The Effect of Interactions Between Capsaicin and Dairy Proteins on Oral Burn

Open Access
- Author:
- Farah, Brigitte
- Graduate Program:
- Food Science
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 09, 2022
- Committee Members:
- Robert Roberts, Program Head/Chair
John Neil Coupland, Thesis Advisor/Co-Advisor
Helene Hopfer, Committee Member
John Hayes, Thesis Advisor/Co-Advisor
Federico Miguel Harte, Committee Member - Keywords:
- Capsaicin
Dairy Proteins
Casein
Hydrophobic
Chemesthesis - Abstract:
- Consumption of the hydrophobic compound capsaicin causes a burning sensation via activation of the TRPV1 receptor in the mouth. Folk wisdom and controlled psychophysical experiments suggest milk is very good at reducing burn after consumption, but specific mechanism(s) remain unknown. It is widely assumed that this reduction arises from partitioning of the hydrophobic agonist into the lipid phase of the milk, as capsaicin is more soluble in lipids than in water. However, it has been found that full fat milk is no better than skim milk at reducing burn when served as rinses following capsaicin exposure. This leads to speculation that the observed reduction may be due to sequestration of capsaicin by milk protein rather than partitioning into the lipid phase. It is generally assumed that to be perceived, a non-volatile molecule must dissolve in saliva to reach receptors; this also implies the concentration of the molecule in the aqueous phase determines its perceived intensity. Accordingly, reduction of the free, aqueous concentration of capsaicin, either by binding or partitioning, may reduce the perceived oral burn. While prior studies have demonstrated the ability of milk proteins to bind small hydrophobic molecules, this has not been explored directly with capsaicin, and few studies have investigated the effect of protein binding on sensory responses. Here, micellar casein and whey protein isolate were selected as model proteins, as they are the most abundant proteins found in cow’s milk. I hypothesize that dairy proteins will bind capsaicin, decreasing the aqueous concentration and the perceived oral burn. The ability of milk proteins to reduce the oral burn from capsaicin was tested both when presented concomitantly in mixtures and sequentially as post-exposure rinses. In the first study, a convenience sample of moderate chili consumers evaluated the oral burn of aqueous micellar casein (n = 89) and whey protein isolate (n = 78) solutions mixed with capsaicin. The samples contained either 0, 2, or 5% protein with a 5 ppm capsaicin concentration, in addition to a 5% protein (no-capsaicin) control. Participants rated the oral burn of each sample on a general Labeled Magnitude Scale (gLMS) every 10 seconds over 2 minutes, with appropriate breaks in between samples to allow any remaining burn to dissipate. Both micellar casein and whey protein isolate reduced the oral burn from capsaicin. The effect of micellar casein was dose-dependent, as burn was reduced more with increasing protein concentration. Compared to the 0% micellar casein sample, the burn of the sample with 2% micellar casein was reduced by 20% and the burn of the 5% micellar casein was reduced by 43%. Conversely, the effect of whey protein isolate was not dose dependent, with equal reduction in burn for both concentrations tested of 23-25%. A convenience sub-set of the participants from the first study (n = 68 of 167) evaluated milk protein solutions as rinses following capsaicin exposure. Participants sampled 5 ppm aqueous capsaicin five times, each paired with a rinse, rating oral burn on a gLMS every 10 seconds for 2 minutes. The rinses were 2 and 5% micellar casein or 2 and 5% whey protein solutions, and a water control. All protein rinses were comparable to the water rinse (control), but the 5% micellar casein rinse was significantly more effective than the water rinse. Specifically, at 1 minute there was a 27% reduction in burn for the 5% micellar casein rinse compared to the water rinse. The second aspect of my research assessed the ability of protein to bind of capsaicin in aqueous solutions. The concentration of capsaicin in the aqueous phase of micellar casein and whey protein isolate solutions was measured by extraction into hexadecane, an immiscible, non-polar solvent. The concentration in the aqueous phase was assumed to be representative of the unbound capsaicin; therefore, a lower aqueous concentration would indicate greater protein binding. Solutions containing 0 to 5% protein with 5 ppm capsaicin concentrations were extracted, and the concentration in the hexadecane phase was measured via fluorescence spectrophotometry. Capsaicin concentration in the hexadecane phase decreased with increasing protein concentration, providing evidence for protein binding. Micellar casein was more effective at binding capsaicin compared to whey protein isolate. Notably, the capsaicin concentrations in hexadecane, which were assumed to reflect the unbound capsaicin concentrations, were directly proportional to the group averages of maximal burn determined in the sensory portion of the work. In summary, the work described here demonstrates that milk proteins can bind capsaicin, reducing the aqueous capsaicin concentration. In addition, I found the perceived burn is proportional to the unbound capsaicin concentration. Further, these findings suggest that the ability of milk to reduce the burn from capsaicin may be due, at least in part, to protein binding, rather than lipid partitioning, as is commonly assumed.