Let them eat fruit!

Jo Smewing defines the role of texture analysis in gaining competitive advantage within the world of fruit

Only relatively recently have modern techniques in texture analysis been implemented in the fruit sector. The embracement of more advanced testing methods and technologies has been led by increased fruit consumption in the consumer sphere, in turn boosted by increased health awareness and a global trend towards healthier living. Historically considered a ‘low value-added’ commodity, agricultural produce such as fruit has failed to attract the same level of investment and sophistication of analysis into the correlations between structural characteristics and consumer acceptability as processed foods have.

In addition, tangential developments such as sustainable farming and the organic movement not only add an ethical dimension to the consumer purchasing decision, but also exaggerate the already fierce competition: where once seasonality and geographical provenance of fruits alone dictated supply, globalisation and the ‘superfruit’ trend – an example of increased general interest in health – have seen new and exotic specimens join the race for market share. Add to the above a debate on GM foods, and it becomes apparent this business requires more than sun, water and fertile ground to be fruitful.

Fruit producers and processors of all sizes are taking advantage of the most up-to-date methods of quality control with a view to overcoming the market constraints outlined above. The concept of fruit quality relies on fulfilment of a number of criteria. Of these, a parameter known as ‘firmness of fruit’ is perhaps most influential.  Its role in the perception of ‘good’ versus ‘bad’ quality fresh fruit is manifest in consumer behaviour when purchasing; with an initial scan of the produce on display, the shopper filters out ‘bad’ quality fruit on aesthetic grounds. But it is the subsequent squeezing of the fruit in the palm of the hand that ultimately holds the key to a fruit’s destiny – in the bag or left on the shelf. This instinctive, subjective test of firmness is related to the ‘eating maturity’ of a fruit, which softens as it ripens as a result of mechanical responses within the fruit structure.

Stages of physiological development, damage, fibrosity and turgidity also influence fruit firmness.   Each of these factors is linked with a particular stage of a fruit’s life-cycle. Measuring the textural characteristics of fruits at each of these stages provides valuable insight into the physical effects of processes such as harvesting, transport, storage and further processing. In turn, this information gives processors the power to cultivate a fruit ingredient that matches the very exacting specifications of a given food or drink manufacturer or retailer.

As the number of properties that can be measured is varied, so are the corresponding test methods. A TA.XTPlus texture analyser accommodates attachments and fixtures to suit both the sample and the objective of a test.

A test of fruit firmness, much like the consumer’s selection process described above, is a compression test. The deformation response of a whole fruit is measured after compression between a large cylinder probe or platen. The resulting force deformation characteristics (beyond the elastic limit) of the fruit are useful in determining bruising potential, in turn informing logistics and transport methods. Magness-Taylor probes are a widely accepted choice of measuring device when it comes to testing skin strength and flesh firmness of whole fruits. Being lowered into a sample to the point of piercing its skin or flesh, the penetration test allows flexible and repeatable testing of pitted fruits, large berries, starchy or pasty fruits (e.g. bananas), as well as fruits with a core and citrus fruits.

An obvious yet highly valued property of any fruit is its volume. Until recently, this external characteristic has been measured by basic means of water displacement. Though straightforward and simple, the test produces a single isolated figure. More advanced methods involving computer-assisted recording of multiple parameters mean it is now possible to scan a sample digitally, from which an on-screen 3D image of the fruit is generated. As such, the contours of the fruit can be used as parameters to calculate its precise dimensions [see graph A]. Laser-based scanners with associated intuitive software, such as the Volscan Profiler, can have vast implications on internal and external operations – from sourcing to quality control and assessment.

Dried fruits, often falling under criticism for loss of piece integrity, present a number of challenges as a result of the processes the fruits undergo: brittleness and toughness are not uncommon. Monitoring the textural characteristics of this form of fruit highlights the pros and cons of a given drying or packaging choice – get it right, and a dried fruit will deliver all the health benefits and mouth-feel a consumer could wish for. Get it wrong, and the consumer may struggle to separate the fruit from its packaging.

A penetration or blade test assesses stickiness and hardness of dried fruits. In the first instance, a straightforward penetration test indicates firmness/softness: a 6mm cylinder probe compresses a sample until a determined resistance is achieved (e.g. 100g). The distance necessary for the probe to travel until this resistance is achieved illustrates how firm or soft a dried fruit sample is [see graph B]. A subsequent phase of the penetration test measures stickiness, whereby stickiness is defined by the work necessary to overcome the attractive forces between the surface of the food and the surface of the probe. Alternatively, cutting tests using a Craft Knife or Kramer shear cell imitate the slicing action of teeth – peak cutting force and energy are typically recorded here. Such tests are particularly suited to firmer fruit pastes, whose acceptance in cereal bars and confectionery items may depend on the mechanical properties of the paste.

Fruit leathers, essentially flattened layers of fruit puree or concentrated juice, have proven popular for snacking and often feature in children’s’ lunchboxes and gym bags for a mouth-watering sugar boost. In the quest for optimum ‘bite’ in these products, a blade test mimics the action of biting, thus representing a useful indicator of eventual eating experience. Biextensional properties of fruit peels or leathers and edible films can also be assessed. Here, a Film Support Rig holds the sample film above the surface of the texture analyser, while a stainless steel probe is lowered through an aperture in the rig surface, beyond and below the level of the sample. A top plate holds the sides of the sample in place, as the film’s resilience and ‘burst strength’ are accurately and objectively recorded. This test can also provide excellent indictors of the fruit product texture under ambient and refrigerated storage conditions, for instance.

In recent times, as time and money have become precious, canned and frozen fruit have regained their share of the market as economical and convenient guarantee of daily fruit intake. Unfortunately, texture more than anything suffers as a result of the canning and freezing process of fruit – the former causes fruit to lose integrity, the latter its crispness upon thawing. Though difficult challenges to overcome, monitoring of fruits at the various process stages can lead to a deeper understanding of textural parameters. Commonly fresh strawberries are harder than tinned or frozen/thawed varieties, for instance, but quick (or cryogenic) freezing minimises structural damage compared to the standard batch freezing process.

In applications where fruits are highly refined (i.e. further removed from their natural state), there is a myriad of factors that has a potential impact on the final texture profile. Achieving the desired composition and balanced texture then becomes a more delicate process, as alteration of one ingredient can affect another. For example, the addition of a starch or similar ingredient performs a necessary stabilising function in fruit-based preparations such as jams, jellies and preserves, but can at the same time impart undesirable influences on texture.

A home-made feel is popular with consumers of such products, and quality is often judged on piece integrity and fruit-to-syrup/jelly ratio. For manufacturers and product developers, objective assessment of texture tells of a fruit’s processing tolerance and stability, as well as the effects of various manufacturing methods and ingredients.

In jarred, homogenous products, a back extrusion test is carried out by fixing a disc to the TA.XTPlus texture analyser: the disc moves down into the product to a chosen distance, and then withdraws upwards at a chosen speed or to a specified distance. The resulting curve is taken as an indication of product consistency. For testing non-homogenous samples, such as jam or marmalade, a Multiple Puncture probe can be used. Testing within the product’s standard vessel eliminates the risk of structure disruption in transferring the product into a testing container.

With the exception of the consumer’s keen eye and firm touch, methods of testing fruit quality through texture nowadays bear little evidence to agricultural roots. With the help of advances in technology, new apparatus is producing results that are precise, reliable and repeatable. Innovative, flexible means of assessment are becoming accessible to the industry as a whole. Equally, the knowledge gained is proving an invaluable source in optimising business processes. And all of the above for the greater good – let them eat fruit.