CITRUS QUALITY MANAGEMENT SYSTEMS
At Citrus Australia, we recognise quality is important and that is why we have developed a simple set of standards outlined in the following manual. Called the Australian Citrus Quality Standards, they aim to assist growers, packers, marketers and retailers deliver quality citrus that Australian consumers can enjoy with confidence. We aim to assist you in keeping your valuable consumers happy and coming back for more!
Quality of Fresh Citrus Fruit
Fresh citrus fruit, comprising oranges, grapefruit, mandarins, limes, lemons and minor varieties, are among the most popular fruits. With increasing year round competition from other fruits, maintaining this market position will require that fresh fruit quality be optimal. Determinants of fruit quality can be divided into those affecting external quality and those defining internal quality. Both of these are critical, since external quality influences initial purchasing decision, while internal quality determines consumption and repeat sale.
CITRUS QUALITY INSPECTION
Citrus is the most economically important fruit crop in the world. In citrus, the concept of fruit quality comprises several other aspects intimately related to human health apart from physical attributes and diet components. Citrus is an excellent model to study fruit quality because of its peculiar fruiting, singular biochemistry and economical relevance. A citrus breeding programme starts with the selection of suitable parents and the planning of controlled crosses. Information on the breeding value of available parents and the heritability of specifi c characters is important in a plant breeding programme to aid the breeder in parent selection and the planning of controlled crosses. Major goals of variety breeding in citrus are mostly related to fruit quality, productivity and harvesting period. In a broad sense, citrus fruit quality includes many physical attributes like fruit color, fruit size, easy of peeling and seedlessness. These traits have become paramount in commercial citrus types and new cultivar being developed through plant breeding and selection of new sports. This paper focus on four main citrus characteristics that responsible for fruit quality and are the basis for judging the product acceptability by consumers. We also discuss the variety strategy for citrus quality improvement.
Key determinants of citrus fruit quality: Metabolites and main changes during maturation
Citrus is one of the main fruit crops in the world and widely recognized by their organoleptic, nutritional and health-related properties of both fresh fruit and juice. The genetic diversity among the genus and the autonomous and independent changes in peel and pulp, make the definition of standard maturity indexes of fruit quality difficult. Commercial maturity indexes in the citrus industry are usually based on peel coloration, percentage of juice, soluble solids/acidity ratio but their relevance may differ among varieties and the specific requirements of the markets. There is also a marked influence of environmental and agronomic conditions such as light and temperature, rootstock selection and plant nutrition, among others. Besides commercial requirements, a more comprehensive definition of fruit quality should also consider organoleptic and nutritional properties that are determined by a complex interaction among a number of bioactive components. Citrus fruit are an excellent source of many phytochemical, including ascorbic acid, carotenoids (antioxidant and pro-vitamin A), polyphenols, flavonoids, limonoids, terpenoids, etc., which greatly contribute to the health-related benefits of these fruits. Criteria and definition of the main maturity indexes for citrus fruit worldwide are described, as well as changes during fruit maturation in key components affecting organoleptic and nutritional properties. Moreover, the involvement of hormonal and nutritional signals and their interaction in the regulation of external and internal maturation of citrus fruit, as well as the influence of environmental and agronomic factors are also critically revised and discussed.
Nondestructive Assessment of Citrus Fruit Quality and Ripening by Visible–Near Infrared Reflectance Spectroscopy
As non-climacteric, citrus fruit are only harvested at their optimal edible ripening stage. The usual approach followed by producers and packinghouses to establish the internal quality and ripening of citrus fruit is to collect fruit sets throughout ripening and use them to determine the quality attributes (QA) by standard and, in many cases, destructive and time-consuming methods. However, due to the large variability within and between orchards, the number of measured fruits is seldom statistically representative of the batch, resulting in a fallible assessment of their internal QA (IQA) and a weak traceability in the citrus supply chain. Visible/near-infrared reflectance spectroscopy (Vis–NIRS) is a nondestructive method that addresses this problem, and has proved to predict many IQA of a wide number of fruit including citrus. Yet, its application on a daily basis is not straightforward, and there are still several questions to address by researchers in order to implement it routinely in the crop supply chain. This chapter reviews the application of Vis–NIRS in the assessment of the quality and ripening of citrus fruit, and makes a critical evaluation on the technique’s limiting issues that need further attention by researchers.
