Optimizing Fruit Harvest Timing for Maximum Oil Recovery from Arbequina Olives in the Pothohar Region: A Six-Year Multi-Location Study

Olive oil is consumed internationally and is a major part of the Mediterranean diet. It is globally recognized for its health and economic benefits (Allaq et al., 2022; IOOC, 2018). Olive oil is native to Mediterranean countries, where almost 95% of global production is produced, especially in Spain, Italy, and Greece. However, in recent years, its expansion has been seen in non-traditional countries like America, Australia, and South Asia, especially Pakistan (Torres et al., 2017; Wang et al., 2018). In Pakistan, commercial olive cultivation began in the last two decades and is emerging as a strategic agricultural sector. Currently, more than 50,555 acres of land in Pakistan are under olive cultivation, and the Pothohar Plateau contributes more than 32% of the total plantation (Pak Olive, 2025). This part of the province is the most promising olive-growing area due to having a unique combination of Mediterranean-like climate, suitable topography and Government-led initiatives that have turned virgin lands into valuable economic assets. However, optimal harvest planning remain unexplored, hence limiting the oil yield potential. On the other hand, various developmental projects at Government level, such as “Transformation of Agriculture in Pothohar, Developing Pothohar into an Olive Valley, Establishment of Centre of Excellence for Olive Research & Training (CEFORT) and Promotion of Olive Cultivation at Commercial Scale in Pakistan,” have sensitized the olive expansion, especially with olive cultivars like Arbequina, Koroneiki, BARI Zaitoon-1, Gemlik, and Manzanilla etc. which are suited to arid environment.

Olive fruit harvesting time is the most critical factor which influences both oil quantity and quality. Late harvesting generally improves oil recovery but imposes risk of over-ripeness and oxidative degradation, whereas early harvesting often results in lower oil recovery (Dag et al., 2011; García-Inza et al., 2018). Given Pothohar’s unique agro-climatic variations as compared with the Mediterranean climate, region-specific studies are important. Previous research studies in analogous climates (e.g. Argentine, Chile) recommended the role of genotype-environment interaction (Trentacoste et al., 2011; Rondanini et al., 2014), but Pakistani data is limited.

This work aimed to assess the effect of 8 calendar-based harvesting dates on olive oil yield in the three main olive-growing areas of the Pothohar region. By analyzing six years of field trials, the study provides region-specific, research-based information to optimize harvesting time, oil recovery, and processing efficiency in Pakistan’s emerging olive sector. This study assessed calendar-based harvest dates, but since the maturing period may fluctuate from year to year depending on climatic conditions, it is hoped that the practical suggestions from this study will be accepted by growers and processors under local conditions. The harvested dates observed over the years also suggest that a fixed calendar date cannot be used as a universal criterion, and that the suggested harvesting window should be based on annual fruit maturity and weather conditions. We assumed that the amount of olive oil recovered would increase as harvesting was delayed to an optimum period, then decrease with further delay. The results from this study are expected to help local olive growers select the optimum harvest time to maximize oil recovery and contribute to diversifying Pakistan’s agricultural economy.

Study area

This study was conducted in the three major olive-growing regions of the Pothohar Plateau: Chakwal, Kallar Kahar, and Talagang. These regions have a semi-arid climate, with average annual rainfall of 290 to 600 mm and temperatures ranging from 5 °C (winter) to 45°C (summer).

Experimental design

The experiment was conducted for six consecutive years (2017–2022) using a randomized complete block design (RCBD). In each region, three olive orchards were selected, and each orchard was divided into three blocks containing trees of uniform age and similar growth and management conditions. All experimental orchards were planted with the same olive cultivar, ‘Arbequina’. The experimental trees were 8 years old at the beginning of the study. The trees were planted at 18x18 feet spacing and maintained under similar agronomic and orchard management practices throughout the experimental period. Orchard management included [drip irrigation, fertilizer application (N.P.K), annual pruning, weed management through hoeing, and plant protection practices against olive psyllid], which were kept as uniform as possible across locations and years.

