Measuring Crunchiness as 'Sound Bites'

Continuing the theme of how food manufacturers can appeal to all the senses, Lynda Searby looks at the gastronomic role of sound.

Diners who order oysters at the Fat Duck - Heston Blumenthal’s three-Michelin-starred restaurant in Bray, Berkshire, UK - are given an iPod to listen to while they are eating. It might be a bit of a conversation-killer, but the experimental chef insists that the sound of breaking waves heightens the flavour of his ‘Sound of the Sea’ dish, which features seafood and edible seaweed on a bed of sand-like tapioca.

This may sound a bit far-fetched but, in fact, the idea for the musical accompaniment came out of scientific research Blumenthal carried out with Professor Charles Spence at Oxford University, which revealed that sound can boost taste.

The idea of the research was to investigate whether multi-sensory environmental cues can influence flavour perception. 70 people were given two oysters to eat, one after the other. They then had to rate the intensity of the flavour and how pleasant each oyster was. The first oyster was consumed whilst listening to the sound of seagulls and waves lapping the seashore, the second while listening to farmyard sounds. The oyster eaten whilst listening to seaside sounds was rated as tasting significantly more pleasant.

A similar experiment was conducted with a dish for which the Fat Duck has become famous – Egg and Bacon Ice Cream. This discovered that people found the bacon flavour significantly more intense when the room was filled with the sound of sizzling bacon rather than the sound of chickens clucking in the farmyard. 

“These results demonstrate that our perception of flavour involves not only the multi-sensory integration of all the available cues in the food itself – the oyster’s slippery brininess, the bacon’s meaty odour – but also the multi-sensory integration of some of the cues that are present in the environment in which we happen to be eating,” says Professor Spence. “This might help explain why food and drink that we enjoyed on holiday is often disappointing when tried back home – we are missing all those multi-sensory environmental cues – the smells, the sounds, the sights – that helped make it taste so wonderful.” 

If readers are thinking that this is all a bit leftfield to be of any practical use to the mainstream food industry, it might interest them to know that major food manufacturers are investing resources in this research area.

Unilever, for example, funded a project in which Professor Spence looked at how sound influences people’s perceptions of the freshness of crisps.

Participants in the study bit into 180 Pringles crisps of varying freshness, whilst the sound that each crisp made was changed in real-time using a microphone, computer and headphones. The participants rated Pringles as tasting 15% fresher and crisper when the crisp sounds were made louder, or when just the high frequency components of the biting sound (above 2KHz) were boosted. These results not only earned Professor Spence and Max Zampini the Ig Nobel Prize for Nutrition, but also prompted other large food companies, such as Nestlé, to conduct their own in-house research based on the findings. 

So if consumers perceive a crisp that crunches louder as tasting fresher, this presumably means that if manufacturers can turn up the volume on product crunchiness, they are more likely to attract loyal consumers. 

But how do you know when you’ve hit the ideal crunch volume? One way is to use a trained tasting panel to determine whether the crunch is loud enough. Another, more repeatable, reliable and scientific approach is to use a texture analyser - an instrument which measures sensory attributes of a product by capturing force, distance and time data at a rate of up to 500 points per second.

How a texture analyser could help assess ‘crunch’ 
If a product developer wanted to make their product crunch louder, they would simply put the product under the texture analyser on an appropriate fixture – so, for a biscuit, they would use a three-point-bending rig. They would then position the microphone 1 centimetre from where the biscuits were going to fracture. The acoustic data would then be captured throughout the breaking of the biscuit and would be presented graphically, as a jagged line with some taller peaks. 

To make a louder product you would be looking for the product that generates the highest ‘peaks’ or decibel values, so tall peaks rather than lots of little ones. If a manufacturer wants to compare how crispy different products are, they can count the number of peaks generated and divide this by the number of seconds over which they occur. This will tell how many fractures are produced in a second – and the more there are, the crispier the product.

Sound research at Leeds University 
Another scientist at the forefront of sound research is food physicist Professor Malcolm Povey. In contrast to Professor Spence, his work involves analysing the crispiness or crunchiness of foods using instrumentation equipment.

Historically, it was thought that the frequency of the sound generated by biting into a product determined how crispy a product was – products that produced a higher frequency sound were thought to be crispier. However, Povey has since proved that what actually differentiates a crispy food from a non-crispy food is the rate at which the fracture events occur. 

“Both crispy and crunchy foods emit sounds in bursts and the human perception of crispiness/crunchiness correlates very highly with the rate at which the bursts appear and to a lesser extent their amplitude,” he explains.
 
“The reason for this is because crispy/crunchy foods break in the mouth through crack propagation and a lot of the energy released by the crack produces sound. Since the crack propagates at a very high speed (too high for even high speed cameras) the sound is produced in a very short space of time – i.e. as a pulse. Slowed down and plotted onto a graph, the pulses can be seen as a series of tall peaks, but actually last only for milliseconds.The more peaks, the crispier it is – it's as simple as that." 

This research has practical implications for food manufacturers. Povey used a texture analyser equipped with an Acoustic Envelope Detector to analyse acoustic emissions from food products during deformation. This technology is commercially available to food manufacturers for use in their product development laboratories.

Povey has also used his own equipment to measure sound waves in the ultrasonic region, going beyond the range of human hearing. In this work he found that the energy produced by the crack of a biscuit breaking in the mouth is released as distinct pulses of ultrasound – sound waves generated at frequency levels more usually associated with bats, whales and dolphins. Whilst the ear can detect these very short pulses, they sound different to ordinary sound tones.

“It’s a good job we can’t hear all the energy in these pulses,” says Povey, “as they would damage our ears if we did. They’re enormously loud bangs – often way beyond safe decibel levels.”

The reason the ultrasonic region is of interest to the food industry is that manufacturers could potentially ‘fingerprint’ the acoustic profile or ‘crunch’ of a product.

So in the future, it could be that it will no longer be about patenting a secret recipe, but about patenting a crunch. 

In the 1980s, Kellogg’s tried to trademark the specific crackling sound made by its cornflakes. That might yet happen. 

The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.