on the oscillation frequency, i.e. both scale as \omega^\alpha, over a wide range of frequencies (essentially all frequencies) . This is an empirical observation although much further work has proposed physical reasoning for the observation.

Here are some references that may help:

The effect of high intensity ultrasonic pre-treatment on the properties of soybean protein isolate gel induced by calcium sulfate. http://www.sciencedirect.com/science/article/pii/S0268005X13000313

Disulfide bond- stabilized physical gels of an asymmetric collagen-inspired telechelic protein polymer.

http://pubs.rsc.org/en/content/articlelanding/2013/sm/c3sm50641j/unauth

Forces Involved in Soy Protein Gelation: Effects of Various Reagents on the Formation, Hardness and Solubility of Heat-Induced Gels Made from 7S, 11S, and Soy Isolate

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2621.1985.tb10461.x/abstract

Determination of sulfhydryl groups and disulfide bonds in heat-induced gels of soy protein isolate

http://pubs.acs.org/doi/abs/10.1021/jf00079a038

The standard analysis of gel point applies a criterion developed by Winter & Chambon (HH Winter, F Chambon - Journal of rheology, 1986): in a linear viscoelastic rheology test, at the gel point the elastic (G') and loss (G'') moduli of a material have the same exponential dependence on the oscillation frequency, i.e. both scale as \omega^\alpha, over a wide range of frequencies (essentially all frequencies) . This is an empirical observation although much further work has proposed physical reasoning for the observation.

you can determine the temprature depencency of gels by DSC method or by observation the change of viscoelestic moduls in the Rheometer's Rheogram

I think Rheometer is the best instrument to determine the gel point. It gives the information both about gelation temperature and gelation time.

Jeffrey Morris is correct. The material at its gel point has very distinct properties which can be used to detect the gel point and to see how stiff the material is during the transition. The findings have been confirmed many times as you can see at http://che.umass.edu/news/winters-milestone-paper-hits-coveted-1000-citation-mark

Gel point is the point where elastic and viscous modulus meets together. It is very disstinct if you use rheometer and especially for your specimen will show excellent gel point. However, you try temperature ramp say starting from 25C to 90C at a heating rate of 2-5C/min. Good luck. DSC will show melting point but you need powder for DSC as your sample contains huge moisture content. Good luck.

Thanks for all good comments. I found that, as the methods suggested by Jeffrey Morris sir and H. Winter sir, are standard rheological methods determining gel points for Physical as well as Chemical gels. However, preliminary experiments such as time sweeps at different frequency ranges (within LVR) were conducted and results indicating instant gel formation. Therefore, predetermination of exact gel point (i.e, gel time) makes it difficult to get to that point. So how can I determine gel time in case of instant bio-polymer gel?

Perhaps this reference might also help you to understand the gel concept. Picout, D. R., & Ross-Murphy, S. B. (2003). Rheology of biopolymer solutions and gels. 43 The Scientific World Journal, 3, 105–21.

Thank you Javier sir for your concern, indeed critical gel portion from the above mentioned research article is quite useful. Although, in our case time sweep was done at different frequencies, but cross over point between G' and G'' was not seen (as mentioned in H. H. Winter and F. Chambon, J. Rheol. 30, 367 1(986)). Whereas G'>G'' throughout time sweep. Therefore is there any possibility to apply Winter-Chambon method or I have to consider any other parameters for estimation of gel time?

Dear Kiran,

Dr. Jasim Ahmed kindly send me this paper, I was thinking it could also be useful for you, regards

Javier

Please find the mentioned article

Dear Kiran

I just wonder how you prepare your aloe sample, according to what you said, it seems to me that you already have a gel when you put it into the rheometer, it is obvious that if you want to determine a gel point, you´ve got to start with a dispersion or a sol (liquid) and look for the gel formation "in situ"...

Dear Javier Sir,

Thank you for your concern. I had certainly followed different methods for gel formation. Where the obtained rheological results were showing increased gel strength compared to existing methods.

Further, gel time determination were carried out for reconstituted gels (at both lower and higher concentrations, respectively) at frequency 0.1, 1 and 10 rad/s for a constant stress value of 1Pa (selected from LVR region). However time sweep data (at single frequency) shows G'>G" and no cross over points detected. Sir is there any changes need to do during experimentations such as selection of stress or frequency ?

Dear Kiran,

as far as I am aware, you´ve got to start with very low stresses (perhaps even lower than 1 Pa) and low frequencies, and then increase frequencies as needed, if you see form the very beginning that your sample shows that G´>G", you might perhaps like running a preliminary test of melting your sample and then run a temperature sweep at constant stress and frequency...at some point the sample should "gel" again....

It is very easy you need to prepare your solution, perform LVE test on Rheometer . γL…limiting value of the linear viscosity range (LVE)

The amplitude sweep is carried out, mainly to determine the limit of the LVE range. As long asthe γ amplitudes remain below the limiting value γL the G’ and the G’’ curves show aconstantly high plateau value. That means that the structure of the sample keeps stable.Above γLthe structure of the sample is going to be more and more irreversible destroyed.G’ > G’’… in this range the elastic behavior dominates over the viscous behavior and thesample behaves as a solid or as a gel, and shows form stability G’’>G’ … in this range the viscous behavior dominates over the elastic behavior. The sample shows the character of a liquid.

inbuilt function fits amplitude test and determines LVE region, then perform frequency sweep at a fixed stress (determined above in LVE) . The instrument will plot G’ and G" as a function of angular freuquncy. The point at which G’ = G" is referred to as gel point.

Gel point can also be determined by Temperature sweep if the gel is thermo-reversible.

I think that there is not such an EXACT METHOD, I do agree with Oyais Chat, he talks about an ordinay and quite common method to find the gel point....

Dear Oyais,

It was clearly mentioned in earlier research articles (Analysis of linear visco-elasticity of a cross linking polymer at the gel point ; Can gel point of a cross linking polymer be detected by the G-G cross over), that time of interaction [G'=G''] during frequency sweep might be close but not identical with the transition time. And also stated that instant of gelation is strictly material dependent and can not dependent on the frequency of the rheological test (unless there is a influence of flow and vitrification). Therefore I understand that to identify the exact gel point (i.e., gel time) one must perform time sweep experiment at constant frequency.

I do agree with your gel point (i.e., gel temperature) determination by temperature sweep experiment.

Dear Javier sir,

Thank you for your suggestion. As I have re-conducted the experiments at low stress i.e 0.00298 Pa (instrument minimum level), at which higher concentration shows cross over within 100 sec, but in case of lower concentration this shows the phase angle increasing with time (>2hrs) indicates no cross over. The exact reason I have not understood, but it might be due to semi dispersion nature of gel at low concentrations decreasing the gel strength.

Dear Kiran

remember that to have a cross-over you need a gel to be formed, If it happens that your sample has a very low solids concentration, you have to make sure that it can form a gel, if it doesn´t, I don´t think you´ll have any cross-over between G´and G", thus an increase in phase angle is an indication that you have a more viscous sample

Dear Javier sir

I understand your point of view, I will try with intermediate concentration ranges (i.e, at or above critical gel concentration).

Dear Kiran,

That is very sensible, when you see that your sample does not give any elastic response, then you do not have a gel, but a solution...when you detect a "threshold" that gives a gel, then you might be very close to the minimum "solids concentration" that yields a gel.

good luck!!