Harvesting schedule

For this experiment, eight harvest dates were tested at 10-day intervals from September 15 to November 25 (T1 to T8) (Table 1). Harvesting was done annually at all sites. Fruits were considered green at the start of the harvest and gradually turned dark purple as the harvest progressed. Fruit maturity was assessed visually on the basis of fruit skin color.

Sample selection and processing

Five trees were randomly chosen and tagged from each selected block. 10kg of olives were taken from each tree; 1350 kg of olives were collected annually (3 districts × 3 orchards × 3 blocks × 5 trees × 10kg of olives), and after 6 years, 8100 kg of olives were collected.

Fruit was harvested and delivered to the Centre of Excellence for Olive Research & Training (CEFORT) within 24 h for the extraction of oil by cold extraction process (Servili et al., 2013) in a PIERALISI (Fattoria) mill.

The fruits were harvested and, at CEFORT, initially washed to remove any dust, debris, or impurities. The cleaned fruits were then crushed with the olive pits in a hammer mill to get a homogeneous olive paste. The paste was then malaxed to promote coalescence of small oil droplets into larger ones, aiding oil separation. The malaxed paste was then horizontally centrifuged to remove the oil from the solid pomace and plant water. Oil was finally subjected to vertical centrifugation to obtain the final olive oil and remove residual moisture.

Oil Recovery Calculation

Oil recovery was expressed as:

Oil recovery (L 100 kg^-¹) = [Volume of extracted oil (L) / Weight of fresh olives (kg)] × 100

Statistical analysis

For statistical analysis, a three-way ANOVA was performed to evaluate the effects of treatments (harvest date), year and location, including their interactions. Duncan’s Multiple Range Test (DMRT) in SPSS V25 was used to obtain significant results (p < 0.05). The data were presented graphically using a boxplot showing the spread and distribution of oil recovery across treatments and locations.

Oil recovery trends across harvest dates

The olive oil recovery data from six years (2017 to 2022) showed that the oil recovery rate consistently increased from mid-September (T1) to mid-November (T7) at all three sites in Pothohar, then decreased on 26th November (T8) at the end of the harvesting period (Figure 1). In terms of oil recovery, Chakwal improved from 8.5% at T1 to 16.2% at T7, and Kallar Kahar from 8.2% at T1 to 15.9% at T7. This was observed in Talagang as well. Delayed harvesting increases oil recovery, mainly due to the progressive accumulation and maturation of oil in the olive mesocarp. The level of extractable oil tends to rise as the fruit develops from green to purple/dark as it ripens. Therefore, the later the harvest date, the higher the amount of extractable oil from the fruit and, thus, the higher the oil recovery. The same trend of an increase in oil content as the fruit matures has been reported for other olive cultivars such as Arbequina (García-Inza et al., 2014; Berbel and Posadillo, 2018).

Table 1: Fruit Harvest-date treatments and locations evaluated during the six-year study (2017–2022)

The small decrease at T8 could suggest that maximum oil accumulation was reached by late November. During maturation, fruit oil content may also change, adding to oil accumulation and affecting apparent recovery during mechanical extraction. Thus, T7 showed better recovery, indicating that fruit maturity and extractable oil content in mid-November were favorable under the climatic conditions of the Pothohar region.

A three-way ANOVA was used to further examine these patterns. The results showed that harvest date, location, and year significantly affected olive oil recovery (Table 2). The interaction harvest date × location also suggests that the response to delayed harvest date may vary by location, necessitating a different optimum harvest period in some areas. The same year-to-year and cultivar variability in response to harvest date has been described in other olive growing areas of the Mediterranean region.