Citrus fruit are grown commercially in more than 50 countries around the world and are major commodities in the international trade [1, 2]. In Europe, the exceptional characteristics met by some of these produces have granted them the Protected Geographical Indication (PGI), such as the lemons (Citrus limon (L.) Osbeck) of Menton in France, Sorrento, Amalfi and Syracuse, and the Sicilian blood orange (Citrus × sinensis) in Italy, the “Algarve Citrus” in Portugal, or the “Valencianos Citrus” in Spain.
As non-climacteric, citrus fruit are only harvested at their optimal edible ripening stage, and are required to meet the expectations of the current consumer who demands for fruit not only with the best appearance, flavor, and nutritional properties, but that also comply with safety, traceability, and the sustainability of the cultural practices used. Like any other commodity, citrus fruit are subjected to worldwide standard specifications within the value chain  on their quality attributes (QA). Additionally, there are also adjustments to these requirements on quality and commercial ripening indices, that arise from the respective PGI normative of each commodity, growing regions and destination markets . The main external quality attributes (EQA) accounted for citrus fruit are general appearance, size, weight, and color. Among the internal quality attributes (IQA), soluble solids content (SSC), titratable acidity (TA), juiciness, maturity index (MI; MI = SSC/TA), and the absence of internal defects are the most relevant. Although firmness is not defined quantitatively, it represents an important IQA, since it is a limiting factor regarding postharvest handling, transport and shelf-life, fruit being expected to maintain a good consistency through the whole supply chain.
Once fruit attain the expected IQA, additional factors will condition the harvest of citrus fruit: orchard yield and size, ripening variability, harvest cost, storage conditions, market prices and consumers’ demand. Although dependent on the country, producers are provided with three options to handle these constrains: (i) immediate harvest and marketing; (ii) immediate harvest and cold-storage; or (iii) delayed fruit harvest. Opting for immediate harvest may result in minimal organoleptic quality and low prices, whereas postponing it until favorable market conditions, risks fruit drop, decay and spoilage caused by extreme weather events, pests and diseases . To prevent some of of these consequences, producers resort to the regular use of pesticides, which increase the production costs and impact negatively the environment [6, 7]. Cold-storage is used in some of the major citrus producing countries, such as Spain or South Africa, and require very strict conditions to avoid fruit loss caused by chilling and/or freezing injury . Both, cold-storage and harvest delay may lead to adverse alterations in the citrus-like flavor, and thus fruit quality deterioration, even if MI or SSC remains acceptable for marketing . In all cases, fruit become more susceptible to the occurrence of physiological disorders that cause internal and/or external defects. Among the most typical physiological disorders registered through the supply chain of citrus fruit, there is the section drying, the rind breaking disorder (RBD), the rind pitting disorder (RP), freezing damage, and granulation, as reported for tangerine (Citrus tangerine Tanaka , ‘Nules Clementine’ mandarin (Citrus × clementina) , ‘Marsh’ grapefruit (Citrus × paradisi Macfad.) , sweet lemons (Citrus limettioides Tan.) , and ‘Honey’ pomelo (Citrus maxima Merr.) , respectively. These disorders are difficult to sort out by visual inspection at harvest, but lead to posterior fruit deterioration, limiting their quality, shelf-life, price and acceptance by consumers. In fact, there are strict standards for fruit sorting and grading, which require the detection of some of these disorders, throughout the supply chain, as established by the California Department of Food and Agriculture (CDFA). For exemple, it is not permitted to sell oranges (Citrus sinensis (L.) Osbeck) if, generally, more than 15% of fruits per batch have considerable freezing damage .
Therefore, the ripening of citrus fruit at harvest is a major determinant of their final quality after the whole postharvest handling processes, the occurrence of storage disorders, and the produce shelf-life span . It also affects the rate of fruit loss between the tree and the consumers’ home. Thus, the management and the decision capacity of the optimal harvest date (OHD) is a critical step in the supply chain. The current approach followed by producers and packinghouses to establish it and therefore, to decide on the harvest, is to collect small fruit sets from the various orchards by the beginning of each variety harvest season, and to use them to determine QA through standard methods, that in most cases are destructive, subjective and very time-consuming.
