Technology Transfer to Improve Fruit
By Terry, Leon
In the first of a six-part series from Cranfield University’s Plant Science Laboratory, Dr Leon Terry looks at research on strawberry quality and using biosensors for fresh-produce quality assessment Strawberries are one of the most important horticultural crops grown in the UK. The rapid growth in sales over the past decade has been underpinned by ongoing research and development both in the UK and overseas. However, technology transfer between research establishments and industry within horticulture, in general, has often been inadequate, partly due to the commonly misperceived view that research and industry have different agendas.
The recent VI International Strawberry Symposium 2008 (see www.iss2008 spain.com/ingles/index.cfm) demonstrated that both basic and applied research can play a significant role in informing industry through targeted knowledge transfer. The conference was held in Huelva, Spain, on 3-7 March. It was attended by some 700 delegates from 60 different countries. The oral presentations were divided into 10 sessions including genetics and breeding, physiology, soil disinfestations, nurseries, crop production, integrated management, ecological production, and post-harvest and quality.
Highlights of the conference included the keynote oral presentation by Professor Douglas Shaw of the University of California (UC) Davis, who has reviewed the UC Davis strawberry breeding programme, which has lasted for 47 years. He showed that the breeding programme had improved a number of key quality indicators including yield, firmness, appearance and size.
This created some discussion outside the conference hall that perhaps these quality attributes were now not always appreciated by the consumer, and that tconsumers increasingly demand improved taste and flavour. It is, of course, understandable why attributes such as firmness and yield have been – and remain – priorities. However, if organoleptic characteristics are not being sought – or, at least, not considered a priority – then one might question whether the consumer is being offered what they ultimately desire.
The sugar:acid ratio
The taste of strawberry fruit is reliant, in part, on the ratio between sugars and acids. Dilution of these attributes in strawberry cultivar Elsanta fruit through overwatering has been shown to reduce the concentration of sugars (fructose, glucose and sucrose) but not of acids (citrate, malate and ascorbate) *.
Moreover, supplying plants with less water during flowering and fruit development increased the concentration of not only sugars by 20 per cent in fruit (fresh weight basis), but also some healthrelated compounds/parameters (total phenolics, total antioxidant capacity (FRAP) and some individual anthocyanins) by as much as 40 per cent.
Reducing water delivery did, however, reduce berry size, and thus the industry needs to question whether the market would be prepared to pay a price premium for smaller berries with higher concentrations of taste- and health-related compounds. Strawberry fruit sales are governed by price, availability and quality – however, has the modern consumer been “trained” into thinking that bigger is best?
Sugar:acid ratios can be used as an important index of consumer acceptability and act as one determinant of strawberry fruit quality (eg riper fruits have a higher sugar content). Sugars are generally responsible for perceived sweetness. Acids can affect flavour directly and can regulate cellular pH, influencing the appearance of fruit pigments (eg anthocyanins).
Total soluble solids (TSS), which is often confused with total soluble sugars or Brix, is ubiquitously used by the strawberry industry to adjudge apparent sugar concentration or “sweetness”. TSS (%) is measured using refractive index, while Brix is technically a measure of the specific gravity.The two terms are commonly used interchangeably.
Titratable acidity (TA) is a measure of the buffering capacity of the fruit and is generally expressed as a percentage of the predominant organic acid (eg citric acid in strawberry).
Some strawberry fruit may have a high TSS value but low acidity and, therefore, taste bland. However, sugaracid ratios are infrequently used due to a requirement for specific instrumentation and semi-skilled analytical scientists (see panel, right).
Recent (as yet unpublished) work conducted within the Plant Science Laboratory at Cranfield University has shown that TSS is a very poor measure of sugars (fructose, glucose and sucrose) in strawberries and some other fresh produce. Results demonstrated that TSS of eight UK-grown strawberry cultivars was weakly correlated with actual sugar content measured using standard high-performance liquid chromatography coupled to evaporative light scattering detection (HPLC-ELS).
NEW TECHNOLOGIES: BIOSENSORS
Quality is the key issue common to all horticultural products. In virtually all cases, fresh-produce quality is set at harvest and inevitably declines during post-harvest senescence (ageing).
In order to evaluate quality, one must be able to measure quality- related attributes. Instrumental measurements are, generally, preferred to sensory evaluations in research and commercial situations because they reduce variation associated with subjective judgement among individuals. This approach can provide a means of transferring objective information on quality throughout the supply chain, so it is fundamental in ensuring greater vertical integration and maintaining trust between supply-chain actors.
