International hazelnut quality characteristics

Niels Maness – Department of Horticulture, Oklahoma State University

HGA newsletter, June 2006

Quality Characteristics and Criteria

In-shell hazelnuts should be properly sized to meet the stated market type and should be properly filled with at least 50% of the shell cavity occupied by nutmeat. Shells should be free of cracks and noticeable mechanical injury, clean, brightly colored and coloring patterns should be characteristic of the stated variety. The pellicle should be smooth and devoid of husk attachments. Kernels should meet the stated market type, be free of any misshapen or underdeveloped kernels and be free of any shell or foreign material and off-odor, off-flavor or mold. Water content of kernels should not exceed 6% if shelled or 7% if in-shell, and the total water content of unshelled nuts should not exceed 10 to 12%. Size is specified with grade as a determinant of quality, and minimum sizes are used for specification of classes “Extra” and “Class I” in international trade. For in-shell markets, larger and particularly rounded types are preferred. Shelled markets accommodate both rounded and oblong types, and size preference is dependent on the intended end use. 


Size is specified in connection with the grade in terms of minimum and maximum diameters which will pass through round openings of a screen and are specified separately for round-type varieties versus long-type varieties. For round-type varieties, Jumbo nuts are those which will not pass through a 56/64 in(22.2 mm) round opening; Large nuts will pass though a 56/64 in (22.2 mm) opening but will not pass through a 49/64 inch (19.4 mm) opening; Medium nuts will pass through a 49/64 inch (19.4 mm) opening but will not pass through a 45/64 in (17.9 mm) opening; Small nuts are all nuts which will pass through a 45/64 in (17.9 mm) opening. For long-type varieties, Jumbo nuts are those which will not pass through a 47/64 in (18.6 mm) opening; Large nuts will pass through a 48/64 inch (19.0 mm) opening but will not pass through a 44/64 in (17.5 mm) opening; Medium nuts will pass through a 45/64 inch (17.9 mm) opening but not through a 34/64 in (13.5 mm) opening; Small nuts will pass through a 35/64 in (13.9 mm) opening.

Optimum Storage Conditions

Soon after harvest nuts should be dried to below 10 to 12% moisture, with kernels below 6 to 7% moisture to deter mold growth. In-shell and un-roasted kernels may be stored for 24 mo with minimal loss in quality at temperatures up to 10 °C (50 ºF). Roasted kernels may only be held for 6 mo prior to development of detectable rancidity stored at 0 °C, 5 °C or 10 °C  (32, 41, or 50 ºF). However, reduced temperature may be effective in combination with other protective measures such as vacuum packaging in extending roasted kernel shelf-life to 1 year or more (Ebraheim et al., 1994).

Chilling Sensitivity:

Hazelnuts are not sensitive to chilling temperatures and are commonly stored at temperatures at or below freezing for long term storage.

Ethylene Production and Sensitivity:

Ethylene may be used as a harvest aide to enhance maturation of husks for earlier harvest (Lagerstedt, 1979). During storage hazelnuts produce very low levels of ethylene.

Respiration Rates:

Properly dried hazelnuts exhibit very low respiration rates during storage.

Physiological Disorders:

Black tips on kernels appears to be associated with nuts having split or weak sutures. It appears to be caused by an oxidation process that occurs on the pellicle only, and may or may not be associated with moldy kernels (Thompson et al., 1996). Twin kernels occur when two kernels develop within one nut shell, and is an undesirable trait because of the small size and irregular shape of affected kernels. Blank nuts are in-shell hazelnuts devoid of normal kernels which result from defective embryo sacs, unviable eggs, failure of fertilization or embryo abortion at varying stages of development. Owing to the alternate bearing cycles for hazelnuts, poorly filled nuts may also result during an overproduction year, and poor kernel fill appears to be a heritable trait (Thompson et al., 1996).

Postharvest Pathology:

The most common decay found in hazelnuts is molds, with Romularia spp. most prevalent throughout nut development and the major pathogen associated with kernel tip mold (see p. 483 of Ebraheim et al., 1997). Although Romularia spp. appears to infect hazelnuts during nut development and may be quiescent prior to maturity and storage, many molds require breakage of the shell to contaminate the nut and thus intactness of the shell offers some natural defense against mold infestation. The dominant fungal flora during storage is Penicillium and Aspergillus spp. A. flavus capable of producing aflatoxin has been isolated from hazelnuts in storage (Eke and Goktan, 1987). Reduction of in-shell moisture content to below 10%, and nutmeat moisture content to less than 6% is an effective means deterring mold growth. Sanitation with chlorine dips may also be effective in reducing the incidence of mold infestation by reducing the amount if innoculum carried into post harvest storage. Because of the high amounts of organic material on the surface of shells, chlorine concentrations should be monitored and replenished as necessary to maintain chlorine at concentrations necessary to kill microorganisms.