However, all QA vary greatly inside the same orchard, either in terms of absolute values and/or in terms of spatial and temporal distribution, and even in the same tree. This has been shown in citrus orchards of ‘Shiranuhi’ mandarin (C. unshiu × C. sinensis) × C. reticulata , ‘Ortanique’ (Citrus reticulata Blanco x Citrus sinensis (L) Osbeck) , mandarin (Citrus reticulata Blanco) , and ‘Newhall’ and ‘Valencia Late’ orange . Multiple factors, such as the level of sunlight exposure and the associated fruit temperature on the tree, fruit yield and size, tree vigor and age, rootstocks, site-specific nutritional requirements and micro topographies within the orchard, are reportedly associated to this variability [21, 22, 23, 24]. Furthermore, the location of the orchards and their edaphoclimatic conditions, as well as the cultural practices also induce variability on the fruit maturation process, leading to different levels of QA and different ripening rates observed for the same cultivar at different sites [20, 21]. Consequently, the number of tested fruits with the standard methods is seldom statistically representative of the orchard, leading to the sub-representation of the effective ripening stage of the fruit within and between orchards, which results in a limited assessment of their ripening, heterogeneous fruit quality, a deficient OHD management and a weak traceability in the citrus supply chain [25, 26, 27].
Overall, there is the need to upgrade the management and the sustainability of citrus fruit supply chain with smart and nondestructive technologies that allow a fast, objective, accurate and extensive assessment of fruit QA and ripening on-tree and in the following postharvest, to replace conventional methods. Their aim would be to deliver the best produce to the markets, and contribute to reduce the current level of food loss around the globe, that involves a large portion of fruit and vegetables [28, 29, 30]. Considering how much of the world’s population lacks food security, and the importance of these commodities in the provision of essential nutrients and vitamins, which could prevent malnutrition, that kind of technologies would comply with the sustainable development goals (SDGs) proposed by the Food and Agriculture Organization (FAO), International Fund for Agricultural Development (IFAD), and the World Food Programme (WFP), in the 2030 Sustainable Development Agenda, which supports a global commitment to end poverty, hunger and malnutrition by 2030, creating a #ZeroHunger world [31, 32].
The large number of reports published in the past two decades, show an active, and highly motivated research concerning the development of various nondestructive technologies for the assessment of quality and ripening parameters of a wide variety of fruit, including citrus [16, 33, 34, 35, 36, 37]. These techniques are used on inline sorting systems, on the bench or in the field and come in many forms, prices and commercial brands. Among them, the visible–near infrared reflectance spectroscopy (Vis–NIRS), is conceivably one of the most suitable and advanced nondestructive technologies currently used to monitoring several horticultural produces. It has been implemented in applications ranging from the inline automated grading systems, assessing up to 10–12 fruit per second, to handheld units suitable for field use, operating in full sunlight and varying ambient temperature [38, 39]. Additionally, it continues to grow stronger as a major investigation topic worldwide, with a major potential for improvement and contribution to the state of the art of precision agriculture and agronomic systems management .
This chapter comprises a brief explanation of Vis-NIRS fundamentals and a review of the various reports on its application published since 2012. Reports published before 2012 were already covered in the last review by  and will not be repeated here, with a few exceptions that represent relevant breakthroughs in the area. It will further attempt a critical evaluation on the limiting issues that need further research, to implement it as an effective nondestructive method to assess these commodities’ quality and optimal ripening.
The authors invite the reader to complement this chapter with some of the most outstanding reviews published throughout the years, by the main researchers working on the subject (but not only in citrus). These reviews comprise the principles of the technique, its various methods and the listing of fruit and the respective QA for which it has provided calibration models [41, 42, 43, 44, 45], the overview on the publications and main research groups in the field , various recommendations for future research activity in the area regarding the adequate experimental design and the reporting requirements , as well as the current real-life applications available on the market that seem to comply with the warranted robustness for the technology to be integrated in the supply chain of many crops, including citrus [38, 39].