Tree Nut Industries Take Varying Approaches to Achieving Nut Quality

By CECILIA PARSONS | Associate Editor

Consistently delivering a top-quality product to domestic and export markets is the goal of tree nut growers and processors.

To achieve that goal, they must determine how and why quality can be impacted during growth, harvest and processing. While still on the tree, quality challenges include insects, disease, nutrition and environmental. Processors maintain quality during handling and storage. Some of those challenges may be beyond grower or processor control, but research is showing proactive management can improve nut quality.

One of the best examples of meeting the insect damage challenge to nut quality is a good control program that takes into account pest pressure and is timed to achieve maximum control possible.

Environmental challenges are a tougher nut to crack.

The pistachio industry has been focusing on early detection of hull breakdown to give growers tools to avoid poor nut quality. Almond processors are looking at wireless technology to detect insect infestation in stockpiles and treatments to manage stockpiles. Walnut industry research is aimed at determining factors that contribute to dark kernels and mold.

In her Pistachio Day presentation, Barbara Blanco-Ulate, UC Davis plant pathologist, said predicting pistachio hull breakdown using biomarkers will allow growers to develop management plans to ensure nut quality. Optimal harvest timing can also help with costs, she said. Biomarkers are molecular or physical signatures that can be measured in the field to anticipate large physiological changes like hull ripening and breakdown. Pistachio hull breakdown occurs after nut ripening and leads to higher nut susceptibility to insect infestation and fungal decay.

Blanco-Ulate said her California Pistachio Research Board-funded project led her to re-think nut growth stages. She noted that shell hardening and kernel growth happen at the same time. Hull ripening and kernel maturation start once the kernel reached the maximum size. There is also a peak in volatiles that happen just prior to hull ripening. Hull softening and hull coloration to red are reliable biomarkers of hull breakdown. Shell split occurs in parallel with hull softening.

Another finding in her research showed a faster hull breakdown in the Golden Hills variety compared to Kerman. At mid-August, Golden Hills had the highest level of VOCs, while Kerman VOCs peaked two weeks later at a much lower level. This faster rate of hull degradation leaves nuts open to insect damage and fungal growth and is the reason growers of this variety are urged to harvest early.

Blanco-Ulate and her research team also found that bloom time and temperature during early nut growth impact blanking and nut quality at harvest. Late bloom nuts have higher incidence of blanks and filled nuts without splits. With normal bloom, blank percentage was at 18%. Late bloom had 37% blanks. Normal bloom produced 94% shell split while late bloom looked had 55% shell split. Late bloom nuts have significantly harder hulls and softer shells. Blanco-Ulate also noted that it is not always the case that kernel expansion forces shell splitting. It is more than physical force, she noted.

Physical Measurements of Almond Quality

The almond industry is exploring genetics and chemical markers in kernels that contribute to taste and shelf life. Researchers are looking