Preliminary assessment of vegetable oil adulteration of pistachio paste by near infrared spectroscopy

Introduction P istachio (Pistacia vera L.) is a tree, native to Asia Minor, which was introduced in Italy early in the first century AD. Today, pistachio is cultivated in many countries within the northern hemisphere; nearly 50% of worldwide production comes from Iran, followed by Turkey (15%) and Italy (<1%). China, Syria, Greece and the United States are among the top contributors of pistachio production. Italian and Turkish varieties are considered the world’s best pistachios, thus justifying their high prices—approximately E25 and E18 per kilogram, respectively. Pistachio paste, obtained from roasted and refined pistachio seeds, is a widelyused ingredient in the confectionery industry. Ice cream manufacturing in Italy alone consumes hundreds of tons of this paste annually. Pistachio pastes typically have a lipid content ranging from 40% to 50%. However, the high price of pistachio paste relative to other low-cost lipid sources, in addition to some processing advantages, has resulted in an increase in intentional, undeclared vegetable oil-based adulterants (e.g., sunflower oil—about E1 per kg) at levels of up to 15% in marketed products. Sterol composition is often used as a marker to determine adulteration of foodstuffs with vegetable oils. The present preliminary study has been carried out in order to evaluate the potential of NIR spectroscopy as a screening tool for undeclared adulteration of pistachio paste in the confectionery industry. Δ7-stigmastenol as an adulteration marker Typical fatty acid composition analysis is not suitable for the assessment of adulterated food matrices. For one thing, the composition variability is typically quite small. On the other hand, quantitative parameters of the adulterants, such as sunflower oil, tend to vary with climate, geographic origin and seed maturity, and are easily altered by refining and fractionation. Triglyceride evaluation also poses challenges. The differences in triglyceride composition that would be expected for low-level adulteration would be quite small relative to the differences in composition caused merely by natural variation. In this situation, the sterol fraction was considered to provide the greatest specificity for the adulterant. The sterol fraction is a good fingerprint for each oil, not influenced by plant selection or hybridisation and it is often used as a tool for detecting adulteration of foodstuffs with vegetable oils. It is used also when they are altered by some processes like bleaching or deodorisation, thanks to their specific isomerisations. For sunflower oil, the sterol D7-stigmastenol naturally occurs at 15% of the total sterol content on average, while for pistachio paste, D7-stigmastenol is found at 0.2–0.8% of total sterol content. This composition is independent of seed origin, the degree of roasting and the refining process. Therefore, D7-stigmastenol composition in adulterated pistachio paste increases in proportion to the adulteration—typically up to 2%. However, the change of expected sterol composition in agreement with sunflower sterol composition confirms the adulteration: this includes a decrease in b-sitosterol and increase in D7-avenasterol, D7-campesterol and stigmasterol content concomitant with rising D7-stigmastenol concentration. The typical ranges for each sterol in unadulterated and sunflower adulterated pistachio paste are provided in Table 1. Among these sterols, D7-stigmastenol has been identified as the marker of choice due to its high specificity and the magnitude of composition change associated with adulteration. Analysis of the sterols is usually carried out by gas chromatography with field ionisation detection (GC-FID), according to the AOCS Official method. Other chromatographic methods have been proposed for this kind of analysis, but they lack the sensitivity, broad selectivity and range of linear response required.