Objective fresh-produce quality assessment can be either destructive or non-destructive. Currently, most nondestructive techniques are not yet appropriate for large-scale commercial use. Various methods using chlorophyll fluorescence, delayed light emission, electronic nose technology, laser vibrometry, nuclear magnetic resonance imaging, optical tomography, ultrasound and X- ray are either still in their infancy or are currently too expensive and/or unreliable to be adopted into most routine quality control (QC) operations. Despite this, some imaging technology (eg infra- red spectroscopy/reflectance) is close to industrial adaptation. However, in the short to medium term, QC improvements for fresh produce should also be based on established technology that is proven and inexpensive.
The inherent specificity, selectivity and adaptability of biosensors make them ideal candidates for use by the food industry. Potential applications in the supply chain range from testing of foodstuffs for maximum pesticide residue verification through to the routine analysis of analyte(s) concentrations – such as glucose, sucrose and alcohol – that may be indicators of food quality and acceptability.
Biosensor formats include simple “one-shot” disposable devices that can be used either in the field or integrated into more sophisticated laboratory instruments. Until now the main impact of these devices has been in the medical diagnostics field (eg blood- glucose biosensor used by diabetics). However, with ongoing technical development, the fresh produce industry will be one of the prime beneficiaries from biosensor technology in the future.
The total soluble solids (TSS) measure is generally poorly correlated to sugars for many fresh produce types, but is still used by the industry on a daily basis. What is needed is a simple, low- cost alternative to TSS, and also a replacement to the cumbersome and time-consuming nature of titrations, which would significantly enhance both the number and extent of tests carried out.
Determination of sugar concentration (rather thanTSS) and non- volatile acids (including ascorbic acid) in fruits would provide a significant improvement over current QC operations. Better feedback vertically through the supply chain caused by more informative QC would ensure fewer rejections and reduce waste. Fruit that is not of sufficient quality would be used for another purpose or customer.
Biosensors may offer one opportunity to fulfil this niche by enhancing the relevance and extent of QC tests being carried out through measuring specific target analytes that are directly related to produce quality. Biosensors are ideal for discrimination of fruit on the basis of taste- and health-related compounds1.
Work at Cranfield University has successfully demonstrated the novel use of biosensors for quantification of fruit quality attributes (ascorbate, beta-D and total D-glucose and sucrose) in tropical fruit (pineapple, mango and pawpaw)2, malate in tomatoes and pome fruit3 and pungency (pyruvate) in onions4. Ultimately, biosensor technology could also be used by growers and breeders themselves, to assess fruit prior to harvest and hence harvest fruit at the optimum time. Successfully completed and current biosensorbased projects conducted within the Plant Science Laboratory at Cranfield University include developing biosensors for the UKonion industry (cf HortLink HL0164 and HL0181) and UK soft fruit industry. The latter project is funded by the Horticultural Development Council through its 2006 Studentship Scheme (CP-43) and GlaxoSmithKline (PhD student: Jordi GineBordonaba).
1. Terry et al, 2005. The Application of Biosensors to Fresh Produce and the Wider Food Industry. J. Agric. Food Chem.53, 1309- 1316
2. Jawaheer et al, 2003. Development of a Common Biosensor Format for an Enzyme-based Biosensor Array to Monitor Fruit Quality. Biosens. Bioelectron. 18, 1429-1437 3. Arif et al, 2002. L-Malic Acid Biosensor for Field-based Evaluation of Apple, Potato and Tomato Horticultural Produce. Analyst 127, 104-108
4. Abayomi et al, 2006. Development of a Disposable Pyruvate Biosensor to Determine Pungency in Onions (Allium cepa LJ. Biosens. Bioelectron. 21, 2176-2179; Abayomi & Terry, 200 7. A Pyruvate Dehydrogenase-based Amperiometric Biosensor for Assessing Pungency in Onion (Allium cepa L.). Sens. & Instrumen. Food Qual. 1, 183-187
Test of quality: biosensors have been used to assess attributes of tomatoes and onions
Strawberries: consumers increasingly demand improved taste and flavour, so research into boosting these organoleptic characteristics is important
* Terry et al, 2007. Effect of Deficit Irrigation and Inoculation with B. cinerea on Strawberry (Fragaria x ananassa) Fruit Quality. J. Agric. Food Chem.55, 10812- 10819
Dr Leon Terry is head of the Plant Science Laboratory at Cranfield University in Bedfordshire.
Copyright Haymarket Business Publications Ltd. May 15, 2008
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