Table 4.

Relevant individual differences likely to influence perception of whey protein-derived mouthdrying in older adults (↑ increases with age; ↓ decreases with age; n/a not applicable).

Category	Factors	Effect of Age	Effect on Mouthdrying	Food Matrix	Methodology Limitations
Physiology	Age 1*	n/a	oWhey protein fortified products cause mouthdrying which may be influenced by age	WPB [89,143], cakes and biscuits [90]	oInconsistent results between studies could result from differences in test sensitivity used (for example, paired comparison test vs generalised labelled magnitude scale)
	Appetite 1,2*#†	↓	oONS and whey protein-fortified products can increase perceived thirst, reduce hunger and prospective consumption	Cupcakes [90], ONS [87,88]	oSelf-report using visual analogue scale. Appetite was not measured at subsequent meals
	Dental status 1,2*†	↓	oPoor dental status could make consumption of solid foods more difficult and therefore negatively impact product liking	Cakes and biscuits [90], meat and cereal [28]	oSelf-report questionnaire or limited oral parameters measured
	Saliva flow 1,2*#	↓	oSaliva flow can decrease with age, however whether this influences subsequent perception is relatively unclear	WPB [89,123], cakes and biscuits [90], meat and cereal [28]	oVolunteers may have been too healthy to demonstrate an effect of saliva flow
	Detection thresholds to sensory stimuli 1#	↑	oDetection thresholds for many stimuli (such as tastants and volatile compounds) increase with age and perception increases (at different rates depending on the stimuli) with stimuli intensity. Studies to date suggest that perceived mouthdrying initially increases with protein concentration until a plateau is reached.	WPB [83,123,139,140]	oNo defined mouthdrying threshold method has been developed
Social	Culture 2*	n/a	oCultural groups have different food oral processing behaviour and this could influence food choice and mouthfeel perception	18 different food products varying in physical properties [162] carrot, cheese and sausage [163]	oOnly limited populations have been studied (for example Dutch nationality and Caucasian ethnicity compared with Chinese nationality and Asian ethnicity)
Preferences	Food preference and neophobia 2‡	No set direction	oFood preferences and neophobia could influence compliance with ONS and whey protein-fortified products	n/a [161,164]	oSelf-report questionnaire
	Mouth behaviour 1*	Not known	oMouth behaviour could influence texture perception of whey protein-fortified products and may alter with age	Cakes and biscuits [90]	oSelf-report questionnaire

¹ Refers to studies using whey protein food matrices; ² refers to factors that may influence whey protein derived mouthdrying but have currently not been investigated within a whey protein food matrix. Study type: * younger adults (18–35 years) and older adult (over 65 years) study; # younger adults only (20–60 years); † older adults only (87: 60–75 years; 28: over 65 years); ‡ other: (161: children aged 9–12 years and parents; 164: n/a review paper). All volunteers considered healthy unless otherwise stated.