Table 2: Analysis of variance table

MS error Df Mean CV

3.671839 286 9.620347 19.91824

Table 3: Mean oil percentage by year (Duncan’s grouping)

Table 4: Mean oil percentage by location

Year-to-Year variability

Year also significantly affected olive oil recovery percentage (Table 3). The data showed that maximum oil recovery was obtained during 2021 (10.70%) and 2017 (10.50%), whereas the minimum percentage of oil was recorded during 2019 (8.6%).

These differences likely reflect variation among climatic conditions, particularly rainfall and temperature fluctuation during fruit maturity (Trentacoste et al., 2012; Rondanini et al., 2014). According to Dag et al. (2011), the exceptional yield during 2021 may have resulted from optimal pre-harvest rains as reported in analogous arid regions.

Location effects and interactions

Main effect of location

No significant differences (p = 0.269) were observed among the three locations (Chakwal, Kallar Kahar & Talagang) in oil recovery (Table 4). These results are consistent with findings of homogeneous Mediterranean regions (IOOC, 2018) and suggest a uniform agro-climatic suitability across Pothohar’s olive growing area.

Treatment by year interaction

Significant differences were recorded for treatment by year interaction, highlighting that optimal harvesting time varies during different years. During 2021, the highest oil recovery (13.13%) was obtained at T4 (15 October), most probably due to synergistic effects of optimum temperature and timely rainfall (García-Inza et al., 2018). On the other hand, the lowest oil recovery (5.27%) was obtained at T1 (15 September) during 2019, highlighting the risks of early harvesting under suboptimal conditions (Dag et al., 2011). Borges et al. (2019) reported similar findings in their two-year experiments on Arbequina and Hojiblanca olive varieties.

Treatment into location interaction

Location-specific responses were recorded for the treatment-by-location interaction. The highest yield (11.82%) was recorded in Chakwal under T8 (25 November), whereas Kallar Kahar showed average performance (9.99%). This may be due to microclimatic differences in temperature during late harvesting (García-Inza et al., 2018).

The six-year, multi-location study in the Pothohar region shows that the harvesting period significantly influences olive oil recovery. Among eight evaluated dates for olive harvesting, Mid-November (T7 15 November) consistently produced the maximum olive oil content (11.04%), making it the most suitable harvesting date for the ‘Arbequina’ olive cultivar under Pothohar conditions. Although some year-to-year and location-specific differences were recorded, the general trend of increasing olive oil content from Mid-September to Mid-November remained consistent. Implementing this harvest schedule can significantly enhance olive oil yield in the Pothohar region, supporting the sustainability and profitability of Pakistan’s emerging olive sector. Future research should examine how harvest timing affects olive oil quality parameters and assesses the applicability of these findings to other olive cultivars and regions of Pakistan.

The authors sincerely acknowledge the Centre of Excellence for Olive Research & Training (CEFORT) at BARI, Chakwal, for providing the facilities for olive oil extraction. The authors also express their gratitude to the selected olive growers of Chakwal, Kallar Kahar, and Talagang for providing olive fruit essential for the successful conduct of this research.

This six-year, multi-location study identifies mid-November (15 November) as the best harvesting time for ‘Arbequina’ olives, providing novel, region-specific evidence to maximize oil recovery under Pothohar conditions.

Humara Umar: Conducted data collection and data entry; prepared the abstract, methodology, results and discussion; and contributed to the conclusion and references.

Inam Ul Haq: Conceived the research idea; contributed to research planning and interpretation of results; provided technical guidance throughout the research and manuscript preparation; critically reviewed the manuscript; and supervised the overall development of the article.

Shahid Ali: Served as the corresponding author; contributed to the interpretation and discussion of results; critically reviewed the manuscript; and contributed to improving its scientific content.

Attiq Ur Rehman: Performed statistical analysis; contributed to the interpretation of results; provided scientific input; and improved the presentation of the research findings.

Generative AI and AI-assisted technology statement

The authors declare that no generative AI and AI assisted technology was used in the creation of this manuscript.

Conflict of interest

The authors have declared no conflict of interest.